Quick answer: A BTU (British Thermal Unit) measures heat energy, and BTU/h is the rate an air conditioner or heater moves it. To size equipment, first estimate the room's load in watts, then convert:
BTU/h = cooling load (W) × 3.412 | Tons of refrigeration = BTU/h ÷ 12,000
As a rough starting point, a well-insulated room needs roughly 20–25 BTU/h per square foot. A 215 sq ft (20 m²) south-facing bedroom with average insulation works out to about 7,300 BTU/h — 0.61 tons, which rounds up to a 1-ton (12,000 BTU) unit. Orientation, window ratio, ceiling height, and insulation level all shift this by 15–30%, which is why a flat per-square-foot rule tends to oversize equipment.
Enter your room details below to get cooling BTU/h, tonnage, kW, and separate heating BTU/h based on ASHRAE unit-index factors. For the full method, see the cooling load guide.
Input Parameters
About This Calculator
This BTU calculator provides both cooling and heating estimates using professional-grade methods:
- Cooling: Unit-index method with orientation, insulation, window ratio, and temperature corrections
- Heating: Heat-loss method with wall, window, and infiltration components
- Results shown in BTU/h (imperial) and kW (SI) simultaneously
- Equivalent tonnage calculation (1 ton = 12,000 BTU/h)
Results are for preliminary sizing only. Final equipment selection should be done by a qualified HVAC engineer.
How this BTU calculator works
This tool estimates the cooling and heating capacity needed for your room in BTU/h (British Thermal Units per hour). For cooling, it starts from a base cooling index per square meter, then adjusts for building type, orientation, insulation, window area, and temperature difference. For heating, it calculates wall, window, and infiltration heat losses.
Understanding BTU
- 1 BTU = the energy to raise 1 lb of water by 1°F
- 1 ton of cooling = 12,000 BTU/h ≈ 3.517 kW
- A typical bedroom (150 sq ft) needs about 5,000–6,000 BTU/h for cooling
- Room AC units range from 5,000 BTU/h (window units) to 60,000 BTU/h (central systems)
FAQ
How many BTUs per square foot do I need?
A common rule of thumb is 20 BTU/sq ft for cooling, but this varies widely based on climate, insulation, and windows. This calculator provides a more accurate estimate using your specific conditions.
Should I buy an AC with more BTUs than calculated?
Slightly above is fine (within 10-15%), but significantly oversizing leads to short-cycling, poor dehumidification, and higher energy costs. Proper sizing is key.
Why are cooling and heating BTUs different?
Because the temperature differences, solar gain patterns, and heat flow directions differ between summer and winter. Your home may need more heating BTU in cold climates or more cooling BTU in hot climates.