How the BTU calculation works
A BTU (British thermal unit) is the heat needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour: how much heat they can move out of a room (or into it) every hour. Sizing means matching that rating to the heat your room actually gains on a hot afternoon or loses on a cold night.
This calculator uses the industry rule-of-thumb method that HVAC contractors, Energy Star's room air conditioner chart and appliance retailers all build on. For cooling, the starting point is 20 BTU per square foot of floor area with an 8-foot ceiling. Each factor below then nudges that number up or down. For heating, the starting point depends on your climate: roughly 30, 40 or 55 BTU per square foot for hot, moderate and cold regions, adjusted for insulation. The result is rounded to the nearest 100 BTU and shown with a ±10% range, because no square-footage method can see your window area, air leakage or thermostat habits.
The factors, and how much each one matters
| Factor | Cooling | Heating | Why |
|---|---|---|---|
| Floor area | × 20 BTU/sq ft | × 30 / 40 / 55 BTU/sq ft by climate | More floor means more air and more wall exposed to outside. |
| Ceiling above 8 ft | × height ÷ 8 | × height ÷ 8 | A 10 ft ceiling holds 25% more air than an 8 ft one. |
| Very sunny room | +10% | −5% | Solar gain through glass is a load in summer and a gift in winter. |
| Heavily shaded room | −10% | +5% | Same effect, reversed. |
| Poor insulation | +15% | +20% | Thin walls, single-pane glass and drafts leak heat both ways. |
| Good insulation | −10% | −15% | Modern, sealed construction with double-pane windows. |
| Climate zone | Hot +10%, cold −10% | Sets the base factor | Design temperatures differ by 30°F+ across the US. |
| Each person beyond 2 | +600 BTU | ignored | A resting adult gives off roughly 400–600 BTU/hr. |
| Kitchen | +4,000 BTU | ignored | Range, oven and refrigerator all reject heat into the room. |
Full formulas are documented in plain English in the sizing guide and in the calculator's “How we got this number” panel.
BTU by room size: quick chart
Average conditions assume two people, 8 ft ceilings and a moderate climate. Click any size for full tables and unit recommendations.
| Room size | Cooling (shaded, good insulation) | Cooling (average) | Cooling (sunny, poor insulation) | Buy | Heating (moderate) |
|---|---|---|---|---|---|
| 100 sq ft | 1,600 | 2,000 | 2,500 | 5,000 BTU | 4,000 |
| 150 sq ft | 2,400 | 3,000 | 3,800 | 5,000 BTU | 6,000 |
| 200 sq ft | 3,200 | 4,000 | 5,100 | 5,000 BTU | 8,000 |
| 250 sq ft | 4,100 | 5,000 | 6,300 | 5,000 BTU | 10,000 |
| 300 sq ft | 4,900 | 6,000 | 7,600 | 6,000 BTU | 12,000 |
| 400 sq ft | 6,500 | 8,000 | 10,100 | 8,000 BTU | 16,000 |
| 500 sq ft | 8,100 | 10,000 | 12,600 | 10,000 BTU | 20,000 |
| 600 sq ft | 9,700 | 12,000 | 15,200 | 12,000 BTU | 24,000 |
| 800 sq ft | 13,000 | 16,000 | 20,200 | 18,000 BTU | 32,000 |
| 1,000 sq ft | 16,200 | 20,000 | 25,300 | 24,000 BTU | 40,000 |
| 1,200 sq ft | 19,400 | 24,000 | 30,400 | 24,000 BTU | 48,000 |
| 1,500 sq ft | 24,300 | 30,000 | 38,000 | 30,000 BTU | 60,000 |
| 2,000 sq ft | 32,400 | 40,000 | 50,600 | 48,000 BTU | 80,000 |
Reading the result
The big number is the load: the BTU per hour your room needs on a design day. The “buy this size” box rounds that up to the nearest capacity actually sold. Window and portable units come in 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000 and 24,000 BTU steps; mini-splits in 9,000, 12,000, 18,000, 24,000 and 36,000; central systems in half-ton (6,000 BTU) increments.
When the load sits just above a class boundary, say 6,300 BTU, the calculator will recommend 8,000 BTU but tell you a 6,000 BTU unit is a legitimate choice. The closer-to-load option is usually right for cooling, because an oversized air conditioner short-cycles and leaves the room humid. For heating, a 10–20% margin over the load is harmless and gives faster warm-up.
One more note on portable air conditioners: since 2017 the US Department of Energy rates them by SACC (seasonally adjusted cooling capacity), which accounts for the heat their exhaust hose leaks back into the room. A portable labelled 10,000 BTU (ASHRAE) is typically 6,000–7,000 BTU SACC. Compare the SACC figure against your result. The window vs. portable guide explains the details.
Frequently asked questions
How many BTU do I need per square foot?
For cooling, the standard rule is 20 BTU per square foot of floor area with an 8-foot ceiling, then adjust: +10% for a very sunny room, −10% for heavy shade, +600 BTU for each person beyond two, and +4,000 BTU if the room is a kitchen. For heating, use 30 BTU per square foot in hot climates, 40 in moderate climates, and 55 in cold climates, adjusted for insulation.
What size air conditioner do I need for a 300 sq ft room?
About 6,000 BTU under average conditions (300 × 20). With a sunny exposure or poor insulation that rises to 6,600–6,900 BTU, so an 8,000 BTU window or portable unit is the usual buy; a 6,000 BTU unit works if the room is shaded and well insulated.
Is it better to oversize or undersize an air conditioner?
Slightly undersized is better than oversized. An oversized AC cools the air quickly and shuts off before it has removed much humidity, leaving the room cold but clammy, and the constant on-off cycling wears the compressor and wastes electricity. Choose the unit class closest to your calculated BTU, not the next one up 'to be safe'.
How many BTU is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour. So an 18,000 BTU unit is 1.5 tons, 24,000 BTU is 2 tons, and 36,000 BTU is 3 tons. The term comes from the heat needed to melt one ton of ice in 24 hours.
How many BTU is a 1,500 watt space heater?
1,500 watts is about 5,118 BTU per hour (1 watt = 3.412 BTU/hr). Electric resistance heaters convert essentially all their electricity to heat, so a 1,500 W heater covers roughly 100–150 sq ft in a moderate climate, or up to 170 sq ft in a hot one.
Does ceiling height change the BTU calculation?
Yes. The 20 BTU per square foot rule assumes 8-foot ceilings. For taller ceilings multiply by the height divided by 8: a 10-foot ceiling adds 25%, a 12-foot ceiling adds 50%, because you are cooling that much more air.
Is the BTU calculator accurate enough to buy an air conditioner?
For a single room unit (window AC, portable, mini-split, or space heater), yes: the rule of thumb lands within one unit size of a full load calculation in the large majority of homes. For whole-house central systems, use it as a sanity check and have a contractor run an ACCA Manual J calculation before buying.
What is the difference between BTU and BTU/hr?
A BTU is a quantity of heat (the energy to raise one pound of water by 1°F). BTU per hour is a rate, which is what air conditioner and heater capacities describe. In everyday usage 'a 10,000 BTU air conditioner' means 10,000 BTU per hour; this site follows that convention.