How to use the room heat loss calculator
A room heat loss calculation tells you how much heat a room needs on the coldest design day to stay at a comfortable temperature. It is the starting point for sizing radiators, underfloor heating, heat pumps and electric heaters, and for comparing the effect of new windows or insulation. This calculator follows the standard steady-state method used in heat loss surveys: fabric loss through each element plus the loss from air changes.
- Choose units. Metric mode uses metres, W/m²K and °C and reports watts. US mode uses feet, BTU/h·ft²·°F and °F and reports BTU per hour. Both show the other unit too.
- Enter the room size and the length of wall that faces outside. Only exterior walls lose heat to the outside.
- Enter the window and door areas in those walls and the U-value of each element. They are taken out of the gross wall area.
- Enter the roof and floor U-values, or zero if there is a heated room above or below.
- Set the temperatures and air change rate. Use your local outdoor design temperature rather than a typical day.
Heat loss formulas
Fabric loss = Σ (U × A) × ΔT, where ΔT = inside − outside temperature
Net wall area = exterior wall length × height − windows − doors
Ventilation loss = 0.33 × ACH × V × ΔT (W, metric) · 0.018 × ACH × V × ΔT (BTU/h, ft³ and °F)
Total = fabric + ventilation · 1 W = 3.412 BTU/h
The 0.33 factor is the heat capacity of air, about 0.33 Wh per cubic metre per kelvin; 0.018 is the imperial equivalent in BTU per cubic foot per °F. The roof and floor are taken as the room's floor area. This is a simplified method: it treats the ground like outdoor air and ignores thermal bridges, so use a ground-floor U-value that already accounts for the soil and add a margin for bridging in older buildings.
| Element | Older, uninsulated (W/m²K) | Modern, insulated (W/m²K) |
|---|---|---|
| Solid or cavity wall | 1.5–2.1 | 0.18–0.30 |
| Window | 4.8 (single) / 2.8 (old double) | 0.8–1.4 |
| Roof or loft | 2.0 | 0.11–0.16 |
| Ground floor | 0.7 | 0.13–0.25 |
| External door | 3.0 | 1.0–1.8 |
To convert, 1 W/m²K = 0.176 BTU/h·ft²·°F. An R-38 ceiling is about U 0.026 in imperial units, and a modern code wall around U 0.06.
Worked examples
A corner living room in metric
A 5 × 4 m room with a 2.5 m ceiling has 9 m of exterior wall, or 22.5 m² gross. Taking out 3 m² of windows leaves 19.5 m² at U 0.30, which is 5.85 W/K. The windows add 3 × 1.4 = 4.2 W/K, the roof 20 × 0.16 = 3.2 and the floor 20 × 0.25 = 5.0, for 18.25 W/K in total. At 21 °C inside and −3 °C outside, ΔT is 24 K, so the fabric loss is 438 W. One air change per hour in the 50 m³ room adds 0.33 × 1 × 50 × 24 = 396 W. The total is 834 W, about 2,850 BTU/h, or 42 W per square metre of floor.
A bedroom in feet
A 16 × 14 ft bedroom with 8 ft ceilings has 30 ft of exterior wall, 240 ft² gross. With 30 ft² of windows at U 0.30, the net wall is 210 ft² at U 0.06, giving 12.6 + 9.0 BTU/h·°F. An R-38 ceiling adds 224 × 0.026 = 5.8, and the floor is over a heated room. At 70 °F inside and 10 °F outside, the fabric loss is 27.4 × 60 = 1,645 BTU/h. Half an air change per hour in the 1,792 ft³ room adds 0.018 × 0.5 × 1,792 × 60 = 968 BTU/h, so the room needs about 2,610 BTU/h, or 766 W.
Who this calculator helps
- Architects testing how window sizes and insulation levels change the heating load at concept stage.
- Heating engineers and heat pump installers doing a quick room-by-room check before a full survey.
- Retrofit designers showing clients the effect of new glazing, loft insulation or draught-proofing.
- Homeowners sizing a radiator or electric heater for one room.
Practical tips
- Work out wall and roof U-values from the layers with the R-value and U-value calculator, and compare insulation options with the insulation calculator.
- Large windows dominate the loss in well-insulated rooms. Check the window-to-wall ratio early.
- Use the ventilation calculator to set a realistic air change rate for mechanically ventilated homes, and the BTU calculator for the cooling side.
- Run every heated room and add them up for the whole-house load; don't size a heat pump from one room multiplied by the floor area.
Planning a retrofit with new windows and finishes? Show the client the result on a photo of their room with Room Redesign.
