Heat loss
Heat loss calculator
Add up the heat a room or a whole house loses through each wall, window, door, roof and floor, plus the air that leaks in and out. Pick a construction for a typical U-value or type your own.
Design heat loss
Heat loss at 21 °C inside and −3 °C outside.
6.65 kW
6,650 W · 22,700 BTU/h design heat loss
- Temperature difference24 °C−3 °C → 21 °C
- Heat-loss coefficient277.2 W/°Closs per degree of difference
- Per floor area69 W/m²of heated floor
Where the heat goes
| Element | Area | U (W/m²K) | U × A (W/°C) | Heat loss |
|---|---|---|---|---|
| Walls | 90 m² | 0.7 | 63 | 1,510 W |
| Windows | 18 m² | 2.8 | 50.4 | 1,210 W |
| Doors | 2 m² | 3 | 6 | 140 W |
| Roof | 48 m² | 0.4 | 19.2 | 460 W |
| Floor | 48 m² | 0.66 | 31.7 | 760 W |
| Thermal bridges | 206 m² | 0.15 | 30.9 | 740 W |
| Air changes | – | 1 ach | 76 | 1,820 W |
| Total | 277.2 | 6,650 W |
Q = Σ(U × A) × ΔT + y × ΣA × ΔT + 0.33 × n × V × ΔT = 277.2 W/°C × 24 °C ≈ 6,650 W
Estimate the yearly running cost
How to use the heat loss calculator
- Set the indoor temperature and your area's outdoor design temperature.
- Enter the heated floor area and ceiling height, which give the air volume, then choose how draughty the space is.
- For each outside wall, window, door, roof and floor, enter its area and pick the construction. The U-value fills in; replace it with a figure from a survey, an energy certificate or a product label if you have one.
What the parts mean
A U-value is the heat in watts that passes through one square metre for each degree of difference between inside and outside, so each element loses U × A × ΔT. Wall area is measured with its windows and doors in it; the calculator takes those out. Only surfaces facing outside, the ground or an unheated loft count: a wall shared with a heated home next door loses almost nothing.
Air leakage needs heating too. Warming a cubic metre of air by one degree takes about 0.33 watt-hours, so ventilation loses 0.33 × n × V × ΔT watts, where n is air changes per hour and V the volume.
Worked example
A 1970s semi-detached house, 96 m² over two floors, heated to 21 °C at −3 °C outside (ΔT 24 °C): 90 m² of filled cavity wall at 0.7 loses 1,512 W, 18 m² of older double glazing at 2.8 loses 1,210 W, the door 144 W, a loft with 100 mm insulation 461 W, an uninsulated solid floor about 760 W, thermal bridges 742 W and one air change an hour in 230 m³ 1,825 W. That is about 6.65 kW in all.
Typical U-values used here
These are defaults that UK energy assessors use when they cannot measure a building, and limits from the building regulations. Real walls and windows vary, so treat them as typical values and use your own figures where you have them. US window labels give the U-factor in BTU/h·ft²·°F: switch the units above to enter it directly.
| Element | Construction | W/m²K | BTU/h·ft²·°F | Source |
|---|---|---|---|---|
| Walls | Solid brick, under 200 mm thick, uninsulated | 2.50 | 0.440 | RdSAP 10 T13 |
| Solid brick, 200–280 mm (about 9 in), uninsulated | 1.70 | 0.299 | RdSAP 10 T13 | |
| Solid brick, 280–420 mm, uninsulated | 1.40 | 0.247 | RdSAP 10 T13 | |
| Solid wall with 100 mm internal or external insulation | 0.32 | 0.056 | RdSAP 10 T6 | |
| Cavity wall, unfilled (built before 1976) | 1.50 | 0.264 | RdSAP 10 T6 | |
| Cavity wall, filled later (built before 1976) | 0.70 | 0.123 | RdSAP 10 T6 | |
| Cavity wall as built 1983–1990 | 0.60 | 0.106 | RdSAP 10 T6 | |
| Cavity wall as built 1996–2002 | 0.45 | 0.079 | RdSAP 10 T6 | |
| Wall as built 2012–2022 | 0.28 | 0.049 | RdSAP 10 T6 | |
| Timber frame as built 1967–1975 | 0.80 | 0.141 | RdSAP 10 T6 | |
| New wall in an existing home (2021 rules, England) | 0.18 | 0.032 | ADL 2021 T4.2 | |
| Windows | Single glazing | 4.80 | 0.845 | SAP 10.2 T6e |
| Single glazing, metal frame | 5.70 | 1.004 | SAP 10.2 T6e | |
| Older double glazing (air, 12 mm gap) | 2.80 | 0.493 | SAP 10.2 T6e | |
| Older double glazing (air, 6 mm gap) | 3.10 | 0.546 | SAP 10.2 T6e | |
| Double glazing, low-E, argon, 16 mm gap | 1.70 | 0.299 | SAP 10.2 T6e | |
| Triple glazing, low-E, argon, 16 mm gaps | 1.30 | 0.229 | SAP 10.2 T6e | |
| New window (2021 rules, England) | 1.40 | 0.247 | ADL 2021 T4.2 | |
| Doors | Solid wooden door | 3.00 | 0.528 | SAP 10.2 T6e |
| Door fitted 2007–2011 | 2.00 | 0.352 | RdSAP 10 T26 | |
| New door (2021 rules, England) | 1.40 | 0.247 | ADL 2021 T4.2 | |
| Roofs and lofts | Loft with no insulation | 2.30 | 0.405 | RdSAP 10 T16 |
| Loft, 50 mm insulation | 0.68 | 0.120 | RdSAP 10 T16 | |
| Loft, 100 mm insulation | 0.40 | 0.070 | RdSAP 10 T16 | |
| Loft, 150 mm insulation | 0.30 | 0.053 | RdSAP 10 T16 | |
| Loft, 200 mm insulation | 0.21 | 0.037 | RdSAP 10 T16 | |
| Loft, 270 mm insulation | 0.16 | 0.028 | RdSAP 10 T16 | |
| Loft, 400 mm or more | 0.11 | 0.019 | RdSAP 10 T16 | |
| Exposed floors | Over a garage or open air, uninsulated | 1.20 | 0.211 | RdSAP 10 T20 |
| Over a garage or open air, 100 mm insulation | 0.30 | 0.053 | RdSAP 10 T20 |
Ground floors lose less heat per square metre the larger they are, because most of it escapes near the edges. The calculator uses the method of ISO 13370 with the soil and wall assumptions of RdSAP 10: an uninsulated solid floor of a 48 m² ground floor with 22 m of exposed edge comes out at about 0.66 W/m²K, while a 120 m² floor with 44 m of edge comes to about 0.57.
Air changes
The CIBSE Domestic Heating Design Guide assumes at least 0.5 air changes per hour in habitable rooms. Older homes leak more: SAP 10.2 counts an open chimney as 80 m³ of air an hour on its own, which is two air changes in a 40 m³ living room. The 1 to 2 options are rough bands for typical homes; a blower-door test gives the real figure.
Sources
- BRE, RdSAP 10 Specification (9 June 2025): wall U-values (Tables 6 and 13, England), roof U-values (Table 16), exposed floors (Table 20), thermal bridging factor y (Table 21), doors (Table 26) and the ISO 13370 ground-floor parameters (section 5.12).
- BRE, SAP 10.2 (11 April 2023): Table 6e window and door U-values (wood or PVC-U frames unless stated).
- HM Government, Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments), Table 4.2: limiting U-values for new walls, windows and doors in existing homes in England.
- MCS, Heat Load Calculator: ventilation rates: minimum 0.5 air changes per hour (CIBSE Domestic Heating Design Guide) and infiltration from chimneys and flues (SAP 10.2 Table 2.1).
- Ventilation factor 0.33 Wh/(m³·K): density of air (about 1.2 kg/m³) × its specific heat (about 1,005 J/(kg·K)) ÷ 3,600 s.
Questions
What is a U-value?
It is the rate heat passes through a building element, in watts per square metre for each degree Celsius of difference across it (W/m²K). Lower is better: an uninsulated solid brick wall is around 1.7, a new wall in England must reach 0.26 or better, single glazing is 4.8 and modern triple glazing about 1.3.
How do I calculate the heat loss of a room?
Multiply each outside surface's area by its U-value and add them up, add 0.33 × air changes per hour × room volume for ventilation, then multiply the total by the temperature difference. The calculator does this for you and shows each part.
How do I convert a U-value between W/m²K and BTU units?
Multiply W/m²K by 0.1761 to get BTU/h·ft²·°F, or divide by 0.1761 to go back. A window rated U-0.30 on a US label is about 1.7 W/m²K. Switching the units in the calculator converts every field.
Why is the air change loss so large?
Every bit of cold air that leaks in has to be warmed. In the 1970s semi example above it is more than a quarter of the total, and draughty homes lose more. Sealing gaps, fitting chimney balloons or closing unused flues and adding draught strips cut it, while a modern airtight home relies on controlled ventilation instead.
Is this good enough to size a heat pump or boiler?
It is a good way to see where heat goes and to check a quote, but installers size heat pumps room by room from measured areas, with adjacent unheated spaces and higher temperatures in bathrooms, to EN 12831 or ACCA Manual J. Ask for that calculation and compare its total with yours.