Heat Load Calculator (U-value method)
Heat loss = U × area × ΔT for each surface, summed.
Result
Heat loss
2,100 BTU/hr
Wall: 1,200; window: 900.
- Wall U × A × ΔT0.06 × 400 × 50 = 1,200 BTU/hr
- Window U × A × ΔT0.3 × 60 × 50 = 900
- Total heat loss2,100 BTU/hr
Step-by-step
- Wall loss = 0.06 × 400 × 50 = 1,200 BTU/hr.
- Window loss = 0.3 × 60 × 50 = 900 BTU/hr.
- Total = 2,100 BTU/hr.
How to use this calculator
- Enter wall area and U-value (or R-value's inverse).
- Enter window area and U-value.
- Enter ΔT (indoor − outdoor).
About this calculator
Heat loss through walls and windows: each surface loses heat at U × area × ΔT. Lower U-value = better insulation. R-value is the inverse (R = 1/U). Modern walls: R-19 (U≈0.05). Modern double-pane low-e windows: R-3 (U≈0.30). Single-pane windows: R-1 (U≈1.0) — they leak heat ~3× faster.
Frequently asked
U = heat-flow rate per ft² per °F. R = 1/U. R-30 wall has U-value ≈ 0.033.
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