justyourcalculatorBTU Calculator
How many BTUs your air conditioner or heater needs for a room or home.
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BTU Calculator
Air conditioner and heater size for a room
Cooling needed
9,000 BTU/h
Choose a 10,000 BTU unit
- Tons of cooling1 ton = 12,000 BTU/h
- 0.75
- Kilowatts
- 2.64 kW
- Floor area37.2 m²
- 400 sq ft
Sizing starts from the ENERGY STAR room air conditioner chart and adjusts for your inputs (our multipliers: ±10% for sun and climate, -10%/+15% for insulation). A load calculation (Manual J) is best for whole-home systems.
How it was calculated
- ENERGY STAR base for 400 sq ft: 9,000 BTU/h
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How the btu calculator works
A BTU (British thermal unit) is the heat needed to warm one pound of water by 1 °F. Air conditioners and heaters are rated in BTU per hour; 12,000 BTU/h is one ton of cooling, about 3.5 kW.
The air conditioner tab starts from the ENERGY STAR sizing chart (for example 12,000 BTU for 450 to 550 sq ft), scales it for ceiling height, and adjusts for insulation, sun, climate, extra people (600 BTU each beyond two) and kitchens (4,000 BTU).
The temperature tab uses volume × temperature difference × insulation factor, a common quick estimate for space heaters and window units.
Using and checking your result
Published by JustYourCalculator. Check the units and assumptions above, and compare a known example before relying on the output. Calculations use browser arithmetic and may round displayed values.
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