Heat Loss Calculator

How to estimate heat loss

1. Enter the inside and outside design temperatures and the room volume and air changes.
2. Enter the areas and U-values of the walls, windows, roof and floor.
3. Read the heat loss in watts and BTU per hour to size a heater or radiator.

Frequently asked questions

Where do I find U-values?

From building specifications or typical values: an insulated modern wall is about 0.2 to 0.35 W/m²K, double glazing about 1.2 to 2.8, and an uninsulated solid wall 2 or more.

How is air leakage counted?

As 0.33 × air changes per hour × volume × temperature difference, in watts.