Air Conditioner Size Calculator: What Size AC?

What size air conditioner do you need?

Start at 20 BTU per hour for every square foot of floor. A 20 × 15 ft room is 300 sq ft, so 6,000 BTU/h — and that is the figure ENERGY STAR publishes for a room with an eight-foot ceiling, average light and two people in it.

Then correct for the things that room does not have in common with the table. Multiply by the ceiling height over eight feet, add 10% for a very sunny room or take 10% off a shaded one, add 15% if the place is draughty or single glazed, add 600 BTU for every person past the first two, and add a flat 4,000 BTU if it is a kitchen.

Round the answer up to a size that is sold: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000. Rounding down leaves the unit running flat out on the hottest day; rounding up two steps is worse than rounding down one, and the reason is on this page.

The 20 rule

Cooling starts at 20 BTU per hour for every square foot of floor, measured at an eight-foot ceiling. Everything else on this site is that number with the room's own corrections applied — sun, insulation, people, climate.

floor area sq ft × 20 × ceiling ÷ 8 × sun × insulation = BTU per hour

A room this big usually takes

3,000 BTUat 150 sq ft
5,000 BTUat 250 sq ft
7,000 BTUat 350 sq ft
9,000 BTUat 450 sq ft
11,000 BTUat 550 sq ft
14,000 BTUat 700 sq ft

Every sizing question on this site is the same three steps in the same order: work out the floor area, multiply by a rate per square foot, then correct for what makes this room different from the average one. Cooling uses 20 BTU per hour per square foot. Heating uses a rate that depends on how cold your winters get. A dehumidifier uses a chart rather than a rate. What never changes is the last step — the answer gets rounded to a size somebody actually sells.

The table below is the one worth keeping. It is the base estimate before any correction, which is where every conversation with a salesperson starts.

Room	Floor area	Cooling needed	Size to buy
Small bedroom	150 sq ft	3,000 BTU/h	5,000 BTU/h
Large bedroom	250 sq ft	5,000 BTU/h	5,000 BTU/h
Living room	350 sq ft	7,000 BTU/h	8,000 BTU/h
Open-plan kitchen and dining	450 sq ft	9,000 + 4,000 BTU/h	14,000 BTU/h
Small studio flat	550 sq ft	11,000 BTU/h	12,000 BTU/h
Large open-plan floor	700 sq ft	14,000 BTU/h	14,000 BTU/h

Read the last two columns together. The gap between them is the reason a room that needs 9,000 BTU and a room that needs 11,000 BTU can end up buying the same machine — and the reason the kitchen row jumps two sizes on one correction.

Frequently asked questions

How many BTU do I need per square foot?

Twenty BTU per hour per square foot is the working number, and it comes from the ENERGY STAR room air conditioner chart. It assumes an eight-foot ceiling, ordinary light and two occupants. Hot, humid climates push it slightly higher and mild coastal ones slightly lower, but the corrections that actually move the answer are ceiling height, sun and insulation — not the base rate.

Is it better to buy a bigger air conditioner than I need?

No, and this is the one place where the intuition is backwards. An oversized unit cools the air fast, hits the thermostat setpoint and shuts off before it has removed the moisture, so the room ends up cold and clammy. Those short cycles also wear the compressor and use more electricity than one longer run. Two sizes up is a worse mistake than one size down.

Does ceiling height change the size I need?

Yes, because you cool a volume rather than a floor. Every published chart assumes eight feet, so a ten-foot ceiling is 25% more air in the same footprint and the estimate goes up by the same quarter. Vaulted ceilings are worse than the arithmetic suggests, since the warm air collects up there and the thermostat never sees it.

What size air conditioner for a 300 sq ft room?

6,000 BTU/h before corrections, which is a size sold everywhere. Put four people in it and it becomes 7,200 and you buy the 8,000. Give it west-facing glass with no shade and 6,600 still fits the 8,000. That is why the corrections are worth two minutes: three plausible versions of the same room land on two different units.

How do BTU, tons and kilowatts relate?

One ton of refrigeration is 12,000 BTU per hour — the old figure for the cooling given off by a ton of ice melting in a day. One kilowatt is 3,412 BTU per hour. So a 12,000 BTU mini-split is a one-ton unit and 3.5 kW of cooling, and the same machine is advertised all three ways depending on which country’s catalogue you are reading.

Does this replace a proper load calculation?

For a window unit, a portable or a single mini-split head, this is the calculation everyone actually uses and it is the right one. For a whole-house system, a contractor should run an ACCA Manual J load calculation, which measures every wall, window and duct rather than assuming a rate per square foot. Use this to sanity-check the quote you are given — if a contractor’s number is far above what this shows, ask which room drove it up.

How we calculate this

Cooling capacity starts from the ENERGY STAR room air conditioner sizing rate of roughly 20 BTU per hour per square foot of floor area at an eight-foot ceiling. Ceiling height scales the result by height ÷ 8, because the load follows the air volume. Sun and insulation apply the published corrections of ±10% and +15%/−10%. Occupancy adds 600 BTU per hour for each person past two, and a kitchen adds a flat 4,000 BTU per hour — both figures as published by ENERGY STAR. The result is then rounded up to the nearest capacity manufacturers actually build, since a 7,300 BTU air conditioner does not exist. Conversions use 12,000 BTU/h per ton of refrigeration and 3,412.14 BTU/h per kilowatt. Every calculation runs in your browser: nothing is stored, and nothing is sent anywhere.

Worked example

With Room length (ft) 20, Room width (ft) 15, Ceiling height (ft) 8, Sun on this room Average light, Insulation Average — typical home, People in the room 2, this page works out 6,000 BTU/h ≈ 1.8 kW. The room works out at 6,000 BTU/h — 6,000 is the nearest size actually sold.

Floor area300 sq ft
In tons of refrigeration0.5
Working rate20 BTU/h per sq ft

Sources:

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