How to use the BTU calculator
- Choose US (feet) or metric (meters).
- Enter the room's length, width and ceiling height.
- Rate the insulation and sun exposure. A room with big west- or south-facing windows and no shade counts as very sunny.
- Enter how many people usually use the room at the same time.
- Tick the box if the room is a kitchen or open to one.
- Pick your winter climate for the heating estimate.
How the BTU estimate is calculated
The starting figure comes from the ENERGY STAR sizing chart for room air conditioners on energystar.gov, which assumes an 8 ft ceiling. ENERGY STAR's own adjustments are the ones for shade or sun (10% either way), extra people (600 BTU/h each beyond two) and kitchens (4,000 BTU/h). The ceiling and insulation factors and the heating rule are common rules of thumb added by this calculator, not ENERGY STAR figures. A "ton" of cooling is 12,000 BTU per hour, the rate needed to melt a ton of ice in a day.
Worked example
The defaults are a 15 × 18 ft room with an 8 ft ceiling, average insulation and sun, 2 people and no kitchen:
- Floor area: 15 × 18 = 270 sq ft
- The chart gives 7,000 BTU/h for 250 up to 300 sq ft
- No ceiling, insulation, sun, people or kitchen adjustments apply
- Cooling: 7,000 BTU/h, or 7,000 ÷ 12,000 = 0.58 tons (2.05 kW)
- Room units come in steps, so the nearest common size at or above it is 8,000 BTU/h
- Heating in a moderate climate: 270 × 40 = 10,800 BTU/h (3.17 kW)
If the same room is very sunny and four people use it, cooling becomes 7,000 × 1.1 + 2 × 600 = 8,900 BTU/h. As a poorly insulated kitchen it would be 7,000 × 1.2 + 4,000 = 12,400 BTU/h.
ENERGY STAR room sizing chart
| Area to cool | Capacity | Area to cool | Capacity |
|---|---|---|---|
| 100 up to 150 sq ft | 5,000 BTU/h | 550 up to 700 sq ft | 14,000 BTU/h |
| 150 up to 250 sq ft | 6,000 BTU/h | 700 up to 1,000 sq ft | 18,000 BTU/h |
| 250 up to 300 sq ft | 7,000 BTU/h | 1,000 up to 1,200 sq ft | 21,000 BTU/h |
| 300 up to 350 sq ft | 8,000 BTU/h | 1,200 up to 1,400 sq ft | 23,000 BTU/h |
| 350 up to 400 sq ft | 9,000 BTU/h | 1,400 up to 1,500 sq ft | 24,000 BTU/h |
| 400 up to 450 sq ft | 10,000 BTU/h | 1,500 up to 2,000 sq ft | 30,000 BTU/h |
| 450 up to 550 sq ft | 12,000 BTU/h | 2,000 up to 2,500 sq ft | 34,000 BTU/h |
Why not just buy a bigger unit?
An oversized air conditioner cools the air quickly and then shuts off before it has removed much moisture, so the room feels cold and clammy. It also cycles on and off more, which wastes energy and wears the compressor. A unit that's too small runs constantly and may never reach the set temperature on hot days. Aim for the size the estimate suggests and round up only to the next common size.
This is not a Manual J calculation
Contractors size central air conditioners, heat pumps and furnaces with a room-by-room load calculation such as ACCA Manual J. It accounts for your local design temperatures, window area and type, wall and attic insulation values, air leakage, duct losses and the direction each wall faces. This calculator is a quick check for a single room or a window, portable or mini-split unit. For whole-house equipment, ask an HVAC professional for a load calculation.
Frequently asked questions
How many BTUs do I need per square foot?
Roughly 20 BTU/h per square foot for cooling, but the ENERGY STAR chart isn't linear: small rooms need more per square foot and large open areas less. A 270 sq ft room needs about 7,000 BTU/h (26 per sq ft), while a 1,000 sq ft area needs about 21,000 BTU/h (21 per sq ft).
How many BTUs is a ton of air conditioning?
One ton is 12,000 BTU per hour, so a 2-ton system delivers 24,000 BTU/h and a 3-ton system 36,000 BTU/h. Room air conditioners are usually labeled in BTU/h instead of tons.
How do I convert BTU/h to kilowatts?
Multiply BTU/h by 0.000293, or divide by 3,412. A 12,000 BTU/h unit has about 3.5 kW of cooling capacity. That's the cooling output, not the electricity it uses, which is typically a third or less of it.
Does a high ceiling need a bigger air conditioner?
Yes, because there's more air to cool. The chart assumes 8 ft ceilings, so the calculator scales the capacity up in proportion for taller ceilings: a 10 ft ceiling adds 25%.
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