Room Heat Loss Calculator (UK): Watts, kW and BTU

Enter the size of the room, the area of external wall, windows and door, the U-values and the inside and outside temperatures. You get the heat lost through each part and through ventilation, and the total in watts, kW and BTU an hour.

UK edition. Switch to the US edition for feet, inches and US gallons.

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Your project

Room length

Room width

Ceiling height

Length of external wall

The length of wall that has the outside on the other side.

Window area

m²

The total area of windows in the external wall.

External door area

m²

The ceiling has the roof or the outside above it

Tick for a top floor room under a roof or a loft.

The floor is on the ground

U-value of the external wall

W/m²K

Use your own figure from a survey or an EPC. The default is the limit for a new home in Approved Document L; an older solid wall is about 2 and a filled cavity about 0.5 or better.

U-value of the windows and door

W/m²K

The default is the limit for a new home. Old single glazing is about 5, and modern double glazing about 1.2 to 1.6.

Inside temperature

°C

21 °C is the usual design temperature for a living room, and 18 °C for a bedroom.

Outside design temperature

The coldest day to design for: about −3 °C in much of the UK. Your heating installer has the figure for your area.

Air changes in an hour

How often the air in the room is replaced by outside air through gaps and vents. About 1 for a modern room, 1.5 to 2 for a draughty one.

Results

Example job shown. Change the numbers to match yours.

Temperature differenceInside less outside: 24 °C
External wall, less windows and door: 7.60 m²
Lost through the wallU-value × area × temperature difference: 47 W
Lost through the windows and door: 77 W
Lost through the roof or ceiling: 0 W
Lost through the floor: 0 W
Lost through ventilation and draughts0.33 × air changes × volume × temperature difference: 266 W

Total

Heat loss of the room

390 W

In kilowatts: 0.39 kW
In BTU an hour1 W = 3.412 BTU an hour: 1,332 BTU/h

This is the steady heat loss at the design temperature: the heat the radiator or heater must put in to hold the room at the inside temperature. Each part loses its U-value times its area times the temperature difference, and the ventilation loses 0.33 watts for each cubic metre of air changed an hour, for each degree. Walls shared with a heated room lose little and are left out. This is a room by room estimate; for a whole house, a boiler or a heat pump, a heat loss survey by an installer is the right way, and it also includes thermal bridging, which this page ignores. Allow a margin of 10 to 20% when you pick a radiator.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

Worked example: a room 4 × 3.5 × 2.4 m has 4 m of external wall, 9.6 m² less 2 m² of window, so 7.6 m². At 21 °C inside and −3 °C outside the difference is 24 °C. The wall at U 0.26 loses 47 W, the window at U 1.6 loses 77 W, and the ventilation at one air change an hour loses 0.33 × 1 × 33.6 × 24 = 266 W. The total is 390 W, which is 0.39 kW or 1,332 BTU an hour. With an old solid wall (U 2) and single glazing (U 5) it is 871 W. A top floor room under a roof adds 54 W, a ground floor 60 W, and two air changes make the total 656 W.

Heat loss of a room 4 × 3.5 m with one 4 m external wall and 2 m² of window, at 21 °C inside and −3 °C outside

Watts for different wall and window U-values and air changes.

Wall U-value	Window U-value	Air changes	Heat loss
0.26	1.6	1	390 W
0.5	1.6	1	434 W
2	1.6	1	708 W
2	5	1.5	1,004 W
0.26	1.2	0.5	238 W

Tips before you buy

Use the U-values for your own walls and windows from a survey, an EPC or the manufacturer: the defaults are for a new home.
Measure only the walls that have the outside on the other side. A wall to a heated room loses little.
The air changes matter a lot in a small room: seal the draughts first, and the heat loss falls.
Pick a radiator that gives out the heat loss with a margin of 10 to 20%, at the water temperature of your system.
For the whole house or a heat pump, ask for a full heat loss survey.

Tools & materials for this project

Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.

Room thermometer and humidity gauge Draught excluder strip Thermal imaging camera Radiator reflector foil Window insulation film

Frequently asked questions

How do I calculate the heat loss of a room?

Add up U-value × area × temperature difference for each wall, window, roof and floor, and add 0.33 × air changes × volume × temperature difference for the ventilation. For this room it is 390 W.

What temperatures should I use?

About 21 °C inside for a living room and 18 °C for a bedroom, and a design outside temperature of about −3 °C in much of the UK. Your installer has the figure for your area.

What U-values should I use?

Your own, from a survey or an EPC. A new home is at 0.26 for the wall, 0.16 for the roof, 0.18 for the floor and 1.6 for windows and doors, and an older solid wall is about 2.

How many BTU is that?

Multiply the watts by 3.412: 390 W is 1,332 BTU an hour.

Does this size a boiler?

No. A boiler or a heat pump is sized from a heat loss of the whole house, with the hot water, and by an installer. This is for one room, to help choose a radiator or a heater.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.