Heat Loss Calculator
Estimate the rate of heat loss through a wall, window, roof, or whole building using surface area, insulation value, and the temperature difference.
What this calculator does
The heat loss calculator estimates how much heat escapes through a building surface — a wall, window, roof section, or an averaged whole-building envelope — for a given temperature difference between inside and outside.
The formula
This uses the standard building-science formula Q = U x A x ΔT, where Q is heat loss in BTU per hour, U is the surface’s U-value (how much heat passes through per square foot per degree, the inverse of R-value), A is the surface area, and ΔT is the difference between indoor and outdoor temperature. A larger area, a higher U-value (less insulation), or a bigger temperature swing all increase heat loss.
This is a simplified single-surface estimate for general planning. A full home heat-loss (or Manual J) calculation also accounts for infiltration, multiple assembly types, and orientation — consult an HVAC or energy professional for a complete assessment.