How the heat loss calculator works
Heat flows out of a room in two ways. It conducts through the walls, glass and roof, at a rate set by each surface's U-value, and it is carried away by cold outside air that leaks or is drawn in and has to be warmed. The calculator works out each path separately for the design temperature difference, the gap between the indoor temperature you want and the cold-day outdoor figure used for heating design in your region.
Heat loss (W) = Σ (U × area × ΔT) + 0.33 × air changes per hour × volume × ΔTThe U-value is the heat in watts that passes through one square metre for each degree of difference; a lower number means better insulation. The factor 0.33 is the energy needed to warm one cubic metre of air by one degree, in watt-hours. Imperial inputs are converted exactly (1 BTU/h·ft²·°F = 5.678 W/m²K) and the answer is given in BTU/h, with 1 W = 3.412 BTU/h.
Worked example
A 5 × 3 m corner bedroom with a 2.5 m ceiling has two outside walls totalling 20 m², of which 3 m² is double glazing, and a ceiling to an insulated loft. With 20 °C inside and −10 °C outside, ΔT is 30 K.
- Walls: 17 m² × 0.35 × 30 = 178.5 W
- Windows: 3 m² × 1.4 × 30 = 126 W
- Ceiling: 15 m² × 0.20 × 30 = 90 W
- Air: 0.33 × 0.5 × 37.5 m³ × 30 = 185.6 W
The total is 580 W, or about 1,980 BTU/h, which is 39 W per square metre of floor. Notice that the leaking air costs more than the whole wall, and the small window loses almost as much as 17 m² of insulated wall.
Typical U-values
| Element | U (W/m²K) | U (BTU/h·ft²·°F) |
|---|---|---|
| Solid brick wall, uninsulated | about 2.0 | 0.35 |
| Cavity wall with fill, or 2×4 stud wall with R-13 | 0.45–0.55 | 0.08–0.10 |
| Recent insulated wall | 0.15–0.30 | 0.03–0.05 |
| Single glazing | about 5.5 | about 1.0 |
| Older double glazing, air filled | about 2.8 | about 0.50 |
| Double glazing, low-e and argon | 1.1–1.4 | 0.20–0.25 |
| Triple glazing | 0.6–0.8 | 0.11–0.14 |
| Ceiling with no loft insulation | about 2.3 | 0.40 |
| Loft with 250–300 mm mineral wool (about R-38) | 0.14–0.16 | about 0.026 |
For glazing, use the whole-window U-value including the frame, not the centre-of-glass figure.
What the result is for, and what it leaves out
- Radiator sizing. European radiators are rated at ΔT50 (EN 442), meaning water about 50 °C warmer than the room. With a heat pump running at 45 °C flow, a radiator gives only about a third to a half of its catalogue output, so it must be larger.
- Floors and party walls. A floor over a cellar or on the ground, and walls to an unheated garage, also lose heat; add them with the same U × area × ΔT rule using a smaller temperature difference.
- Thermal bridges. Lintels, slab edges and balconies bypass the insulation; allow about 10% extra in older houses.
- Heat-up margin. Rooms heated only in the evening need spare power to warm up; a 10–20% margin is common.
For choosing a boiler or heat pump for a whole house, a room-by-room calculation to EN 12831 in Europe or ACCA Manual J in North America, done by a heating engineer, is the reference.
A reproducible calculation with complete inputs
For a reproducible room heat loss calculator example, use the metric input system and enter Outside wall area (without windows): 17 m²; Wall U-value: 0.35 W/m²K; Window and glazed door area: 3 m²; Window U-value: 1.4 W/m²K; Roof or ceiling area to a cold space: 15 m²; Roof or ceiling U-value: 0.2 W/m²K; Room volume: 37.5 m³; Air changes per hour: 0.5 per hour; Indoor temperature: 20 °C; Outdoor design temperature: -10 °C. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Total heat loss | 580.125 | 580 | W |
| Total in BTU/h | 1979.46887775 | 1979 | BTU/h |
| Walls | 178.5 | 178 | W |
| Windows | 126 | 126 | W |
| Roof or ceiling | 90 | 90 | W |
| Ventilation and draughts | 185.625 | 186 | W |
Frequently asked questions
How do you calculate heat loss of a room?
Multiply each outside surface's area by its U-value and by the indoor-outdoor temperature difference, then add 0.33 × air changes × volume × temperature difference for air leakage. The sum is the heat loss in watts.
What is a good heat loss per square metre?
Recent well-insulated homes lose about 30–50 W/m² of floor on a design cold day; older homes with little insulation often need 80–120 W/m².
How do I convert U-value to R-value?
R = 1 ÷ U in the same system. A U-value of 0.35 W/m²K is R 2.86 m²K/W, about R-16 in US units; multiply metric R by 5.678 to get US R.
How many watts of heat per m³ of room?
As a rough check, 25–40 W/m³ for a modern room and 40–60 W/m³ for an older one, but the U-value method is far more accurate because it counts the actual walls and windows.
Why does ventilation add so much heat loss?
Every cubic metre of cold air that replaces warm air must be heated: at half an air change per hour, a 40 m³ room swaps 20 m³ an hour, costing about 200 W at a 30 °C difference.