Heat Loss Calculator
How many watts each room loses on a cold day, and the total for the house. Enter the rooms one at a time: the walls, windows, floor and roof that face the cold, and the air that leaks through them. The answer is what your radiators, boiler or heat pump have to replace.
This is a planning estimate. It is not an MCS heat loss design, and a heat pump installation needs a full room-by-room survey from an MCS-certified installer. Use it to see roughly what to expect and to check the figure you are given.
The house
Design outdoor temperature: . 99.6% means only about 35 hours a year are colder. The figure drops 0.6 °C for every full 100 m you sit above the weather station.
Ground floor
Enter the whole ground floor here. A floor loses heat mostly round its edges, so the U-value depends on the house's footprint. Exposed perimeter counts the outside walls only and leaves out party walls. Floor U-value: .
Rooms
External wall length is the length of wall facing outside, measured inside the room. Party walls are shared with a neighbour's house.
How the heat loss is worked out
The method is the room-by-room steady-state calculation set out in the CIBSE Domestic Heating Design Guide and the MCS heat pump standard MIS 3005-D. It is a simplified form of BS EN 12831-1, which a full MCS design follows in detail. Each room loses heat two ways: through the fabric, and in the air that leaks out and has to be replaced by cold air from outside.
Fabric loss
U is how easily heat passes through the element, in watts per square metre per degree. A is its area in square metres. ΔT is the room temperature minus the temperature on the other side. For outside walls, windows, doors and the loft that is the design outdoor temperature. A solid ground floor loses to the ground, which stays near the year's average temperature, so it uses that instead. A party wall or a wall to a garage is taken as losing to a space at 10 °C.
Ventilation loss
0.33 is the heat needed to warm one cubic metre of air by one degree, expressed per hour (1.2 kg/m³ × 1,005 J/kg K ÷ 3,600). The air change rate is how many times an hour the room's air is replaced by draughts and ventilation. Older houses leak more, and kitchens and bathrooms are given more because they are ventilated to clear steam.
Room total and the whole house
The whole-house figure is the sum of the rooms. It is the heat the house needs on the design day, the cold day that is only colder for about 35 hours in a typical year (the 99.6th percentile). A boiler or heat pump is sized against this figure.
Example: a 1900s terrace living room
A living room in a 1900s mid-terrace in London: 4.2 m deep, 3.6 m wide, 2.7 m ceilings. One 3.6 m wall faces the street with 2.2 m² of older double glazing. The 4.2 m wall on the other side is a party wall. It sits on a suspended timber floor, with the whole ground floor 40 m² and 10 m of exposed perimeter, and there is a heated bedroom above.
- Design outdoor temperature, London at the 99.6th percentile: -3.0 °C. Living room design temperature 21 °C, so ΔT = 24 K.
- External wall, solid 9 inch brick: (3.6 × 2.7 - 2.2) = 7.52 m² × 1.7 × 24 = 306.8 W
- Window: 2.2 m² × 2.8 × 24 = 147.8 W
- Suspended floor: U = 0.5084 for a 40 m² floor with 10 m of perimeter, so 15.12 m² × 0.5084 × 24 = 184.5 W
- Party wall to a neighbour at 10 °C: 11.34 m² × 1.33 × 11 = 165.9 W
- Thermal bridging: 0.15 × 24.84 m² of exposed surface × 24 = 89.4 W
- Fabric total: 894.5 W
- Ventilation: 0.33 × 1.5 air changes × 40.82 m³ × 24 = 485.0 W
- Room heat loss: 1,379 W, or 4,707 BTU/h. About 91 W per square metre of floor.
That is the example loaded in the calculator above. Change the windows to 2022 glazing and the loss falls by 74 W. Fill the gaps round the sash and floorboards to bring the room from 1.5 air changes down to 1.0, and it falls by 162 W. Ventilation is over a third of this room's loss.
Where the U-values and temperatures come from
You choose the location, age, walls, glazing, loft and floor, and can override each room's temperature and air changes. Every default comes from the source a heating designer would use, read on 28 September 2026.
| Input | Source | Examples |
|---|---|---|
| Design outdoor temperature | CIBSE Guide A (2015) Table 2.5, as used by the MCS Heat Load Calculator, with its altitude rule | London -3.0 °C, Birmingham -5.1 °C, Glasgow -5.6 °C, Plymouth -1.5 °C |
| Room temperatures | CIBSE Domestic Heating Design Guide (2026) Table 2-2, via MCS | Living 21 °C, bathroom 22 °C, bedroom 18 °C (21 °C from 2007 builds) |
| Air changes per hour | CIBSE Domestic Heating Design Guide Table 3.8, as carried in the MCS heat pump calculator | Living room 1.5 for homes built before 2003, 1.0 for 2003 to 2006, 0.5 after |
| Walls, loft, windows, doors | RdSAP 10 specification (BRE, June 2025), by age band | Solid brick 1.7, unfilled cavity 1.5, filled cavity 0.7, 100 mm loft 0.40, single glazing 4.8 |
| Ground floor | BS EN ISO 13370, as implemented in RdSAP 10 | Worked out from the floor's area, perimeter and insulation |
| Thermal bridging | RdSAP 10 Table 21 | 0.15 W/m²K on exposed area before 2003, 0.08 from 2007 |
What this calculator leaves out
- Heat moving between rooms held at different temperatures, such as a 22 °C bathroom next to an 18 °C bedroom. It cancels out across the house but shifts a few tens of watts between rooms.
- Open fireplaces and chimneys. For a room with an open flue, a heating design uses 2 to 5 air changes an hour, depending on the room's size and whether the flue has a throat restrictor; enter your figure under "override".
- Intermittent heating. If the heating goes off overnight, a designer may add a margin so rooms warm up in time. At the 99.6% design day MCS does not require one.
- Rooms with sloping ceilings, dormers and bay windows. Approximate them with an equivalent box.
The air change table is the conservative part. It is what MCS designs used for years, but a draught-proofed older house often measures well under it. We ran the eight-room mid-terrace that ships with OpenEnergyMonitor's open source heatlossjs tool through both. With the same inputs the two agree to within 0.5%. With our defaults this calculator reads 73% higher. The largest single cause is the air changes: its owner measured 0.6, against our 1.0 to 3.0. If you have had an air-tightness test, override the air changes with the measured figure. An installer's MCS design for an older house can come out lower than this calculator for the same reason, and that alone is no sign of undersizing.
Heat loss questions
How do you calculate heat loss for a room?
Add up the fabric loss and the ventilation loss. For each wall, window, floor and ceiling that faces somewhere colder, multiply its U-value by its area and by the temperature difference across it. Then add 0.33 times the air changes per hour times the room volume times the temperature difference for the air that leaks out. The result is in watts; multiply by 3.412 for BTU per hour.
What outside temperature should I use for a heat loss calculation?
Use the design outdoor temperature for where you live. MCS uses the CIBSE figures for 14 UK weather stations, the temperature that is only undercut for about 0.4% of the hours in a year: -3.0 °C in London, -5.1 °C in Birmingham, -4.5 °C in Manchester, -5.4 °C in Edinburgh and -1.5 °C in Plymouth. Take off 0.6 °C for every full 100 m your house sits above the station. The old flat -3 °C figure is too mild for most of the country.
What is a typical heat loss per square metre?
It varies with the age of the house and with the room. In the test houses behind this calculator, a bedroom in a house built since 2012 comes out at 35 W/m², a whole 1930s cavity-wall semi at 90 W/m², the 1900s terrace living room in the worked example at 91 W/m², and a single-glazed Victorian bedroom under an uninsulated loft at 161 W/m². Use a rule of thumb only as a sanity check. Rooms with the most outside wall and window lose more per square metre than rooms in the middle of the house.
Can I use this for building regulations or an MCS heat pump installation?
No. Building regulations compliance for a new home or extension is shown with a SAP calculation under Part L, which is a different, whole-house energy model. For a heat pump, an MCS installer measures every element, checks the construction and signs off a room-by-room design they are accountable for; you need that design for the Boiler Upgrade Scheme grant. This calculator follows the same room-by-room method, but its ventilation and neighbour assumptions are the older, more cautious CIBSE ones, so an installer's MCS figure for a pre-2000 house can come out lower. That on its own is not a sign of undersizing.
Why is my heat loss higher than my gas bill suggests?
The design heat loss is the peak the house needs on one of the coldest days of the year, with every room at its design temperature. Most of the winter is much milder, rooms are not all heated at once, and people, cooking and the sun add free heat. The design defaults are also deliberately cautious, especially the air change rates, so a well draught-proofed house will usually need less than the figure shown.
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