Heating Load Calculator
A heater must replace heat as fast as a room loses it. This calculator estimates that rate from two physics-based parts: transmission loss through walls, windows, and roof, given by U-value times area times the temperature difference, and infiltration loss from air leaking in and out, given by room volume, air changes per hour, the volumetric heat capacity of air, and the temperature difference. Enter your envelope figures and design conditions to see the heating load in BTU per hour and in watts, the basis for sizing equipment.
Heating load formula
Transmission loss = U-value * area * temperature difference
Infiltration loss = 0.018 * volume * air changes per hour * temperature difference
Total load (BTU/hr) = transmission loss + infiltration loss
Total load (watts) = total BTU/hr / 3.412142
The constant 0.018 is the volumetric heat capacity of air in BTU per cubic foot per degree Fahrenheit. The factor 3.412142 converts BTU per hour to watts (1 watt equals 3.412142 BTU/hr).
Heating load context
- U-value is the reciprocal of R-value; lower U means better insulation.
- Design temperature difference is the indoor set point minus the outdoor design temperature.
- Outdoor design temperatures come from ASHRAE or local building-code data.
- Air changes per hour depend on how tight the building is; tighter homes leak less.
- This is a simplified estimate; a full residential design uses ACCA Manual J.
Heating load: frequently asked questions
How is heating load calculated?
Heating load is the rate of heat loss the heater must replace. It is the sum of transmission loss through the envelope (U-value times area times temperature difference) and infiltration loss from air leakage (volume times air changes per hour times a specific-heat constant times temperature difference). This calculator adds both and reports BTU per hour and watts.
What is a U-value?
The U-value (overall heat transfer coefficient) is the rate of heat flow through a building element per unit area per degree of temperature difference, in BTU per hour per square foot per degree Fahrenheit. Lower U-values mean better insulation. It is the reciprocal of the R-value.
What temperature difference should I use?
Use the design temperature difference: your indoor set point minus the outdoor design temperature for your area. Outdoor design temperatures come from ASHRAE or local code data. Enter the difference directly so the result matches your climate.
Where does the infiltration constant 0.018 come from?
It is the volumetric heat capacity of air in BTU per cubic foot per degree Fahrenheit at typical conditions, approximately 0.018. Multiplying room volume, air changes per hour, this constant, and the temperature difference gives the air-change heat loss in BTU per hour.
Is this a full Manual J load calculation?
No. This is a simplified envelope-plus-infiltration estimate useful for understanding the drivers of heat loss. A full residential load calculation follows ACCA Manual J and accounts for solar gain, internal gains, and detailed assemblies.
Official sources
- U.S. Department of Energy: Heating and cooling guidance.
- National Institute of Standards and Technology: Unit definitions (BTU, watt).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.