Window Heat Loss Calculator

Enter the room dimensions, number of external walls, window and door sizes, insulation levels, and the indoor/outdoor design temperatures. The calculator applies the formula Q = U × A × ΔT for each building element, adds ventilation and infiltration losses, and gives you the total heat loss in Watts, kW, and BTU/hr.

The core fabric heat loss formula is Q = Σ(U × A × ΔT), where U is the thermal transmittance of a building element (W/m²K), A is its area (m²), and ΔT is the temperature difference between inside and outside (K). Ventilation heat loss uses Q = 0.33 × n × V × ΔT, where n is the air change rate and V is the room volume.

Use the heating design temperature for your location — typically the 99% or 99.6% winter design condition from sources like CIBSE Guide A (UK), ASHRAE Fundamentals (US), or your national building code. Our calculator includes a location lookup to suggest the appropriate value, but you can always override manually.

A U-value (W/m²K) measures how much heat passes through one square metre of a building element for every degree of temperature difference. Lower U-values mean better insulation. Unlike calculators that use vague labels like 'poor' or 'good', we display the actual U-value used in the calculation so you can verify and adjust it.

This calculator uses principles informed by recognised standards including ACCA Manual J, BS EN 12831, and CIBSE guidelines. It provides a Manual J-style estimate suitable for preliminary sizing and comparison. For official Manual J compliance — typically required for code submissions or equipment warranties in the US — a professionally reviewed calculation using certified software is recommended.

Yes. Heat pumps typically operate at lower flow temperatures (35–45°C) compared to boilers (60–80°C). This means a radiator's heat output at ΔT30 is significantly lower than at ΔT50. Our calculator shows radiator requirements at multiple ΔT values so you can size correctly for heat pump systems.

Online calculators provide estimates based on the inputs and assumptions you provide. Accuracy depends on the quality of your input data — particularly U-values, room dimensions, and air tightness. For most domestic radiator sizing, a well-configured online calculation is adequate. For complex buildings, regulatory submissions, or large installations, a professional survey and calculation is recommended.

Cold outside air that leaks into the building through gaps, cracks, and openings must be heated to room temperature. This infiltration heat loss can represent 15–25% of the total, especially in older buildings with poor air sealing. Excluding it would significantly underestimate your heating requirement.