BTU Calculator — Room Size
Match a room to the right air conditioner capacity using the same chart Energy Star publishes — square footage to BTU/hr, then adjusted for sun, people, kitchens, and ceiling height.
Pick the nearest common size at or above the adjusted number. This is a rule-of-thumb sizer for single rooms — not a Manual J load calculation. For whole-house systems, additions, or extreme climates, have an HVAC pro run the load calc.
Chart and adjustment rules from Energy Star (energystar.gov/products/ room_air_conditioners, fetched 2026-09-01): shaded rooms −10%, very sunny +10%, +600 BTU per person beyond 2, +4,000 BTU for kitchens, scale capacity for ceilings above 8 ft. Cooling only — heating sizing uses a different climate-based method.
What a BTU is, and why the number matters
A British thermal unit is a measure of heat energy. Air conditioners are rated in BTU per hour — a rate, not a total: a 12,000 BTU/hr window unit removes 12,000 BTU of heat for each hour it runs. Sizing is about matching that rate to the room.
Get the match right and the unit runs in long, steady cycles. Oversize it and it does not. A unit that is too large drops the temperature in a few minutes, hits its setpoint, and shuts off. That is short cycling, and it is a comfort problem before it is a cost problem: the compressor never runs long enough to pull moisture out of the air, so the coil stays wet and the room feels clammy even though the thermometer reads fine. The repeated starts also wear the compressor faster than steady running does.
How the Energy Star chart works
The chart is a lookup, not a formula. Find the room area, read across to the capacity, then apply the adjustments for your specific room. It opens at 5,000 BTU/hr for a room of 100–150 sq ft, steps up through 6,000, 7,000, 8,000, and 9,000 at 400 sq ft, reaches 14,000 at 550–700 sq ft, and tops out at 34,000 BTU/hr for 2,000–2,500 sq ft.
Two things about that chart are easy to miss: the steps are not proportional, and every band assumes an 8-ft ceiling.
The 8-ft ceiling assumption
The chart is built for rooms with 8-ft ceilings. A room with 9-ft or 10-ft ceilings holds more air, and more air means more heat to move, so the chart reads low for it. The correction is a straight scale: take the chart capacity and multiply it by your ceiling height divided by 8. A 9-ft ceiling is 9 ÷ 8 = 1.125; a 10-ft ceiling is 10 ÷ 8 = 1.25. A room that charts at 9,000 BTU/hr under an 8-ft ceiling needs about 11,250 BTU/hr under a 10-ft ceiling. The calculator applies this step for you.
Adjustment rules
Energy Star gives four adjustments on top of the chart:
- Sun exposure. Heavily shaded room: reduce 10%. Very sunny: increase 10%.
- Occupancy. More than two people regularly in the room: add 600 BTU/hr per additional person.
- Kitchen install. Add 4,000 BTU/hr for cooking heat.
- Ceiling height. Above 8 ft, scale capacity by height ÷ 8.
The percentages hit the chart number first; the flat additions come after. Concretely, a 396-sq-ft room that is very sunny is 9,000 × 1.10 = 9,900 BTU/hr. Four people instead of two adds 600 × 2 = 1,200, for 11,100. Set a kitchen in it and you add 4,000 more.
Cooling load vs heating load
Cooling and heating are not mirror images, which is why this is a cooling calculator. A cooling load is heat coming in — sun through glass, hot outside air leaking past the envelope, body heat, appliances. Sun exposure does a lot of that work, which is why the chart asks about shading first. A heating load is the opposite problem: heat leaking out, sized from your climate's outdoor design temperature, the insulation and window values, and the size of the structure. That is why the same room wants a specific cooling number and a differently derived heating number.
The shortcut rules you will run into online are worth recognizing for what they are. The flat "multiply square footage by 20" rule lines up with the middle of the Energy Star chart, so it works on an average room and misses on a sunny or tall one. Northern heating guides quote roughly 25–30 BTU/hr per square foot. Both are recorded here for context only. For a whole-house system, an addition, or a borderline room, have a pro run an ACCA Manual J load calculation.
Worked example — 18 ft × 22 ft kitchen, very sunny
A 22-ft-deep kitchen runs 18 ft wide under an 8-ft ceiling, faces the afternoon sun, and holds two people. Work it in the same order the calculator does:
- Area: 18 × 22 = 396 sq ft
- Chart band: 350–400 sq ft → 9,000 BTU/hr
- Very sunny: 9,000 × 1.10 = 9,900 BTU/hr
- Kitchen install: 9,900 + 4,000 = 13,900 BTU/hr
- Nearest standard sizes: 12,000 and 14,000 → pick 14,000
Two occupants means no occupancy adder — the rule only starts past two. Put the same kitchen under a 10-ft ceiling and the height scale comes last: 13,900 × 1.25 = 17,375 BTU/hr, which lands on 18,000.
Common mistakes
- Sizing on square footage alone. The chart number is a starting point, not an answer. Skip the sun, occupancy, kitchen, and ceiling adjustments and you are guessing.
- Ignoring ceiling height. The chart assumes 8 ft. Vaulted and 9-ft or 10-ft rooms hold more air and read low; scale by height ÷ 8.
- Adding people and kitchens twice. The chart already carries an assumption of two occupants and no kitchen load. Add only the difference — people past the second, and 4,000 BTU/hr if the unit sits in the kitchen.
- Buying "one size up" out of habit. Going up a size does not add margin, it adds short cycling: less dehumidification and more compressor starts.
- Confusing cooling and heating numbers. A 12,000 BTU/hr air conditioner is a cooling rating; a similarly rated heater is a different load under different conditions.
BTU sizing FAQ
How many BTU per square foot do I need?
There is no single number — you read it off the chart: 5,000 BTU/hr for a room of 100–150 sq ft, up to 34,000 BTU/hr at 2,000–2,500 sq ft, all at an 8-ft ceiling. The online shortcut of roughly 20 BTU/hr per square foot lands near the middle of the chart.
Does ceiling height change how many BTU I need?
Yes. Every band on the Energy Star chart assumes an 8-ft ceiling. Above that, scale the chart capacity by your ceiling height divided by 8: a 9-ft ceiling is ×1.125, a 10-ft ceiling is ×1.25, and a 12-ft ceiling is ×1.5.
Can I just buy a bigger unit to be safe?
No. An oversized air conditioner cools the room in a few minutes, hits its setpoint, and shuts off. Those short cycles never run the coil long enough to pull moisture out of the air, so the room stays humid even though the thermostat reads fine.
Do I size a heater the same way?
No. This chart sizes cooling load — heat coming into the room from sun, people, and appliances. Heating load is heat leaking out, sized from your climate, the insulation, and the window area. For a whole-house system or an addition, have a pro run an ACCA Manual J load calculation.