BTU heat load calculator
Areas and U-values of walls, windows and roof, volume, air changes and temperatures: you get heat loss by component and the total heating load.
Total heat loss
12351.953 BTU/h
- Total heat loss12351.953 BTU/h
- Wall losses3616.87 BTU/h
- Window losses2524.985 BTU/h
- Roof / ceiling losses2695.592 BTU/h
- Ventilation and infiltration losses3514.506 BTU/h
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How it's calculated (visible formula)
Q = Σ U × A × ΔT + 0.34 × ACH × volume × ΔT (W); 1 W = 3.412 BTU/h
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure wall, window and roof areas exposed to outside.
- Assign each a U-value.
- Set indoor and outdoor design temperatures.
- Transmission loss = U × A × ΔT.
- Add ventilation and infiltration losses.
Worked example: 970 sq ft home, ΔT 40 °F (22 °C)
- Walls: 80 m² × 0.6 × 22 = 1,056 W
- Windows: 739 W; roof: 792 W
- Air: 0.34 × 0.6 × 230 × 22 = 1,032 W
→ 3.62 kW (about 12,400 BTU/h) of heat loss.
Common mistakes
- Using the insulation U-value instead of the whole wall assembly.
- Ignoring infiltration around old windows.
- Using average winter temperature instead of design temperature.
Pro tips
- Use whole-assembly U-values, not just the insulation.
- Add 10–15% for thermal bridges in older homes.
- Windows are often the weakest link.
- Include slab-on-grade losses with an equivalent U.
From calculation to quote
Add labor, overhead and your margin, and generate the professional PDF.
FAQs
- How do I convert R-value to U?
- U (W/m²K) = 5.678 ÷ R (US).
- What U-value is a window?
- Single glazing 5.7; double 2.8; low-e double with thermal break 1.4–1.8 W/m²K.
- What air change rate?
- 0.3–0.6 ACH for tight homes; 1 or more for older houses.
- What outdoor design temperature?
- The ASHRAE 99% heating design temperature for your location.
- How do I get a wall's U-value?
- U = 1 ÷ total R of all layers including air films.
- What is infiltration?
- Uncontrolled air leaking through gaps, outlets and windows.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.