Heat Loss Calculator
Estimate room, zone, or whole-house heat loss from size, temperature difference, insulation, glazing, and air leakage.
This is a quick heat loss estimate, not a full room-by-room heat loss survey.
Results
This is a quick heat loss estimate, not a full room-by-room heat loss survey.
How to use this calculator
- 01Choose dimensions or known floor area.
- 02Enter the ceiling height, indoor design temperature, and outdoor design temperature.
- 03Choose the insulation, window/glazing level, air leakage, and room or space type.
- 04Read the heat loss in BTU/hr, W, and kW, plus the recommended heat output range.
If you need heating and cooling loads together, use the HVAC Load Calculator. For a simpler room BTU estimate, use the BTU Calculator.
Understanding the math
The estimate starts with a base heat loss rate at a standard temperature difference. It then scales that rate by the actual indoor/outdoor temperature difference and adjusts for insulation, glazing, air leakage, and room type.
heat loss = area x base rate x temperature factor x envelope factors temperature factor = actual temperature difference / standard delta watts = BTU/hr / 3.412142 kW = BTU/hr / 3412.142
A detailed wall calculation uses area x U-value x temperature difference. This quick calculator uses insulation and glazing choices instead of asking for every wall, roof, floor, and window U-value.
Once the heat loss is known, the Furnace Size Calculator can help translate heating output into furnace input BTU.
Heat loss quick reference
Use these ranges as a rough check. Real heat loss can move outside the table when local design temperatures, window area, insulation, air leakage, or room layout is unusual.
| Space | Area | Typical heat loss | Rate |
|---|---|---|---|
| Small bedroom | 100–180 sq ft | 2,500–5,000 BTU/hr | 22–35 BTU/hr per sq ft |
| Living room | 250–500 sq ft | 6,000–14,000 BTU/hr | 24–38 BTU/hr per sq ft |
| Bathroom | 50–120 sq ft | 2,000–5,000 BTU/hr | 30–50 BTU/hr per sq ft |
| Extension / high glass | 150–350 sq ft | 6,000–18,000 BTU/hr | 35–60 BTU/hr per sq ft |
| Small home / zone | 600–1,200 sq ft | 20,000–50,000 BTU/hr | 30–50 BTU/hr per sq ft |
| Whole house estimate | 1,200–2,500 sq ft | 40,000–100,000 BTU/hr | 30–55 BTU/hr per sq ft |
Frequently asked questions
How to calculate heat loss?
A quick heat loss estimate starts with area or volume, then adjusts for indoor/outdoor temperature difference, insulation, glazing, air leakage, and room type. The result is the heat output needed to hold the indoor design temperature.
How to calculate heat loss through a wall?
The simple wall formula is heat loss = area x U-value x temperature difference. This calculator keeps the workflow simpler by using insulation and glazing inputs instead of asking for every U-value.
How to calculate BTU heat loss?
BTU heat loss is usually shown as BTU/hr. If you already know the heat loss in watts, multiply watts by 3.412142 to get BTU/hr.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, reference values, and examples follow the active unit system where applicable.