Heating Load Calculator
Size a heater from floor area, indoor-outdoor design temperature difference and envelope insulation quality, with the U-factor model documented.
Last updated: 2026-09-15
How the calculation works
- The envelope is reduced to a single U factor based on insulation quality: 0.06 (good), 0.08 (average), 0.12 (poor) BTU/h·ft²·°F.
- Load = area × U × design ΔT gives the heat needed to hold the indoor design temperature at the outdoor design temperature.
- The result is converted to kW for comparison with electric heaters and heat pump ratings.
Formula
Load = Area × U × ΔT U = overall envelope heat-transfer coefficient (BTU/h·ft²·°F)
| Symbol | Meaning | Unit |
|---|---|---|
U | Whole-envelope U factor | BTU/h·ft²·°F |
ΔT | Design temperature difference | °F |
Area | Conditioned floor area | ft² |
Worked example
Interpreting the result
The result is the heat input needed at design conditions — the coldest reasonable weather, not the absolute record. Equipment should be sized near this figure; much larger furnaces short-cycle and cost more. Heat pumps should be checked against their capacity at the local design temperature, not their 47 °F rating.
Assumptions
- Whole-envelope U factor lumping walls, roof, floor and glazing into one coefficient.
- Design conditions per local climate data, not extreme records.
Limitations
- No infiltration/ventilation included — add roughly 10% for typical airtightness, more for fireplaces or leaky older homes.
- Not valid for passive-solar or high-performance envelopes with U below 0.04.
- Use Manual J for final equipment selection.
Frequently asked questions
How do I find my design temperature difference?
Take your indoor design temperature (typically 68–70 °F / 20 °C) and subtract the 99th-percentile winter design temperature for your location, published by ASHRAE or your national weather service.
Should I oversize my furnace?
No — size to the calculated load with a small margin. Oversized furnaces short-cycle, waste energy and create uneven temperatures.