BTU Calculator
Enter your room area, ceiling height, sun exposure and how many people use it. The calculator gives the cooling capacity in BTU per hour, with tons and kilowatts alongside. It is a rule-of-thumb estimate for choosing an air conditioner, not a Manual J load calculation.
Last updated November 4, 2026
Standard is 8 ft (2.4 m)
The first two are included
Optional: 0 for none, e.g. 10 for a hot climate
Result
8,000 BTU/h
About 0.67 tons of cooling. A rule-of-thumb estimate, not a Manual J load calculation.
| Cooling needed | 8,000 BTU/h |
|---|---|
| Tons of cooling | 0.67 |
| Kilowatts | 2.34 kW |
Show the work
Formula: BTU/h = ENERGY STAR chart value for the area x sun x (ceiling / 8) x climate + 600 x (people - 2) + kitchen 4,000
- Base: 300 sq ft falls in the ENERGY STAR room air conditioner chart band for 8,000 BTU/h.
- Adjust: 8,000 x 1 (average sun) x 8/8 (ceiling) x 1 (climate) = 8,000 BTU/h.
- People: max(0, 2 - 2) x 600 = 0 BTU/h.
- Kitchen: none, +0 BTU/h.
- Total: 8,000 + 0 + 0 = 8,000 BTU/h (rounded); ÷ 12,000 = 0.67 tons; x 0.29307107 W = 2.34 kW.
Result: 8,000 BTU/h
How the cooling estimate is worked out
BTU/h = chart value for the area × sun × (ceiling ÷ 8) × climate + 600 × (people − 2) + 4,000 if kitchen
The starting point is the ENERGY STAR room air conditioner chart (a capacity for each band of floor area, such as 8,000 BTU/h for 300 to 350 sq ft), for a room with 8 ft ceilings. A shaded room is reduced by 10 percent and a very sunny one raised by 10 percent. Every person beyond the first two adds 600 BTU/h, and a kitchen adds 4,000 BTU/h for the heat from cooking.
This calculator also scales the area part by the ceiling height divided by 8 ft, and lets you add or remove a percentage for your climate. Those two steps are our own extensions of the simple rule. Metric areas are converted at 1 m² = 10.7639 sq ft.
Cooling capacity is also given in tons, where 12,000 BTU/h is 1 ton, and in kilowatts, where 1 BTU/h is 0.29307107 W.
See how we calculate for the formulas, sources and review schedule behind every calculator.
What this estimate is and is not
A real load calculation, such as ACCA Manual J, looks at insulation, windows, orientation, air leakage and local weather; this tool does not. Use it to get in range before shopping.
The result steps up at each area band edge. Outside the chart (under 100 or over 2,500 sq ft) the load is scaled in proportion.
Choosing a unit
Pick a unit rated close to the result. An oversized unit shuts off before it removes humidity, and an undersized one runs constantly. Room units come in steps such as 5,000, 6,000 or 8,000 BTU/h.
How to use the BTU Calculator
- Choose imperial (sq ft, ft) or metric (m², m).
- Enter the room area and ceiling height.
- Pick the sun exposure and enter the number of people.
- Switch on the kitchen if the room has one, and add a climate percentage if you want.
- Read the BTU/h, tons and kW, then use Copy, Print or Share.
Worked example
A 300 sq ft room with 8 ft ceilings, two people and no kitchen is in the 300 to 350 sq ft band, which needs 8,000 BTU/h, or 0.67 ton and 2.34 kW.
| Sun exposure | BTU/h | Tons | kW |
|---|---|---|---|
| Shaded | 7,200 | 0.6 | 2.11 |
| Average | 8,000 | 0.67 | 2.34 |
| Sunny | 8,800 | 0.73 | 2.58 |
Frequently asked questions
How many BTU do I need per square foot?
It depends on the floor area, not one constant: ENERGY STAR lists 5,000 BTU/h for 100 to 150 sq ft and 8,000 for 300 to 350 sq ft, for an 8 ft ceiling.
How many BTU is one ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, so a 6,000 BTU/h room unit is half a ton.
Is this the same as a Manual J load calculation?
No. Manual J uses insulation, windows and weather. This is a quick estimate for a room air conditioner.
What does the climate adjustment do?
It adds or removes a percentage of the area-based load, for example plus 10 in a very hot place. Leave it at 0 to use the plain rule.
Is my data stored when I use the calculator?
No. Everything is calculated in your browser, and nothing you type is saved or uploaded.