What Is the Radiator BTU Calculator?
A British Thermal Unit (BTU) measures heat energy. When sizing a radiator, you need to know how many BTUs per hour are required to comfortably warm a room. Undersized radiators leave a room cold; oversized ones waste energy. This calculator estimates the heat output your radiator should provide based on the room volume and how the room is used.
How to Use It
Enter the room length, width and height in metres, then choose the room type. The calculator multiplies the room volume by a heat-loss factor that reflects typical requirements for that type of space, returning the recommended output in both BTU per hour and watts.
The Formula
The core estimate uses the room volume and a room-type factor \(f\) given in BTU per cubic metre:
$$\text{BTU} = L \times W \times H \times f$$where \(L\) = length, \(W\) = width and \(H\) = height (all in metres). Typical factors are roughly \(135\) for bedrooms, \(153\) for lounges, \(108\) for kitchens and \(180\) for bathrooms. To convert to watts, multiply by \(0.293071\):
$$W = \text{BTU} \times 0.293071$$
Worked Example
Consider a bedroom measuring \(4\,\text{m} \times 3\,\text{m} \times 2.4\,\text{m}\), giving a volume of \(28.8\,\text{m}^3\). Using a bedroom factor of \(135\):
$$\text{BTU} = 28.8 \times 135 = 3888\ \text{BTU/hr}$$In watts:
$$W = 3888 \times 0.293071 \approx 1140\ \text{W}$$FAQ
Should I add for windows or poor insulation? Yes. The base estimate assumes average insulation. Add roughly 10–20% for large windows, north-facing walls or older properties.
What if I have two radiators in one room? Split the total BTU between them — together they should meet or slightly exceed the required output.
BTU or watts — which do retailers use? Both are common. This tool shows each so you can match any product listing.
