Free Tool
BTU Calculator — What Size Radiator Do I Need?
Work out exactly how much heat your room needs, allowing for wall construction, glazing and what sits above and below. Takes a minute.
Room dimensions
Measure the whole room, not just one wall
Most UK ceilings are around 2.4 m.
Window area (m2)
Window height multiplied by width. Enter 0 if there is no window.
Type of room
Bathrooms are heated warmer than bedrooms, so need more output.
Type of outside wall
Number of outside walls
What is above the room?
What is below the room?
Type of window
You’ll need
Minimum heat output for this room
0
BTU per hour
Enter your room dimensions
What to look for
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- Outputs are quoted at ΔT50. A lower boiler flow temperature reduces real output.
- You can combine radiators to reach the total requirement.
- Underfloor heating can replace or supplement a radiator.
A planning estimate based on room volume and construction. Pipe sizing, boiler output and system design should be confirmed by a qualified heating engineer.
Radiators and towel rails from Ahmco
We hold accounts with brands including Reina, Vado and Warmup, many for over 15 years, so we can match output and style to your room and budget. Our sister company East & Gray can install it too.
BTU and radiator sizing explained
BTU stands for British Thermal Unit. It is a measure of heat energy — roughly the amount needed to raise one pound of water by one degree Fahrenheit. Radiators are rated in BTU per hour, which tells you how quickly they put heat into a room.
Enough output to replace the heat your room loses. A small insulated en-suite might need around 1,000 BTU, a typical family bathroom 2,000 to 2,500, and a large or poorly insulated room 3,000 or more. The calculator above works out the figure for your specific room.
It starts from the volume of the room — length, width and height — because you are heating air, not floor space. That volume is then adjusted for how much heat escapes: the type of outside wall, how many outside walls there are, what sits above and below, and the size and type of glazing. Windows are calculated separately using their U-value, since glass loses far more heat than a wall.
A radiator with too little output means the room never reaches a comfortable temperature, however long the heating runs. Too much output costs more than necessary, though you can always turn a radiator down. Undersizing is the more common and more expensive mistake, because it is usually discovered after the room is tiled and plumbed in.
Manufacturers quote radiator outputs at a 50 degree difference between the water inside and the air in the room. That assumes a fairly hot boiler. If your system runs cooler, or you have a heat pump at around 45 degrees, the same radiator will deliver noticeably less than its rated figure. Worth checking before you order.
Yes. The figure is a total for the room, so two radiators of 1,200 BTU each will do the same job as one of 2,400. Many bathrooms combine a towel rail with underfloor heating for exactly this reason.
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