Heat Loss Calculator
Estimate room heat loss from wall, window, ceiling, floor and air-infiltration inputs in imperial or metric units.
Default project preview
8,923 BTU/h
Envelope transmission
6,168 BTU/h
Air infiltration
1,944 BTU/h
Temperature difference
50 °F
Room volume
4,320 ft³
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Room length
Enter this project assumption in ft. The current imperial default is 24 ft.
Room width
Enter this project assumption in ft. The current imperial default is 20 ft.
Ceiling height
Enter this project assumption in ft. The current imperial default is 9 ft.
Indoor design temperature
Enter this project assumption in °F. The current imperial default is 70 °F.
Outdoor design temperature
Enter this project assumption in °F. The current imperial default is 20 °F.
Window and door area
Enter this project assumption in ft². The current imperial default is 80 ft². Minimum: 0.
Opaque wall U-factor
Enter this project assumption in BTU/h·ft²·°F. The current imperial default is 0.08 BTU/h·ft²·°F.
Window and door U-factor
Enter this project assumption in BTU/h·ft²·°F. The current imperial default is 0.35 BTU/h·ft²·°F.
Ceiling U-factor
Enter this project assumption in BTU/h·ft²·°F. The current imperial default is 0.03 BTU/h·ft²·°F.
Floor U-factor
Enter this project assumption in BTU/h·ft²·°F. The current imperial default is 0.05 BTU/h·ft²·°F.
Air changes per hour
Enter this project assumption in ACH. The current imperial default is 0.5 ACH. Minimum: 0.
Design reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 40.
How to read the outputs
Envelope transmission
walls, openings, ceiling and floor. Default preview: 6,168 BTU/h.
Air infiltration
0.5 air changes per hour. Default preview: 1,944 BTU/h.
Temperature difference
indoor minus outdoor. Default preview: 50 °F.
Room volume
conditioned volume. Default preview: 4,320 ft³.
Equivalent capacity
2.62 kW. Default preview: 8,923 BTU/h.
Openings area
windows and doors. Default preview: 80 ft².
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Transmission loss = area × U-factor × temperature difference for each surface. Infiltration uses room volume × air changes × air heat-capacity factor. Design reserve is applied last.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
This calculator estimates steady-state building heat loss from envelope U-values, areas, temperature difference and air leakage assumptions. It is a screening model, not a complete room-by-room load calculation.
Calculation steps
- 1Measure each envelope area by assembly type.
- 2Convert R-values to appropriate U-values where justified.
- 3Multiply U × area × design temperature difference.
- 4Add infiltration and ventilation loads without double counting.
Worked numerical example
If an assembly group has UA of 600 BTU/h·°F and the design temperature difference is 50°F, conductive loss is 30,000 BTU/h before ventilation and infiltration are added.
Purchasing and rounding
The result is not direct equipment selection. Heating equipment must be compared with certified output at design conditions and distribution capability.
Interpret the result
Large losses usually point to high area, weak insulation, high design temperature difference or air leakage. Improving one input can be modeled without claiming a guaranteed savings result.
Page-specific assumptions
- Steady design condition.
- Assembly U-values are representative.
- Thermal bridges are included or acknowledged.
- Air-change assumptions are realistic.
Common mistakes
- Using nominal insulation R-value for the whole wall.
- Omitting windows and doors.
- Double counting ventilation and infiltration.
- Adding arbitrary oversizing factors.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Heat Loss Calculator questions
What is a U-factor?+
U-factor describes heat flow through an assembly; lower values generally mean better insulation.
Why does infiltration matter?+
Outdoor air entering through leakage or ventilation must be heated, adding to the design load.
Can this select a furnace or heat pump?+
No. Equipment selection requires a complete building load calculation and performance data at design conditions.