20 × 15 ft room, 8 ft ceiling, normal sun, 2 people
7,000 BTU, so buy an 8,000 BTU unit.
BTU calculator
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Based on the ENERGY STAR room air conditioner sizing chart for an 8 ft ceiling. Taller ceilings scale the load up. A sunny room adds 10%, a shaded room takes off 10%, each person beyond two adds 600 BTU and a kitchen adds 4,000 BTU.
ft
ft
ft
Sunlight
Is it a kitchen?
Cooling needed
7,000 BTU/h
For 300 sq ft. The next common unit size is 8,000 BTU, which is 0.67 tons or 2.34 kW of cooling.
- Suggested unit
- 8,000 BTU
- Tons
- 0.58
- Kilowatts
- 2.05
- Room area
- 300 sq ft (27.9 m²)
ENERGY STAR room size chart
| Room area | BTU per hour |
|---|---|
| 100 to 150 sq ft | 5,000 |
| 150 to 250 sq ft | 6,000 |
| 250 to 300 sq ft | 7,000 |
| 300 to 350 sq ft | 8,000 |
| 350 to 400 sq ft | 9,000 |
| 400 to 450 sq ft | 10,000 |
| 450 to 550 sq ft | 12,000 |
| 550 to 700 sq ft | 14,000 |
| 700 to 1000 sq ft | 18,000 |
| 1000 to 1200 sq ft | 21,000 |
| 1200 to 1400 sq ft | 23,000 |
| 1400 to 1500 sq ft | 24,000 |
| 1500 to 2000 sq ft | 30,000 |
| 2000 to 2500 sq ft | 34,000 |
Sizing an air conditioner for one room
Air conditioners are sized by how much heat they can take out of a room in an hour, measured in BTU. The starting point is the room's floor area, and the ENERGY STAR chart works out to about 20 BTU per square foot for a normal room.
A 20 × 15 ft room is 300 sq ft, which the chart puts at 7,000 BTU. Units are not made in every size, so you would buy the next size up, 8,000 BTU. That is about 0.67 tons or 2.3 kW of cooling.
Then adjust for the room. A ceiling higher than 8 ft means more air to cool. Strong sun through big windows adds 10%. Each extra person adds about 600 BTU, and a kitchen adds 4,000 because of the stove and fridge. A 500 sq ft sunny kitchen used by three people needs about 17,800 BTU, so an 18,000 BTU unit.
Do not round up too far. An air conditioner that is much too big cools the room quickly and stops, leaving it cold and clammy. Whole-house systems should be sized by a contractor with a proper load calculation.
Sources
What this calculator does not account for
- Single rooms only, not whole houses.
- Insulation and climate are not modelled.
How this is calculated
Formula: Cooling load for a room
BTU =chart BTU for the area × (height ÷ 8) × sun factor + 600 × extra people + 4,000 for a kitchenWhat the symbols mean
- chart BTUFrom the ENERGY STAR room sizing chart, about 20 BTU per sq ft.
- sun factor1.1 for a very sunny room, 0.9 for a shaded one.
Good to know
- Tons = BTU ÷ 12,000. Kilowatts = BTU × 0.000293.
- To estimate the running cost, use the electricity bill calculator.
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Worked examples
Each example is worked out by the same code as the calculator above.
25 × 20 ft sunny kitchen, 3 people
17,800 BTU, so buy an 18,000 BTU unit.
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<iframe src="/embed/btu-calculator" title="BTU calculator" width="100%" height="720" style="border:0;max-width:42rem" loading="lazy"></iframe>Questions
How many BTU do I need for 300 sq ft?
About 7,000 BTU, so an 8,000 BTU unit, for a normal room with an 8 ft ceiling.
How many BTU per square foot?
About 20 BTU per square foot for a normal room.
What is a 1 ton AC in BTU?
12,000 BTU per hour. A 1.5 ton AC is 18,000 BTU.
Is a bigger air conditioner better?
No. An oversized unit cools the air too fast and switches off before it removes the humidity, so the room feels damp.
How do I convert BTU to kW?
Multiply by 0.000293. A 12,000 BTU unit is about 3.5 kW of cooling.