BTU SizerBTU per square foot — the full table by room size
The BTU Sizer table on this page pairs every room size with the cooling it needs at 20 BTU per hour for every square foot of floor — the rate behind the ENERGY STAR room air conditioner chart, which assumes an eight-foot ceiling, ordinary light and two people in the room.
Read across for the ceiling you actually have, because you cool a volume rather than a floor: the correction is height divided by eight, so ten feet is 25% more load and twelve feet is 50% more. Then apply the corrections in the second table — 10% either way for sun, 15% more for a draughty shell, 600 BTU per person past two, a flat 4,000 for a kitchen — and round the answer up to a size that is actually sold.
Heating is a different table. The same floor area needs 27 BTU per hour per square foot in Miami and 57 in northern Maine, and all seven zones are below.
Five numbers worth remembering
All five come out of the tables below, and all five are computed from the rates this site’s calculators use rather than typed in by hand.
- 20 BTU per hour per square foot is 215 BTU/h per square metre, or 63.1 watts of cooling for every square metre of floor.
- A 12 ft ceiling adds exactly 50% to the same floor area, because the correction is ceiling height divided by 8.
- Room air conditioners stop at 24,000 BTU/h, which is 1,200 sq ft (111 m²) at this rate — a larger room is a mini split or a central system, not a bigger window unit.
- Cooling rates move only 18 to 22 BTU/h per square foot across the 7 climate zones, a 22% spread, while heating rates move 27 to 57 — 111%.
- One 300 sq ft room can honestly need anything from 4,860 to 13,390 BTU/h — 2.8 times as much — which is the difference between a 5,000 and a 14,000 BTU unit.
BTU by floor area and ceiling height
The base estimate, before any correction, for every room size a single machine can sensibly cover. The last column is the number that decides the purchase: the nearest capacity that is actually built.
| Floor area, sq ft | m² | 8 ft ceiling | 9 ft ceiling | 10 ft ceiling | 12 ft ceiling | Size sold at 8 ft | kW |
|---|---|---|---|---|---|---|---|
| 100 | 9 | 2,000 | 2,250 | 2,500 | 3,000 | 5,000 | 1.5 |
| 150 | 14 | 3,000 | 3,375 | 3,750 | 4,500 | 5,000 | 1.5 |
| 200 | 19 | 4,000 | 4,500 | 5,000 | 6,000 | 5,000 | 1.5 |
| 250 | 23 | 5,000 | 5,625 | 6,250 | 7,500 | 5,000 | 1.5 |
| 300 | 28 | 6,000 | 6,750 | 7,500 | 9,000 | 6,000 | 1.8 |
| 350 | 33 | 7,000 | 7,875 | 8,750 | 10,500 | 8,000 | 2.3 |
| 400 | 37 | 8,000 | 9,000 | 10,000 | 12,000 | 8,000 | 2.3 |
| 450 | 42 | 9,000 | 10,125 | 11,250 | 13,500 | 10,000 | 2.9 |
| 500 | 46 | 10,000 | 11,250 | 12,500 | 15,000 | 10,000 | 2.9 |
| 600 | 56 | 12,000 | 13,500 | 15,000 | 18,000 | 12,000 | 3.5 |
| 700 | 65 | 14,000 | 15,750 | 17,500 | 21,000 | 14,000 | 4.1 |
| 800 | 74 | 16,000 | 18,000 | 20,000 | 24,000 | 18,000 | 5.3 |
| 1,000 | 93 | 20,000 | 22,500 | 25,000 | 30,000 | 21,000 | 6.2 |
| 1,200 | 111 | 24,000 | 27,000 | 30,000 | 36,000 | 24,000 | 7 |
Two things are worth reading twice. The ceiling columns are not a rounding detail — a 400 sq ft room with a twelve-foot ceiling asks for as much cooling as a 600 sq ft room with a standard one, because you condition the air in the space rather than the floor under it. And the gap between the requirement and the size sold is uneven: a room needing 7,000 BTU/h and a room needing 8,000 buy the same machine, while 100 BTU/h more can move you a whole step up.
What each correction does to the same room
Every correction on one 300 sq ft room, one at a time, so the size of each effect is visible instead of implied. The last two rows are the same floor area at its cheapest and at its most demanding.
| What changes | Factor or adder | BTU/h needed | Size sold |
|---|---|---|---|
| Nothing — average light, average insulation, two people | — | 6,000 | 6,000 |
| Sun: mostly shaded | ×0.90 | 5,400 | 6,000 |
| Sun: very sunny | ×1.10 | 6,600 | 8,000 |
| Insulation: poor — draughty or single glazed | ×1.15 | 6,900 | 8,000 |
| Insulation: good — new build or retrofitted | ×0.90 | 5,400 | 6,000 |
| 3 people in the room | +600 | 6,600 | 8,000 |
| 4 people in the room | +1,200 | 7,200 | 8,000 |
| 5 people in the room | +1,800 | 7,800 | 8,000 |
| The room is a kitchen | +4,000 | 10,000 | 10,000 |
| Shaded and well sealed — the cheapest version | ×0.90 ×0.90 | 4,860 | 5,000 |
| Sunny, draughty, 5 people, and a kitchen | ×1.10 ×1.15 +5,800 | 13,390 | 14,000 |
The corrections are worth the two minutes they take. Sun and insulation are multipliers, so they compound; people and cooking are flat additions in BTU, which is why they matter far more in a small room than a large one. A kitchen is the single largest correction in the list, and it is the one most often left out.
Reading it backwards: what each size covers
Half of the people who need this table arrive from the other end — not with a room, looking for a rate, but with a machine in a shopping basket, wondering what it will cool.
| Size, BTU/h | kW | Tons | 8 ft ceiling | 9 ft ceiling | 10 ft ceiling | 12 ft ceiling |
|---|---|---|---|---|---|---|
| 5,000 | 1.5 | 0.4 | 250 | 222 | 200 | 167 |
| 6,000 | 1.8 | 0.5 | 300 | 267 | 240 | 200 |
| 8,000 | 2.3 | 0.7 | 400 | 356 | 320 | 267 |
| 10,000 | 2.9 | 0.8 | 500 | 444 | 400 | 333 |
| 12,000 | 3.5 | 1 | 600 | 533 | 480 | 400 |
| 14,000 | 4.1 | 1.2 | 700 | 622 | 560 | 467 |
| 18,000 | 5.3 | 1.5 | 900 | 800 | 720 | 600 |
| 21,000 | 6.2 | 1.8 | 1,050 | 933 | 840 | 700 |
| 24,000 | 7 | 2 | 1,200 | 1,067 | 960 | 800 |
The ceiling columns are what a catalogue’s “covers up to” figure never says: the same unit covers a fifth less floor under a ten-foot ceiling, and a third less under twelve. Treat these as ceilings on the room rather than targets — a unit at the very top of its range runs at full tilt on the hottest afternoon of the year, and an oversized one is worse than a slightly small one for a different reason entirely.
The heating table is not the cooling table
Cooling barely moves across the United States. Heating doubles, and the same floor area can want twice the output six hundred miles further north.
| Zone | Example cities | Cooling, per sq ft | Heating, per sq ft | Heating load, BTU/h | Furnace sold, BTU/h |
|---|---|---|---|---|---|
| 1 | Miami, Honolulu | 22 | 27 | 48,600 | 60,000 |
| 2 | Houston, Phoenix | 21 | 32 | 57,600 | 60,000 |
| 3 | Atlanta, Los Angeles | 20 | 37 | 66,600 | 80,000 |
| 4 | Baltimore, Seattle | 20 | 42 | 75,600 | 80,000 |
| 5 | Chicago, Boston | 19 | 47 | 84,600 | 100,000 |
| 6 | Minneapolis, Burlington | 18 | 52 | 93,600 | 100,000 |
| 7 | Duluth, northern Maine | 18 | 57 | 102,600 | 120,000 |
The load follows the gap between the temperature you want inside and the coldest it gets outside. In summer that gap is 15 to 25 °F almost everywhere. In winter it runs from 25 °F to 70 °F, and the heating rate follows it directly — which is why a single national rule of thumb works for air conditioners and is useless for furnaces. Climate zones and sizing explains how to find yours and why the boundaries fall where they do.
Cooling that same house is the boring half of the story: it lands between three and three and a half tons in every zone from Miami to Duluth, which is one step on the ladder, and the central AC tonnage calculator will show it for your own floor area. The furnace output doubles across the same seven rows.
Where the rate stops being enough
For one room and one machine, this is the calculation the whole industry uses, and it lands inside a size. For a whole-house system it does not see what actually drives the load: the area of west-facing glass, ducts running through an unconditioned attic, how leaky the shell is. That is what an ACCA Manual J calculation measures, and why a contractor’s number can legitimately differ from the one here. Take the tables as the sanity check they are: a quote at twice these figures needs an explanation, and a contractor who cannot give one is worth a second opinion.
To skip the reading, the air conditioner size calculator applies every correction in the second table for you, the furnace calculator does the same for heating, and the central AC tonnage calculator works on whole-house area instead of one room.
Frequently asked questions
Does the floor area include the closet, the en-suite or the hallway the room opens onto?
Include what shares air with the room and leave out what does not. A walk-in closet with no door, an en-suite that stands open, or a doorway you never close is part of the same volume and belongs in the square footage. A closet that stays shut and has no register in it is not being cooled and should be left out. The test is whether air moves between the two spaces on an ordinary day, not whether an estate agent would count the floor. Where a room opens into a hallway or an open-plan kitchen, add both floor areas together and read the combined figure — and if the kitchen is inside that combined space, add the 4,000 BTU as well.
Why do two BTU charts give different answers for the same room?
Because the base rate is not universal, and this site alone uses three of them. Window units and mini splits are sized at 20 BTU/h per square foot, portable air conditioners at 26 — a portable is rated by SACC, which counts the heat its hose and its own leakage bring back into the room, so it has to be bigger to do the same work. Central systems use the climate-zone rate instead, 18 to 22. A chart that quotes 25 or 30 BTU per square foot has usually folded the corrections for sun, insulation and occupancy into the rate rather than applying them separately, which is fine right up to the point where your room is not the average one it assumed.
What is 20 BTU per square foot in watts per square metre?
215 BTU/h per square metre, which is 63 watts of cooling for every square metre of floor. A 30 m² living room therefore comes to about 6,460 BTU/h, or 1.9 kW, and the nearest room unit sold is 8,000 BTU/h — 2.3 kW. The conversion is exact arithmetic on the same rate rather than a second rule of thumb: one square metre is 10.7639 square feet and one kilowatt is 3,412.14 BTU per hour, so any metric figure on this page can be taken back to the imperial one it came from.
How big a room can a window unit still cover?
About 1,200 sq ft, or 111 m², because the room air conditioner ladder stops at 24,000 BTU/h and that is what 24,000 covers at 20 BTU/h per square foot. Raise the ceiling and the same top size covers less floor: 960 sq ft at ten feet, 800 sq ft at twelve. Past that the honest answer is not a bigger window unit but a mini split, whose heads run to 48,000 BTU/h, or a central system. One machine also stops being able to throw air far enough before the arithmetic runs out — 1,200 sq ft on one unit assumes the cold air can reach the far corner.
Is 20 BTU per square foot enough in Phoenix or Houston?
Near enough, with about 5% on top. Zone 2, which covers Houston and Phoenix, works at 21 BTU/h per square foot on this site’s rates and the hottest zone 1 at 22, while the coldest zones sit at 18 — the whole country inside a 22% spread. The reason is that the temperature difference an air conditioner bridges on a summer afternoon is similar almost everywhere; what changes is how many hours a year it has to bridge it, and that lands on the electricity bill rather than on the size. Heating is the opposite: the same map runs from 27 to 57 BTU/h per square foot.
How we calculate this
Sources:
Cite or reference this page
You are welcome to reference the figures on this page with attribution. Every number is generated from the data source at build time — the methodology above explains how.
“BTU per square foot — the full table by room size.” BTU Sizer, updated Aug 10, 2026, https://btusizer.com/guides/btu-per-square-foot/