Heat Loss Calculator
Heat loss determines how big your heating system needs to be. This calculator estimates heat loss through walls, windows, ceilings, and air infiltration based on your home's construction and local climate.
Enter Your Measurements
Results
How to Use This Calculator
- Estimate total wall area (perimeter × height minus window area).
- Select your wall insulation level.
- Enter total window area and type.
- Enter ceiling area and insulation R-value.
- Enter indoor setpoint and your area's winter design temperature.
- Enter floor area and ceiling height for infiltration calculation.
How Heat Loss Calculation Works
Heat flows from warm to cold through every surface of your home. The rate depends on the surface area, the R-value (thermal resistance) of that surface, and the temperature difference (ΔT) between inside and outside. This calculator computes loss through walls, windows, and ceiling separately, then adds infiltration loss (cold air leaking in through gaps and cracks). The total tells you the minimum heating capacity needed. A typical modern home loses 30,000–50,000 BTU/hr at design conditions; older homes can lose 80,000–120,000 BTU/hr.
Formula
Q = U × A × ΔT for each surface (U = 1/R). Infiltration = 0.018 × Volume × ACH × ΔT. Total = sum of all losses.