BTU Sizer

Refunds and withdrawal

Mon, 10 Aug 2026 00:00:00 +0000

Last updated 2026-08-10.

The short version

You have 14 days from buying Pro to change your mind and get your money back. Press Withdraw from contract on your account page. We confirm by email straight away and the refund goes back to the payment method you used, within 14 days of that email.

You do not need a reason, and we do not ask you to give one up.

Your right of withdrawal

If you are a consumer in the EU, you have a statutory right to withdraw from this contract within 14 days without giving any reason. The period ends 14 days after the day the contract was concluded — that is, 14 days after you bought Pro.

Terms of use

Mon, 10 Aug 2026 00:00:00 +0000

Last updated 2026-08-10.

These terms cover btusizer.com (“the site”). Using the site means you accept them. If you buy Pro, the Pro section below is part of your contract.

Who you are dealing with

Operated by: maicrosites.com
Established in: Slovakia, European Union
Email: info@btusizer.com
Site: btusizer.com

1. What the site is

BTU Sizer is a set of calculators and reference pages. Everything that calculates runs in your browser and is free to use, with no account.

CFM to m³/h Airflow Converter

Sun, 09 Aug 2026 00:00:00 +0000

The conversions in one line each, for the times you want the number without the calculator.

From	To	Multiply by
CFM	m³/h	1.699
CFM	L/s	0.472
m³/h	CFM	0.5886
m³/h	L/s	0.2778
L/s	CFM	2.119
L/s	m³/h	3.6

The last row is the one worth memorising, because it is exact: a litre per second is 3.6 cubic metres an hour, and that falls straight out of 3,600 seconds in an hour and 1,000 litres in a cubic metre. No approximation involved.

Duct Size Calculator

Sun, 09 Aug 2026 00:00:00 +0000

Round duct, at the three velocities that matter. Read down the column that matches the run.

Duct	500 FPM (quiet)	700 FPM (branch)	900 FPM (trunk)
4 in	44 CFM	61 CFM	79 CFM
5 in	68 CFM	95 CFM	123 CFM
6 in	98 CFM	137 CFM	177 CFM
7 in	134 CFM	187 CFM	240 CFM
8 in	175 CFM	244 CFM	314 CFM
10 in	273 CFM	382 CFM	491 CFM
12 in	393 CFM	550 CFM	707 CFM
14 in	535 CFM	748 CFM	962 CFM
16 in	698 CFM	977 CFM	1,257 CFM

The jump from 8 to 10 inch is the one that surprises people: two inches of diameter carries more than half again as much air, because area goes with the square of the radius. It is also why squeezing a run down one size to clear a beam costs so much more than it looks like it should.

Equipment Replacement Tracker

Sun, 09 Aug 2026 00:00:00 +0000

Every piece of equipment with its install date and expected life, so the replacement is a planned purchase rather than an emergency call-out.

Evaporative Cooler Size Calculator

Sun, 09 Aug 2026 00:00:00 +0000

Volume in, CFM out, and the exhaust opening that has to come with it.

Space	Volume	CFM needed	Exhaust opening
12 × 12 ft bedroom, 8 ft ceiling	1,150 cu ft	600	half a window
20 × 15 ft living room, 8 ft ceiling	2,400 cu ft	1,200	one window
24 × 20 ft garage, 10 ft ceiling	4,800 cu ft	2,400	side door ajar
1,000 sq ft house, 8 ft ceilings	8,000 cu ft	4,000	four windows cracked
1,800 sq ft house, 9 ft ceilings	16,200 cu ft	8,100	windows on the far side

The exhaust column is not advice, it is part of the specification. A cooler with nowhere to push the air is a humidifier with a big motor.

Filter and Service Schedule

Sun, 09 Aug 2026 00:00:00 +0000

Set the interval once and get the next twelve filter changes and service dates, with an email before each one.

Free and Pro

Sun, 09 Aug 2026 00:00:00 +0000

Everything that calculates here is free and always will be — no account, no sign-up, no limits on how often you use it. That is not a trial. Search engines send people here to get an answer, and an answer behind a wall is not an answer.

Pro is for what happens around the answer. It remembers your setups on every device you sign in on, turns any result into a page you can send someone, lets you put a tool on your own site, emails you when a date you care about is coming, and takes the ads off every site we run — with one account and one price.

Humidifier Size Calculator

Sun, 09 Aug 2026 00:00:00 +0000

Output is what the room feels; tank size is what your evenings feel like. Both are below.

Floor area	Output needed	Typical machine	Tank you want
up to 300 sq ft	1 gal/day	tabletop	1 gal — daily refill
up to 500 sq ft	2 gal/day	tabletop or small console	1.5–2 gal
up to 1,000 sq ft	3 gal/day	console	3 gal+
up to 1,500 sq ft	4 gal/day	large console	4 gal, or plumbed in
up to 2,500 sq ft	6 gal/day	whole-house bypass	plumbed in
over 2,500 sq ft	9 gal/day	whole-house steam	plumbed in

The last two rows are the honest limit of a portable. A machine that evaporates six gallons a day has to be fed six gallons a day, and carrying that up the stairs in jugs stops being charming in the first week of January.

HVAC Quote Comparison

Sun, 09 Aug 2026 00:00:00 +0000

Equipment, installation, extras and the ten-year running cost of each quote in one table — the total that decides, not the number on the front page.

Radiator Size Calculator

Sun, 09 Aug 2026 00:00:00 +0000

Two tables, and the second is the one people find out about after the radiator is on the wall.

Room	Held at	Multiplier
Living room, dining room	21 °C	1.00
Bathroom	22 °C	1.09
Bedroom	18 °C	0.86
Kitchen	18 °C	0.78
Hall, landing	16 °C	0.71

The kitchen sits below the bedroom at the same temperature because appliances and cooking put heat back into the room, and a kitchen with an oven in it is rarely short of warmth.

Room Air Quality Plan

Sun, 09 Aug 2026 00:00:00 +0000

Purifier CADR for every room in one sheet, totalled — so a whole-house air plan is one table rather than six separate calculations.

Seasonal Changeover Plan

Sun, 09 Aug 2026 00:00:00 +0000

Two dates a year, set once — the spring and autumn changeover, with the checklist for each and an email before it arrives.

SEER, EER and COP Converter

Sun, 09 Aug 2026 00:00:00 +0000

The same machine, read five ways. Pick the row that matches the label you are holding.

SEER	SEER2 (ducted split)	EER	COP	kW/ton
13	12.4	11.2	3.28	1.07
14	13.3	11.8	3.46	1.02
16	15.2	12.8	3.75	0.94
18	17.1	13.7	4.01	0.88
20	19.0	14.4	4.22	0.83
24	22.8	15.4	4.51	0.78

Notice how the EER column flattens while SEER keeps climbing. That is not an artefact — the very high SEER numbers come from variable-speed equipment that is extremely efficient at part load, and part load is most of the season but not the afternoon you bought it for.

Server Room Cooling Calculator

Sun, 09 Aug 2026 00:00:00 +0000

The conversion is exact, so the table is just multiplication — and worth keeping to hand.

Equipment draw	Heat produced	With a 120 sq ft room and one person	Unit to buy
500 W	1,706 BTU/h	2,706 BTU/h	9,000 BTU/h
1 kW	3,412 BTU/h	4,412 BTU/h	9,000 BTU/h
2 kW	6,824 BTU/h	7,824 BTU/h	9,000 BTU/h
3 kW	10,236 BTU/h	11,236 BTU/h	12,000 BTU/h
5 kW	17,061 BTU/h	18,061 BTU/h	18,000 BTU/h
10 kW	34,121 BTU/h	35,121 BTU/h	36,000 BTU/h

Read the draw off the PDU, not off the labels on the back of the servers. Nameplate figures are what the power supply could deliver, not what the machine pulls, and they are usually about double — which is how rooms end up with twice the cooling they need and half the money they had.

Whole-Home Cooling Sheet

Sun, 09 Aug 2026 00:00:00 +0000

Every room in one table — area, ceiling and rate per room — adding up to the cooling load for the whole floor plan, saved and shareable.

About

Sat, 08 Aug 2026 00:00:00 +0000

BTU Sizer is a small independent site that answers one question in thirty-one different shapes: how big should the thing in this room be? Air conditioner, furnace, heat pump, dehumidifier, extractor fan, ceiling fan — each is a different machine with a different rule, and each rule is published somewhere by somebody who tested it.

Why it exists

The sizing question is asked by people who are about to spend real money, usually once every ten years, with no way to tell whether the number they have been given is right. The calculators that already exist fall into two camps. Manufacturers and retailers have one, and it exists to land you on their product page. General-purpose calculator sites have the arithmetic, but they stop at a raw figure — 7,340 BTU per hour — without telling you that no such machine is made and the choice is between 6,000 and 8,000.

Air Changes Per Hour Calculator

Sat, 08 Aug 2026 00:00:00 +0000

The same fan in different rooms is a completely different fan. This is what 100 CFM actually does, room by room.

Room	Volume at 8 ft	ACH from 100 CFM	Target
Cloakroom, 4 × 5 ft	160 cu ft	37.5	8
Bathroom, 8 × 6 ft	384 cu ft	15.6	8
Bedroom, 12 × 14 ft	1,344 cu ft	4.5	4–5
Living room, 16 × 20 ft	2,560 cu ft	2.3	4–5
Open plan, 20 × 30 ft	4,800 cu ft	1.3	4–5

Read the last two rows against the target column and the purifier market makes sense: this is why a unit that transforms a bedroom does nothing noticeable in an open-plan floor, and why CADR is quoted against room size rather than on its own.

Air Purifier Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

The two-thirds rule gives a floor. This table shows what that floor buys you, and what going above it does.

Room	Minimum CADR (2/3 rule)	Air changes/hour	For allergies or smoke
150 sq ft bedroom	100 CFM	~5	200 CFM
250 sq ft bedroom	167 CFM	~5	330 CFM
350 sq ft living room	234 CFM	~5	470 CFM
500 sq ft open plan	334 CFM	~5	670 CFM
800 sq ft floor	534 CFM	~5	1,070 CFM

Two practical notes the numbers do not carry. Rated CADR is measured on the top fan speed, which in most units is loud enough that nobody runs it there — so buy a size up and run it on medium. And a purifier only cleans the air that reaches it, so a unit behind a sofa in the corner of an open-plan room is doing considerably less than its label says.

Baseboard Heater Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Baseboard is the one heater sized in feet, which makes it the one you can check against the wall before you buy. This is the conversion table.

Room	Watts	Feet of baseboard	Usual split
100 sq ft	1,000 W	4 ft	one 4 ft unit
150 sq ft	1,500 W	6 ft	one 6 ft unit
200 sq ft	2,000 W	8 ft	one 8 ft, or 2 × 4 ft
250 sq ft	2,500 W	10 ft	6 ft + 4 ft
300 sq ft	3,000 W	12 ft	2 × 6 ft

Measure the wall under each window before ordering. A room that calculates at twelve feet and has nine feet of clear wall needs a rethink — a taller-profile heater, a hydronic model, or accepting that this room wants a mini split instead.

Bathroom Fan CFM Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Bathrooms come in a handful of sizes and so do fans. Here is the whole decision in one table.

Bathroom	Floor area	Fan by area	Fan by fixtures
Cloakroom, 4 × 5 ft	20 sq ft	50 CFM (minimum)	50 CFM (toilet)
Small, 6 × 8 ft	48 sq ft	50 CFM	100 CFM (toilet + shower)
Standard, 8 × 10 ft	80 sq ft	80 CFM	150 CFM (toilet + shower + bath)
Large, 10 × 12 ft	120 sq ft	use fixtures	150 CFM
Ensuite with jetted tub	140 sq ft	use fixtures	200 CFM

Where the two columns disagree, the fixture method wins — it is what HVI recommends above 100 sq ft, and it is the one that notices a separate shower and bath in the same room.

BTU to kW and Tons Converter

Sat, 08 Aug 2026 00:00:00 +0000

Equipment sizes cluster around a handful of values, and it is worth being able to recognise them in any of the three units.

BTU/h	kW	Tons	Usually seen as
5,000	1.47	0.42	smallest window unit
9,000	2.64	0.75	smallest mini-split head
12,000	3.52	1.0	one-ton unit, common split
18,000	5.28	1.5	large room, small house
24,000	7.03	2.0	two-ton central system
36,000	10.55	3.0	three-ton central system
60,000	17.58	5.0	largest common residential

A useful mental shortcut: BTU ÷ 3,400 ≈ kW, and BTU ÷ 12,000 = tons exactly. That gets you close enough to spot when a quote has been converted wrongly, which happens more often than it should.

BTU, Tons and Kilowatts Explained

Sat, 08 Aug 2026 00:00:00 +0000

Three units, one quantity. A contractor can quote you all three in a single conversation without noticing, and a listing can print two of them for the same machine.

What each one means

BTU per hour is a rate of heat flow: how much heat the machine moves in an hour. One BTU is the energy that raises a pound of water by one degree Fahrenheit — about the heat of a kitchen match. It is the US convention for equipment ratings, and the “per hour” is almost always dropped in conversation, which causes confusion of its own: a “12,000 BTU air conditioner” means 12,000 BTU per hour.

Ceiling Fan Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

The whole sizing table fits in five rows, and the fourth column is the one worth reading twice.

Floor area	Blade span	Typical room	Mount
up to 75 sq ft	36 in	bathroom, small study	flush or short rod
up to 144 sq ft	44 in	bedroom	short rod
up to 225 sq ft	52 in	large bedroom, dining	rod to bring blades to 8–9 ft
up to 400 sq ft	56 in	living room	rod, 18 in from walls
over 400 sq ft	60 in or two fans	open plan	two fans if longer than 18 ft

The mount column is where most ceiling fans are let down. A 52-inch fan flush against an eight-foot ceiling moves noticeably less air than the same fan on a short downrod, because the blades need clear air above them to pull from.

Central AC Tonnage Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Tonnage is the number on the quote, and it is the one homeowners are least equipped to argue with. This table is the sanity check: conditioned floor area against the size that is normally right for it, before insulation and climate pull it either way.

Conditioned area	Zone 2 (Houston)	Zone 4 (Baltimore)	Zone 6 (Minneapolis)
1,000 sq ft	2 tons	2 tons	1.5 tons
1,400 sq ft	2.5 tons	2.5 tons	2 tons
1,800 sq ft	3.5 tons	3 tons	3 tons
2,200 sq ft	4 tons	4 tons	3.5 tons
2,600 sq ft	5 tons	4.5 tons	4 tons
3,000 sq ft	5 tons	5 tons	4.5 tons

Notice how little the columns differ. Summer is roughly the same problem everywhere; winter is not, which is why the furnace calculator asks the same question and gets an answer that doubles across the table.

Climate Zones and Sizing

Sat, 08 Aug 2026 00:00:00 +0000

Two houses of identical size, one in Houston and one in Minneapolis, need almost the same air conditioner and furnaces that differ by two thirds. That asymmetry is the single most useful thing to understand about sizing.

What a climate zone is

The IECC climate zones run 1 to 8 from the Gulf coast to Alaska, and they exist for building codes: insulation requirements, window specifications and equipment efficiency minimums are all written per zone. Sizing rates follow them because they are already the standard way of saying “how cold does it get here”.

Contact

Sat, 08 Aug 2026 00:00:00 +0000

Corrections, questions and requests for a calculator we do not have yet — all welcome at the same address:

info@btusizer.com

Corrections are the most useful mail we get. If a rate, a chart row or a rounding step looks wrong to you, say which page and which number, and if you have the standard or the manufacturer document it came from, send that too. Sizing rules do change: the dehumidifier rating scale moved in 2019 and half the charts on the internet still have not caught up.

Dehumidifier Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

This is the whole chart, on today’s scale. Find the row for your area and the column for how the space honestly feels, not how you would like it to feel.

Floor area	Damp	Very damp	Wet
up to 500 sq ft	20 pints	20 pints	25 pints
up to 1,000 sq ft	20 pints	25 pints	30 pints
up to 1,500 sq ft	25 pints	30 pints	40 pints
up to 2,000 sq ft	30 pints	35 pints	45 pints
up to 2,500 sq ft	35 pints	40 pints	50 pints
over 2,500 sq ft	45 pints	50 pints	70 pints

Damp means a musty smell in humid weather. Very damp means the air feels clammy and there are damp patches. Wet means beading walls, seepage after rain or standing water — and at that point the dehumidifier is managing a symptom while the drainage or the damp-proofing is the actual repair.

Furnace Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Furnace sizing is one table with two axes, and both of them matter. Read down for your climate, across for your house.

Conditioned area	Zone 2	Zone 4	Zone 5	Zone 7
1,000 sq ft	40,000	60,000	60,000	60,000
1,500 sq ft	60,000	80,000	80,000	100,000
2,000 sq ft	80,000	100,000	100,000	120,000
2,500 sq ft	80,000	120,000	120,000	140,000
3,000 sq ft	100,000	140,000	140,000	140,000+

All figures are output BTU per hour, already rounded to a size that is sold. For the model number to shop for, divide by the AFUE: an 80% furnace needs a 25% larger input rating than the output shown, a 96% condensing furnace almost none.

Garage Heater Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Garages come in a handful of standard footprints, and knowing which one you have saves getting the tape measure out.

Garage	Typical size	Floor area	Uninsulated, zone 5
Single	12 × 22 ft	264 sq ft	30,000 BTU/h
Single, deep	14 × 24 ft	336 sq ft	45,000 BTU/h
Two-car	20 × 22 ft	440 sq ft	45,000 BTU/h
Two-car, deep	24 × 24 ft	576 sq ft	60,000 BTU/h
Three-car	32 × 22 ft	704 sq ft	75,000 BTU/h

Insulate the door, the ceiling and the perimeter of the slab and every row moves down at least one size. It is the only room in the house where the insulation decision changes the equipment you buy rather than just the bill.

Heat Pump Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Two numbers decide whether a heat pump installation is a success, and only one of them is on the box.

Outdoor temperature	Conventional single-stage	Cold-climate inverter
47 °F (rating point)	100%	100%
35 °F	~85%	~100%
17 °F	~60%	~85%
5 °F	~45%	~75%
−5 °F	little or none	~60%

The percentages are typical, not universal — every model publishes its own table, and the gap between two units with the same nameplate can be enormous down the left column. Ask for that table before signing, and check it against the coldest design temperature for your area rather than against the average winter day.

Heat Pumps in Cold Weather

Sat, 08 Aug 2026 00:00:00 +0000

A heat pump’s nameplate is a single number measured under one condition. The machine’s actual output changes with the weather, and it changes in the wrong direction: capacity falls as the house’s demand rises.

The rating point is 47 °F

Air-source heat pumps are rated for heating at 47 °F outdoors. That is a mild autumn day, not a January night, and it is the number on the box.

As it gets colder, there is less heat in the outdoor air to move and the compressor works against a bigger pressure difference, so output falls. Meanwhile the house’s demand is rising along a straight line. Typical figures:

How Many BTU Per Square Foot?

Sat, 08 Aug 2026 00:00:00 +0000

Every sizing conversation starts with a rate per square foot, and almost every argument about sizing is really an argument about which rate.

The cooling rate is 20, almost everywhere

Twenty BTU per hour for every square foot of floor is the figure behind the ENERGY STAR room air conditioner chart, and it holds across most of the United States. It assumes an eight-foot ceiling, ordinary daylight and two people in the room.

How to Measure a Room for Sizing

Sat, 08 Aug 2026 00:00:00 +0000

Every calculator on this site takes floor area as its first input, so an area that is wrong by 20% produces an answer that is wrong by a whole size. Measuring is the cheapest accuracy you will ever buy.

Measure the room, wall to wall

Take length and width at floor level, wall face to wall face. Skirting boards are close enough to ignore. What is not close enough to ignore:

How We Calculate

Sat, 08 Aug 2026 00:00:00 +0000

Every calculator on this site runs the same four steps in the same order: area, rate, corrections, ladder. Work out the floor area, multiply by a rate per square foot, correct for what makes this room different from the average one, then round up to a size somebody actually builds.

The formulas

Floor area: length × width, or the area typed straight in. L-shaped rooms are split into rectangles and added.
Cooling load: area sq ft × 20 BTU/h. Multiplied by ceiling height ÷ 8, because you cool a volume rather than a floor.
Heating load: area sq ft × the climate-zone rate below, same ceiling correction.
Electric heat: the same load in watts — BTU/h ÷ 3.412. About 10 W per square foot in an average room, which is where that convention comes from.
Corrections: sun ×0.9 shaded, ×1.1 very sunny. Insulation ×1.15 poor, ×0.9 good. Occupancy +600 BTU/h per person past two. Kitchen +4,000 BTU/h flat.
Air changes per hour: CFM × 60 ÷ room volume in cubic feet.
Rounding: always up to the next capacity manufacturers build. A 7,340 BTU air conditioner does not exist; the choice is 6,000 or 8,000.

The rules that are not area × rate

Four calculators here do not start from floor area, because the machine they size does not care about it.

Humidity and Comfort

Sat, 08 Aug 2026 00:00:00 +0000

Ask somebody to describe an uncomfortable room and they will mention temperature. Measure it and the temperature is usually fine. What is wrong is the moisture, and almost every sizing decision in the house affects it.

The numbers

Aim for 45 to 50% relative humidity in living space year round. The practical boundaries:

Relative humidity	What happens
Below 30%	Dry skin and airways, static, timber and joinery shrink
30–40%	Comfortable, common in winter with heating on
45–50%	The target
55–60%	Starts to feel close; dust mites become happy
Above 60%	Mould risk, musty smell, sleep quality drops

At 78 °F and 45% a room is pleasant. At 78 °F and 65% the same room feels stuffy and the instinct is to turn the thermostat down — which treats the wrong variable and costs money.

Manual J vs the Rule of Thumb

Sat, 08 Aug 2026 00:00:00 +0000

There are two ways to size heating and cooling equipment. One takes thirty seconds and lands within a size for a single room. The other takes a few hours and is what should happen before anybody installs a whole-house system. Knowing which one you are looking at is most of the battle when a quote lands.

What the rule of thumb actually is

A rate per square foot with corrections — the method every calculator on this site uses. It is an average of many real buildings, adjusted for the handful of things that vary most: ceiling height, sun, insulation, occupancy.

Mini Split Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

The mini split ladder has seven rungs and the gaps between them are wide. That single fact explains most of what goes wrong: between a 12,000 and an 18,000 head there is nothing, and a room that calculates at 13,500 has to go one way or the other.

Head size	Suits a room of	Also sold as
9,000 BTU/h	up to 450 sq ft	0.75 ton · 2.6 kW
12,000 BTU/h	up to 600 sq ft	1 ton · 3.5 kW
18,000 BTU/h	up to 900 sq ft	1.5 ton · 5.3 kW
24,000 BTU/h	up to 1,200 sq ft	2 ton · 7.0 kW
30,000 BTU/h	up to 1,500 sq ft	2.5 ton · 8.8 kW
36,000 BTU/h	up to 1,800 sq ft	3 ton · 10.5 kW

Go down rather than up when a room lands between two rungs and the smaller one is within about 15%. An inverter running near the top of its range all afternoon is doing exactly what it was designed for; one running at the bottom of its range is not.

Portable AC Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Portables are the one product on this site where the number printed on the box and the cooling that arrives in the room are different quantities. Since 2017 the US label has been honest about it, but shops, listings and older reviews still quote the old figure — often both, in the same listing, without saying which is which.

On the box (SACC)	Roughly, in old ASHRAE terms	Room it actually suits
6,000 BTU/h	~8,500 BTU/h	up to 230 sq ft
8,000 BTU/h	~11,500 BTU/h	up to 300 sq ft
10,000 BTU/h	~14,000 BTU/h	up to 385 sq ft
12,000 BTU/h	~17,000 BTU/h	up to 460 sq ft
14,000 BTU/h	~20,000 BTU/h	up to 540 sq ft

If a listing quotes a big BTU figure with no mention of SACC, assume it is the middle column. A “14,000 BTU” portable in that sense is a 10,000 machine, and it will disappoint anyone who sized their room against a window-unit chart.

Privacy Policy

Sat, 08 Aug 2026 00:00:00 +0000

Who is responsible for your data

Operated by: maicrosites.com
Established in: Slovakia, European Union
Email: info@btusizer.com
Site: btusizer.com

There is no data protection officer. The site is run by one person and meets none of the conditions in Article 37 GDPR that would require one, so the address above reaches the person who actually decides what happens to your data — not a queue.

What we process

The calculators run entirely in your browser — the room dimensions you type are never sent anywhere.

Range Hood CFM Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Range hoods are sized by what is underneath them, not by the size of the kitchen — which is why a big kitchen with a small hob needs less extraction than a galley with a professional range.

Cooktop	Against a wall	Island	Gas rule (BTU ÷ 100)
24 in	200 CFM	300 CFM	40,000 BTU → 400 CFM
30 in	250 CFM	375 CFM	50,000 BTU → 500 CFM
36 in	300 CFM	450 CFM	60,000 BTU → 600 CFM
48 in	400 CFM	600 CFM	80,000 BTU → 800 CFM
60 in	500 CFM	750 CFM	100,000 BTU → 1,000 CFM

Where the gas column is larger than the others — which it usually is on a serious range — that is the number to buy. And once any of these columns passes 400, budget for make-up air as part of the job rather than discovering it at inspection.

Room Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Rooms in ordinary houses cluster around a few sizes, which makes it easy to sanity-check a measurement before acting on it.

Room	Typical size	Floor area	Volume at 8 ft
Box room	8 × 8 ft	64 sq ft	512 cu ft
Single bedroom	10 × 12 ft	120 sq ft	960 cu ft
Double bedroom	12 × 14 ft	168 sq ft	1,344 cu ft
Living room	16 × 20 ft	320 sq ft	2,560 cu ft
Open-plan kitchen and living	20 × 30 ft	600 sq ft	4,800 cu ft

If your measurement lands far outside the row it should be in, measure again before buying anything. An area that is wrong by 20% moves most of the tools on this site by a full size.

Space Heater Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Electric heating is the easiest arithmetic on this site and the easiest to get wrong in practice, because the physics says one thing and the wiring says another.

Room	Watts needed	BTU/h	Fits one plug?
80 sq ft — small study	800 W	2,700	Yes
120 sq ft — box room	1,200 W	4,100	Yes
150 sq ft — bedroom	1,500 W	5,100	Just
200 sq ft — large bedroom	2,000 W	6,800	No
300 sq ft — living room	3,000 W	10,200	No

Everything below the third row needs either a hard-wired heater on its own circuit or a different kind of machine altogether. A mini split delivers three times the heat per unit of electricity, which changes the bill rather than the physics.

Why an Oversized Air Conditioner Is Worse

Sat, 08 Aug 2026 00:00:00 +0000

Almost everybody’s instinct is that a bigger air conditioner is a safer buy. It is the one place in home equipment where the instinct is exactly backwards, and the reason is worth two minutes.

Cooling and drying are two different jobs

An air conditioner does two things at once. It lowers the air temperature — sensible cooling — and it condenses moisture out of the air onto its cold coil — latent cooling. The first happens within a minute of the compressor starting. The second needs the coil to be cold and wet for a sustained run.

Window AC Size Calculator

Sat, 08 Aug 2026 00:00:00 +0000

Window units are bought in a hurry, usually during the first hot week of the year, and the decision is made standing in an aisle in front of four boxes. These are the four numbers worth having on your phone before you get there.

Unit	Cools about	Window width needed	Typical circuit
5,000 BTU/h	up to 250 sq ft	22–36 in	115 V, shared
8,000 BTU/h	up to 400 sq ft	24–39 in	115 V, shared
10,000 BTU/h	up to 500 sq ft	26–41 in	115 V, own circuit
12,000 BTU/h	up to 600 sq ft	27–42 in	115 V, own circuit
15,000 BTU/h	up to 750 sq ft	28–44 in	115 V or 230 V
18,000 BTU/h	up to 900 sq ft	29–45 in	230 V, dedicated

The “cools about” column is the base rate with no corrections. A sunny top-floor room with four people in it is one line further down the table than its floor area suggests.

Window vs Portable vs Mini Split

Sat, 08 Aug 2026 00:00:00 +0000

For a single room there are three realistic machines, and the choice is usually made on installation constraints rather than performance. It is worth knowing what each constraint costs you.

The short version

	Window unit	Portable	Mini split
Typical cost	$150–500	$300–700	$2,000–5,000 installed
Install	Yourself, an afternoon	Out of the box	Licensed installer
Delivered capacity	Full label value	60–70% of old-style ratings	Full, and modulating
Noise indoors	Loud — compressor is in the room	Loudest — compressor plus fan	Very quiet
Efficiency	Moderate	Lowest	Highest
Window	Blocked	Partly blocked by hose kit	Untouched
Heating too?	Rarely	Some models	Almost always

Why the portable loses capacity

A portable stands inside the room it is cooling. Two things work against it. Its condenser heat leaves through a hose that runs hot across the floor and radiates part of that heat straight back into the room. And a single-hose model takes the air its condenser needs from the room, which means the house pulls in an equal volume of hot outside air through every gap to replace it.