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Heat Loss Calculator

Enter the U-value, area and temperatures to find the heat loss.

How it is calculated

The tool calculates steady conduction loss through one part of the building envelope: Q = U × A × (Tin − Tout). U is the U-value in W/m²K, A the area in m², and both temperatures are in °C. The temperature difference ΔT is shown separately. The daily energy assumes the same ΔT for 24 hours: kWh/day = Q × 24 ÷ 1000. U and area cannot be negative. If the outside is warmer than the inside, ΔT and Q come out negative, which means heat is flowing into the building. The formula covers only conduction through the surface you describe: it leaves out ventilation, air leakage and thermal bridges, and a whole building needs this calculation repeated for each element and then added.

Worked example

Take an external wall of 120 m² with U = 0.35 W/m²K, indoor temperature 20 °C and outdoor −2 °C. Temperature difference: 20 − (−2) = 22 °C. Heat loss: 0.35 × 120 × 22 = 924.00 W. Over a full day at that constant difference: 924 × 24 ÷ 1000 = 22.176 kWh/day. If better insulation lowered U to 0.20, the loss would fall to 0.20 × 120 × 22 = 528 W.

Try this example →

Common mistakes

  • Using the R-value (insulation resistance) instead of the U-value. U is its inverse (U = 1/R), so a high R means a low U, and swapping them gives a wrong loss.
  • Entering U-values in BTU/h·ft²·°F. The tool expects W/m²K, and the area must be in square metres.
  • Treating the result as the full heat demand of a house. It covers one element at one U-value; ventilation and other walls, roof and windows with their own U-values must be added.
  • Multiplying the daily figure by 365 to get an annual cost. It assumes the same ΔT all day, but real outdoor temperatures change, so the year-round total would be overstated.

About this calculator

Steady-state heat loss through a building element follows a simple formula: Q = U × A × ΔT, where U is the element's U-value (thermal transmittance in W/m²K, lower is better insulated), A is its surface area, and ΔT is the temperature difference between inside and outside. Multiplying these gives the heat flow in watts leaving through that wall, roof, floor or window.

Architects and HVAC engineers sum this calculation across every wall, roof, floor and window of a building to size a heating system correctly, and energy auditors use it to identify which parts of a building's envelope are losing the most heat and would benefit most from added insulation.

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