Heat loss calculator
Room-by-room heat loss for a whole property, in watts, with the radiator each room needs at the flow temperature you're fitting. Pick the construction and the region once, then measure each room as you walk round.
- Works out
- Heat loss per room and for the property
- Breakdown
- Walls, windows, doors, roof, floor, air
- Sizes
- Radiators at ΔT 50, 40, 30 or 20
- Presets
- 6 construction types, 8 UK regions
Heat loss
Room by room across a property, with radiators sized for the system you're actually fitting.
1976–1990, or an older house that's had cavity fill
Override for an exposed or high-altitude site.
75/65, traditional boiler. Radiators are catalogued at ΔT50; this corrects the schedule for what the system actually runs at.
Outside walls only.
Override temperature and air changes for this room
Raise it for a draughty room, an open flue or a large opening.
W/m²K. Editing any value switches the construction to custom.
Whole property heat loss
0.8kW
Floor area
14m²
Average
56.1W/m²
Where the heat goes
Radiator schedule
rated at ΔT50 · system 75/65
Whole property heat loss
0.8kW
Floor area
14m²
Average
56.1W/m²
Where the heat goes
Radiator schedule
rated at ΔT50 · system 75/65
- Guidance only. Typical values for a quick survey, not a full design.
- Your survey stays on this device. It's kept as you type, and Save keeps a property by name.
- The same calculator engineers use in OnPoint's Tools.
01What it works out
Where the heat goes, room by room.
Every room loses heat through its walls, windows, doors, roof and floor, and in the air that leaks out. The calculator adds those up for each room on a cold day, then adds the rooms up for the property.
You don't need U-values to hand. Pick when the house was built and where it is, say what each room is used for, and the typical figures fill in. Change any of them if you know better.
When you'd use it
- Sizing a boilerThe property total, in kW, is the figure to size the heat source against.
- Sizing radiatorsEach room's output, corrected for the flow temperature, is the catalogue rating to order.
- Lower flow temperaturesSee how much bigger the radiators need to be on a condensing, low temperature or heat pump system.
- Talking to the customerThe breakdown shows where the heat goes, room by room, so you can show why a room is cold.
02The formula
Fabric, plus air.
A fabric-and-ventilation method, room by room. These are the exact steps the calculator takes, and every preset it fills in is listed below.
01Fabric
loss (W) = area × U-value × (inside − outside)
Worked out for each element. Wall area is the length of outside wall × the room height, less the windows and doors. The roof and floor count only when they're exposed, and use the floor area.
02Ventilation
loss (W) = 0.33 × air changes per hour × volume × (inside − outside)
0.33 is the heat it takes to warm a cubic metre of air by a degree, in watt-hours. The air change rate comes from the room type.
03The room
heat loss = fabric + ventilation
emitter = heat loss × 1.1
The emitter figure adds 10% so the radiator has a little in hand.
04The radiator to order
rating at ΔT50 = emitter ÷ (ΔT ÷ 50)^1.3
Radiators are catalogued at ΔT50. Run one cooler and it gives out less, so the rating you order goes up as the flow temperature comes down.
05The property
property loss = the rooms added together
A plain sum of the rooms. It doesn't net off heat that passes between rooms kept at different temperatures.
The presets it fills in
Typical figures for a quick assessment, not measured ones. Every one of them can be changed in the calculator.
U-values by construction, W/m²K
- Solid wall, uninsulatedPre-1919 brick or stone, single or early double glazing2.14.830.40.7walls 2.1 · windows 4.8 · doors 3 · roof 0.4 · floor 0.7
- Cavity, uninsulatedRoughly 1930–1975, double glazed since1.52.830.40.7walls 1.5 · windows 2.8 · doors 3 · roof 0.4 · floor 0.7
- Cavity, insulated1976–1990, or an older house that's had cavity fill0.62.82.20.350.6walls 0.6 · windows 2.8 · doors 2.2 · roof 0.35 · floor 0.6
- 1991–2005Insulated cavity and loft to the standards of the day0.4522.20.250.45walls 0.45 · windows 2 · doors 2.2 · roof 0.25 · floor 0.45
- 2006–2013Part L era, better glazing and roof insulation0.31.81.80.20.25walls 0.3 · windows 1.8 · doors 1.8 · roof 0.2 · floor 0.25
- 2014 onwardsCurrent new build0.261.41.40.130.18walls 0.26 · windows 1.4 · doors 1.4 · roof 0.13 · floor 0.18
Room types
- Living room21 °C · 1.5/h
- Dining room21 °C · 1.5/h
- Kitchen18 °C · 2/h
- Bedroom18 °C · 1/h
- Bathroom22 °C · 3/h
- Hall / landing18 °C · 2/h
- Study / office21 °C · 1.5/h
- Utility18 °C · 2/h
Inside temperature, then air changes per hour.
Outside, by region
- South West-2 °C
- London & South East-3 °C
- Wales-3 °C
- Northern Ireland-3 °C
- Midlands-4 °C
- North of England-4 °C
- Central Scotland-5 °C
- Highlands & islands-6 °C
Worth lowering for an exposed or high site.
Flow temperature
- ΔT 50 · 75/65, traditional boiler100%
- ΔT 40 · 65/55, condensing75%
- ΔT 30 · 55/45, low temperature51%
- ΔT 20 · 45/35, heat pump30%
How much of its catalogue output a radiator gives.
03A worked example
The front room of a 1930s semi.
4.2 × 3.6 m and 2.4 m high, with two outside walls, cavity with no insulation, a 3 m² bay, a suspended timber floor and a bedroom above. London & South East, so -3 °C outside and 21 °C in, a difference of 24 °C.
- Walls lose the most39% of this room's 1,453 W goes through the walls. That's the conversation to have about why it feels cold.
- On a heat pump, the radiator is 3.3 times the sizeAt ΔT 50 a 1,598 W radiator does the job. At ΔT 20 (45/35, heat pump) the same room needs one rated 5,259 W. Size off the raw heat loss and the room ends up cold.
Heat loss survey
Front room, 1930s semi
Cavity, uninsulated · London & South East, -3 °C outside
Heat loss
1,453 W
Where the heat goes
- Walls566 W39%
- Windows202 W14%
- Floor254 W17%
- Ventilation431 W30%
- Walls 15.72 m² × 1.5 × 24566 W
- Windows 3 m² × 2.8 × 24202 W
- Floor 15.12 m² × 0.7 × 24254 W
- Ventilation 0.33 × 1.5 × 36.29 m³ × 24431 W
- Emitter 1,453 W × 1.11,598 W
The radiator to order, rated at ΔT50
- ΔT 50 · 75/65, traditional boiler1,598 W
- ΔT 40 · 65/55, condensing2,136 W
- ΔT 30 · 55/45, low temperature3,104 W
- ΔT 20 · 45/35, heat pump5,259 W
04Reading the result
Size the boiler to the house, not the radiators.
Radiators get rounded up to what's made and what fits the wall, and at a low flow temperature their ratings are far bigger than the heat loss. Add them up and you'll oversize the boiler. Size the radiators to the rooms, and the heat source to the property total.
Guidance only. This is a room-by-room sizing aid using typical U-values for the construction you pick. A full design should use surveyed construction and the standard the project is being built to. For a gas boiler, follow Gas Safe procedures and the manufacturer's instructions.
- Whole property, kWThe rooms added together. It's the figure to size the boiler or heat pump against.
- Radiator scheduleEach room's loss plus 10%, turned into the ΔT50 rating to order for the flow temperature you picked.
- Where the heat goesEach element's share across the property. The biggest slice is usually the one worth fixing.
- W/m²Loss per square metre of floor. A room far above the others usually means a preset that doesn't match what's there, or a measurement in the wrong unit.
More free calculators.
All free, with no account, and the same ones engineers use inside OnPoint.
OnPoint for your trade
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- Every calculator in OnPointThese four, plus the tightness test, IGEM/UP/1 testing and unsafe situations walk-throughs in the app.
Questions people ask.
Spotted something wrong in a sum? Ring +44 7884 697405 or email us.
How do you calculate heat loss for a room?
Work out each element separately. For each wall, window, door, roof and floor: area × U-value × the difference between the inside and outside temperatures. For the air: 0.33 × air changes per hour × the room's volume × the same difference. Add them up for the room, and add the rooms up for the property.
What U-values does the calculator use?
Typical values for when the house was built. Walls go from 2.1 W/m²K (solid wall, uninsulated) down to 0.26 W/m²K (2014 onwards), with windows, doors, roof and floor to match. You can change any of them if you know the actual construction.
What outside temperature should I use?
The calculator uses a typical UK winter design temperature for the region, from -2 °C in the South West to -6 °C in the Highlands & islands. Lower it for an exposed or high site.
What inside temperatures and air changes does it use?
They come from the room type: living room 21 °C and 1.5 air changes an hour, bedroom 18 °C and 1, bathroom 22 °C and 3, and so on. Raise the air changes for a draughty room, an open flue or a large opening.
Why does the radiator get bigger at a lower flow temperature?
Radiators are rated at ΔT50, with the water about 50 °C warmer than the room. Run one cooler and it gives out less: the output goes with (ΔT ÷ 50) to the power 1.3. At ΔT 20, typical of a heat pump, a radiator gives about 30% of its rating, so you need a much bigger one.
Should I size the boiler to the radiators or the heat loss?
The heat loss. Radiators are rounded up to what's made and what fits, and at low flow temperatures their ratings are far bigger than the room's loss. Size the radiators to each room, and the heat source to the property total.
Is it accurate enough for a heat pump design?
It's a room-by-room sizing aid using typical values, and it adds up the rooms without netting off heat that passes between them. A heat pump or any full design should use surveyed construction and the standard the installation is being designed to.
Is the heat loss calculator free?
Yes. There's no sign-up and no limit on rooms. Your survey is kept on your device as you type, and you can save each property with a name. Nothing you enter is sent to OnPoint.
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