This BTU calculator estimates heating or cooling capacity needs from the size and condition fields on the page. Rules of thumb vary by climate and insulation.
For air conditioner oriented sizing, also see the AC BTU calculator. Room area inputs often start from the square footage calculator.
ENERGY STAR cooling capacity by room size
These capacities assume an 8-foot ceiling:
| Area (sq ft) | BTU per hour |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
The calculator’s average-climate factor of 25 BTU per square foot matches the chart at 400 square feet (10,000 BTU). The chart steps down per square foot for large spaces, though. At 2,000 square feet it is about 17 BTU per square foot, so a flat factor will oversize big open areas.
The four ENERGY STAR adjustments
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two regular occupants: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
BTU calculator worked example: a sunny 14 × 20 ft home office
- Area: 14 × 20 = 280 sq ft, which falls in the 250–300 row, so 7,000 BTU.
- Very sunny: 7,000 × 1.10 = 7,700 BTU.
- Four people work there daily, which is two more than the baseline: 7,700 + 2 × 600 = 8,900 BTU.
- The nearest common size is a 9,000 BTU unit.
The plain area rule (280 × 25 = 7,000) would have picked a unit about 2,000 BTU too small for this room.
Why an oversized unit performs worse
ENERGY STAR warns that an oversized air conditioner cools the room before it has time to remove humidity. That leaves the air “damp and clammy”. It also costs more to buy and wastes energy as it cycles on and off. Pick the size closest to your adjusted number rather than rounding up a full step “to be safe”.
High ceilings, whole-house systems and heating
The chart assumes 8-foot ceilings, which is why this calculator scales by height ÷ 8. For central air, heat pumps or furnaces, U.S. contractors size equipment with an ACCA Manual J load calculation. That accounts for insulation, windows, air leakage and local design temperatures. Use this page for single rooms and first estimates, not for sizing a whole-house system.
Installation affects real capacity
ENERGY STAR notes that a poorly installed room unit can leak as much air as a 6-square-inch hole. Seal it with the supplied insulation, keep it level so it drains, and keep lamps and TVs away from its thermostat.
Before you buy, measure the room twice and write down the details the calculator asks for: floor area, ceiling height, how much direct sun the windows get, how many people use the space and whether it includes a kitchen.
If your number lands between two unit sizes, choose the smaller one when the room is shaded and the larger one when the room faces west or has big windows.
Check the unit’s listed cooling capacity on the box, since the label is what you are matching against your result, and compare the yearly energy cost of each model before you decide. A properly sized unit that runs steadily will keep the room more comfortable, drier and quieter than one that constantly switches on and off.
Last reviewed: October 2026