Window Heat Loss

A window heat loss calculator estimates the thermal energy lost through windows based on their U-value (thermal transmittance), area, and the temperature difference between inside and outside. Use this calculator for accurate, instant results.

The calculator uses the standard published formula for window heat loss. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches, square metres, Celsius, kilowatts. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: Heat loss (W) = U-value × Area (m²) × ΔT (°C). Each step in the calculation is shown so you can verify the result manually.