BTU Calculator
Use this BTU calculator to estimate the cooling capacity a room needs in BTU/h. It starts from the ENERGY STAR room air conditioner sizing chart for your floor area, then adjusts for ceiling height, climate, insulation, sun, people and a kitchen, and shows tons and kW. A BTU, W, kW and tons converter is included. It is a rough estimate, not a Manual J load calculation.
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Rough estimate only
This is a rough cooling estimate for sizing a room air conditioner, not a Manual J load calculation, and it does not size heating, which has different loads. Real loads depend on windows, orientation, air leakage, ducts and local design temperatures. For central air, a furnace or a whole-house system, ask a licensed HVAC contractor for a Manual J calculation.
Recommended cooling capacity
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Tons of cooling
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Power (thermal)
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Base capacity before adjustments
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Basis
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Base capacity comes from the ENERGY STAR room air conditioner sizing chart (100 up to 2,500 sq ft, 8 ft ceiling); outside that range the tool falls back to the rough 20 BTU/h per sq ft rule. Sun (−10% heavily shaded, +10% very sunny), 600 BTU/h per person beyond two and +4,000 BTU/h for a kitchen follow ENERGY STAR. A ceiling above 8 ft scales the base in proportion, and the climate and insulation multipliers (0.9 / 1.0 / 1.1) are our own rough assumptions, not from a standard. The result is rounded to the nearest 100 BTU/h. 1 ton = 12,000 BTU/h and 1 BTU/h = 0.29307 W (thermal power, not electrical input).
BTU/h, W, kW and tons converter
BTU/h
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W
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kW
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tons
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How it works
- How many BTU do I need to cool a room?
- Find the room's square footage and read the ENERGY STAR sizing chart: 100 up to 150 sq ft is 5,000 BTU/h, 300 up to 350 sq ft is 8,000, and 2,000 up to 2,500 sq ft is 34,000. Then adjust for sun, people and a kitchen. This calculator does the lookup and the adjustments for you (cooling, based on an 8 ft ceiling).
- How many BTU per square foot is that?
- A commonly quoted rough rule is about 20 BTU/h per sq ft for cooling. The ENERGY STAR chart works out to roughly 14 to 50 BTU/h per sq ft, higher for small rooms and lower for large ones, which is why this calculator uses the chart first and the 20 BTU/h rule only outside 100–2,500 sq ft.
- How do I convert BTU to tons and kW?
- 12,000 BTU/h equal 1 ton of cooling, so divide BTU/h by 12,000. To get watts, multiply BTU/h by 0.29307: 12,000 BTU/h = 3,517 W = 3.517 kW. These are thermal figures, not the electrical power an air conditioner draws, which depends on its efficiency.
- What adjustments does the estimate include?
- Following ENERGY STAR: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU/h for each person beyond two, and +4,000 BTU/h for a kitchen. The calculator also scales for ceilings above 8 ft and applies rough climate and insulation multipliers of its own (0.9 to 1.1), which are assumptions rather than standard values.
- Is this the same as a Manual J load calculation?
- No. It is a rough room-sizing estimate, mainly for room air conditioners. A Manual J calculation models windows, orientation, air leakage, ducts and local design temperatures for the whole house. Use a licensed HVAC contractor for central air or furnace sizing.