How to use the BTU calculator
- Enter the room size: length and width in feet or meters, or the floor area if you know it. For an L shaped room, work out each rectangle and add them.
- Pick the sun: heavily shaded, normal or very sunny.
- Enter how many people regularly use the room (blank means 2), and tick kitchen if the unit will cool a kitchen.
The result is the recommended cooling capacity in BTU per hour (BTU/h), the next size up in the chart, the same figure in tons and kilowatts, and the BTU per square foot. It follows the ENERGY STAR sizing chart for room air conditioners, such as a window AC cooling one room.
The ENERGY STAR chart: BTU per square foot
ENERGY STAR's step "Find the correct cooling capacity for your room size" gives this chart. The square meters and the BTU per square foot are worked out from it; the first and third columns are as ENERGY STAR prints them.
| Area to be cooled (sq ft) | About m² | Capacity needed (BTU/h) | BTU/h per sq ft |
|---|---|---|---|
| 100 up to 150 | 9.3 to 13.9 | 5,000 | 33.3 to 50.0 |
| 150 up to 250 | 13.9 to 23.2 | 6,000 | 24.0 to 40.0 |
| 250 up to 300 | 23.2 to 27.9 | 7,000 | 23.3 to 28.0 |
| 300 up to 350 | 27.9 to 32.5 | 8,000 | 22.9 to 26.7 |
| 350 up to 400 | 32.5 to 37.2 | 9,000 | 22.5 to 25.7 |
| 400 up to 450 | 37.2 to 41.8 | 10,000 | 22.2 to 25.0 |
| 450 up to 550 | 41.8 to 51.1 | 12,000 | 21.8 to 26.7 |
| 550 up to 700 | 51.1 to 65.0 | 14,000 | 20.0 to 25.5 |
| 700 up to 1,000 | 65.0 to 92.9 | 18,000 | 18.0 to 25.7 |
| 1,000 up to 1,200 | 92.9 to 111.5 | 21,000 | 17.5 to 21.0 |
| 1,200 up to 1,400 | 111.5 to 130.1 | 23,000 | 16.4 to 19.2 |
| 1,400 up to 1,500 | 130.1 to 139.4 | 24,000 | 16.0 to 17.1 |
| 1,500 up to 2,000 | 139.4 to 185.8 | 30,000 | 15.0 to 20.0 |
| 2,000 up to 2,500 | 185.8 to 232.3 | 34,000 | 13.6 to 17.0 |
The BTU per square foot falls as rooms get bigger: from 50 at 100 sq ft to under 14 at 2,500 sq ft, so a single "BTU per square foot" figure does not fit every room. A room of exactly 150 sq ft is read from the "150 up to 250" row. The chart starts at 100 sq ft; for a smaller room the calculator uses its smallest size, 5,000 BTU/h. ENERGY STAR says the capacities are "calculated based on an 8-foot ceiling", so a room with a higher ceiling may need more.
Adjusting for sun, people and a kitchen
ENERGY STAR's next step, "Adjust as necessary", gives four changes, quoted here:
- "If the room is heavily shaded, reduce capacity by 10 percent."
- "If the room is very sunny, increase capacity by 10 percent."
- "If more than two people regularly occupy the room, add 600 BTUs for each additional person."
- "If the unit is used in a kitchen, increase capacity by 4,000 BTUs."
The calculator applies the 10% to the chart figure, then adds the people and the kitchen. Worked examples:
| Room | Sq ft | Chart | Adjusted BTU/h | Next chart size |
|---|---|---|---|---|
| Bedroom, 12 x 15 ft | 180 | 6,000 | 6,000 | 6,000 |
| Same bedroom, very sunny | 180 | 6,000 | 6,600 | 7,000 |
| Same bedroom, heavily shaded | 180 | 6,000 | 5,400 | 6,000 |
| Same bedroom, 4 people | 180 | 6,000 | 7,200 | 8,000 |
| Kitchen, 14 x 20 ft | 280 | 7,000 | 11,000 | 12,000 |
| Living room, 20 x 25 ft | 500 | 12,000 | 12,000 | 12,000 |
The very sunny bedroom works out as 6,000 + 10% = 6,600 BTU/h, and the kitchen as 7,000 + 4,000 = 11,000 BTU/h.
Window AC BTU: picking a unit size
Units are sold in steps, so the calculator also shows the next capacity up in the ENERGY STAR chart. Pick the unit closest to the recommended figure rather than a much bigger one. ENERGY STAR warns that "A unit that is too big will be less effective and offer reduced air conditioner efficiency". It explains that a unit that is too large "will cool the room before it has a chance to remove the humidity", and the cool, moist air leaves the room feeling "damp and clammy".
Check the cooling capacity on the unit's own label or the maker's specification, since model names do not always match the rated BTU/h.
BTU to tons for a mini split
Mini splits and central systems are often described in tons. A ton of refrigeration is 12,000 BTU per hour (NIST lists "ton of refrigeration (12 000 BtuIT/h)"), so tons = BTU/h ÷ 12,000. One BTU per hour is 0.2930711 watt, which gives the kilowatts used on many UK and European labels.
| Tons | BTU/h | kW |
|---|---|---|
| 0.5 tons | 6,000 | 1.76 |
| 0.75 tons | 9,000 | 2.64 |
| 1 ton | 12,000 | 3.52 |
| 1.5 tons | 18,000 | 5.28 |
| 2 tons | 24,000 | 7.03 |
| 2.5 tons | 30,000 | 8.79 |
| 3 tons | 36,000 | 10.55 |
So the 20 x 25 ft living room above, at 12,000 BTU/h, needs about 1 ton of cooling. The chart is for one room; a mini split system serving several rooms should be sized room by room by the installer.
Whole house, garages and heating
This BTU calculator for a room uses a simple chart. It does not look at insulation, windows, climate, ceiling height or air leaks. For central air or a heat pump for a whole home, ask a heating and cooling contractor for a load calculation. In the US the usual method is ACCA's Manual J, which ACCA describes as "the national ANSI-recognized standard for producing HVAC equipment sizing loads".
BTU calculator for a garage: treat the chart figure only as a rough starting point. Garages are often uninsulated, have a large door and may hold a car that is still hot from driving, so they can need much more cooling than a bedroom of the same size. Have a professional size a unit for a garage or workshop.
Heating: this page sizes cooling only. We did not find an official simple chart for heating BTU per room, so the calculator does not give one.
Questions and answers
How many BTU do I need for a 12 x 12 room?
A 12 x 12 ft room is 144 sq ft, which is in the ENERGY STAR "100 up to 150" row: 5,000 BTU/h. Add 10% if it is very sunny (5,500 BTU/h) and 600 BTU/h for each regular occupant over two.
How many BTU per square foot?
It depends on the room size. In the ENERGY STAR chart it runs from 50 BTU/h per sq ft for a 100 sq ft room down to under 14 for 2,500 sq ft. A 500 sq ft room needs 12,000 BTU/h, which is 24 per sq ft.
How do I convert BTU to tons?
Divide by 12,000. 18,000 BTU/h is 1.5 tons, and 24,000 BTU/h is 2 tons. To go from tons to BTU/h, multiply by 12,000.
Is a bigger air conditioner better?
No. ENERGY STAR says a unit that is too big is less effective and less efficient, and that it cools the room before it can remove the humidity, which leaves the room damp. Choose the size closest to the recommendation.
What size mini split do I need for 500 square feet?
From the ENERGY STAR chart, a 500 sq ft room needs 12,000 BTU/h, or 1 ton, before adjustments for sun, people or a kitchen. For several rooms or a whole home, have the installer do a load calculation.
Can I use this for a garage?
Only as a rough guide. Garages are often uninsulated and have large doors, so the room chart can underestimate. Ask a heating and cooling professional to size it.
Sources
Checked on 8 October 2026. If a rule has changed, please tell us.