BTU SizerHow We Calculate
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.
- Evaporative coolers are sized on room volume ÷ 2 — two complete air changes a minute, or 0.5 CFM per cubic foot. A swamp cooler does not remove heat from the air in the room; it replaces that air, so the number that matters is how fast it can do it.
- Radiators use 4.3 BTU/h per cubic foot for a living room held at 21 °C, then a multiplier for what the room is: bathroom ×1.09, bedroom ×0.86, kitchen ×0.78, hall ×0.71. Those follow the target temperature each room is normally heated to. Every printed radiator output assumes delta-T 50; at delta-T 40 the same radiator gives about 80% of it, and on a heat pump at delta-T 25 about half.
- Server rooms are sized on the equipment, not the room: watts × 3.412142, because every watt into IT hardware leaves it as heat, plus 5 BTU/h per square foot for an interior envelope and 400 BTU/h per person.
- Ducts are sized on velocity: cross-section = CFM ÷ FPM, then the diameter of the circle with that area, rounded up to a size sold. 500–700 FPM for branches, 700–900 for a trunk, 600 or less for returns.
Three more are table lookups rather than formulas, because the answer is published as bands rather than as a rate: dehumidifiers (pints per day by area and dampness, post-2019 DOE scale), humidifiers (gallons per day by area) and ceiling fans (blade span by area). For those the tool reports the band value directly instead of interpolating — the bands exist because the machines are sold in steps.
The rates
| IECC climate zone | Example cities | Cooling BTU/h per sq ft | Heating BTU/h per sq ft |
|---|---|---|---|
| 1 | Miami, Honolulu | 22 | 27 |
| 2 | Houston, Phoenix | 21 | 32 |
| 3 | Atlanta, Los Angeles | 20 | 37 |
| 4 | Baltimore, Seattle | 20 | 42 |
| 5 | Chicago, Boston | 19 | 47 |
| 6 | Minneapolis, Burlington | 18 | 52 |
| 7 | Duluth, northern Maine | 18 | 57 |
Cooling barely moves across the zones and heating doubles. That is not a quirk of the table: the load follows the temperature difference the system has to bridge, and in summer that gap is similar almost everywhere while in winter it is not.
The size ladders
Rounding up only means something if the list is the real one. These are the capacities sold:
| Equipment | Sizes |
|---|---|
| Room and window AC | 5,000 · 6,000 · 8,000 · 10,000 · 12,000 · 14,000 · 18,000 · 21,000 · 24,000 BTU/h |
| Mini-split heads | 9,000 · 12,000 · 18,000 · 24,000 · 30,000 · 36,000 · 48,000 BTU/h |
| Central systems | 1.5 · 2 · 2.5 · 3 · 3.5 · 4 · 5 tons |
| Gas furnaces | 40k · 60k · 80k · 100k · 120k · 140k BTU/h output |
| Electric heaters | 400 · 500 · 750 · 1,000 · 1,250 · 1,500 W |
| Dehumidifiers | 20 · 25 · 30 · 35 · 40 · 45 · 50 · 60 · 70 pints/day |
| Humidifiers | 1 · 1.5 · 2 · 3 · 4 · 6 · 9 · 12 gallons/day |
| Evaporative coolers | 1,300 · 2,200 · 3,000 · 3,500 · 4,000 · 4,500 · 5,000 · 5,500 · 6,500 · 7,500 · 8,500 CFM |
| Radiators | 1,000 · 1,500 · 2,000 · 2,500 · 3,000 · 3,500 · 4,000 · 5,000 · 6,000 · 7,000 · 8,000 · 10,000 BTU/h |
| Round duct | 4 · 5 · 6 · 7 · 8 · 9 · 10 · 12 · 14 · 16 · 18 · 20 · 24 in |
Unit conversions
One ton of refrigeration is exactly 12,000 BTU per hour. One kilowatt is 3,412.142 BTU per hour. One square metre is 10.7639 square feet, one cubic metre 35.3147 cubic feet. Converting out and back returns the number you started with — no rounding is baked into the chain.
Airflow is exact in every direction: 1 CFM = 1.699011 m³/h = 0.4719474 L/s. Everything is computed through CFM as the base unit so that rounding happens once, at the end, rather than compounding through a chain.
Efficiency ratings are not all exact. COP = EER ÷ 3.412142 and kW/ton = 12 ÷ EER are definitions. SEER to EER is not: SEER is a weighted seasonal average and EER is a single test point at 95 °F, so the conversion uses the long-standing approximation EER = 1.12 × SEER − 0.02 × SEER². SEER2 and EER2 are taken as 0.95 × the old figure, the US Department of Energy conversion factor for ducted split systems under the 2023 test procedure — packaged and ductless equipment use different factors, and the only authoritative SEER2 number is the one from re-testing.
Where the figures come from
The cooling rate and its corrections follow the ENERGY STAR room air conditioner sizing guidance and the US Department of Energy’s room air conditioner pages. Heating rates follow the IECC climate zone bands. Dehumidifier capacities use the post-2019 DOE test scale; ratings from before that change measure roughly 30% higher for the same machine, which is why old charts oversize badly. Air purifier sizing uses the AHAM two-thirds rule, bathroom fan sizing the HVI recommendation of 1 CFM per square foot with a 50 CFM floor, and range hoods 100 CFM per linear foot of cooktop against a wall or 150 for an island.
For a whole-house system, none of this replaces an ACCA Manual J load calculation, which measures the actual walls, windows, doors and ducts. It is, however, enough to tell whether a quote is in the right postcode — and a contractor who cannot explain a number twice the size of this one is worth a second opinion.
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