Radiator Size Calculator (BTU)
Describe the room and how well it is insulated. The calculator estimates the heat it loses on a cold day, adds a margin, and gives the output to look for in BTU an hour and in watts.
This is an estimate based on typical values for each type of construction. A heating engineer can do a full room by room survey.
Worked examples
A living room
4 m by 3 m by 2.4 m, 4 m of insulated cavity wall, 1.5 square metres of double glazing, ground floor, insulated loft
754 W heat loss, 2,574 BTU an hour, 868 W (2,960 BTU) with a margin
Planned at 21 degrees inside and minus 3 outside.
About Radiator Size Calculator (BTU)
A radiator size calculator estimates how much heat a room needs. A radiator only has to replace the heat that escapes, and that depends on the room's size, its outside walls and windows, whether the floor and ceiling are insulated, and how cold it is outside. Describe your room and the calculator adds up the heat lost through each part, plus the heat needed to warm the fresh air that comes in.
The temperatures used are the usual design figures: 21 degrees for living rooms, 18 for bedrooms and kitchens and 22 for bathrooms, against an outside temperature of minus 3. A margin of 15% is added so the room warms up reasonably fast. The answer is in BTU an hour and watts, which is how radiators are labelled, and a table shows where the heat goes.
How to use Radiator Size Calculator (BTU)
- Choose the type of room and enter its size.
- Enter the length of outside wall and the window area.
- Choose the type of wall, glazing, floor and ceiling.
- Read the output to look for in BTU and watts.
Common uses
- Replacing a radiator
- Heating a new extension or loft room
- Checking if insulation would cut the size needed
- Planning for a heat pump
Frequently asked questions
How many BTU do I need for a room?
It depends on the room. A 4 m by 3 m living room with one insulated outside wall, double glazing and an insulated loft needs about 2,600 BTU an hour without the margin, or about 3,000 with it.
What is a BTU?
A British Thermal Unit is a measure of heat. A radiator rated at 1,000 BTU an hour gives out about 293 watts. Heating engineers use both units.
Why does insulation change the size?
Good walls, windows and loft insulation lose less heat, so a smaller radiator is enough. Poor insulation can double the heat loss.
Is this the same as a heating survey?
No. It is an estimate for one room using typical values, and assumes the rooms next door are heated to a similar temperature. A heating engineer can do a full survey, which matters most for a heat pump.
Are radiators rated at a standard temperature?
Yes. The BTU on a radiator is usually at a high water temperature, and the output falls at lower flow temperatures such as those from a heat pump. Check the output at your system's temperature.