Work out the BTU/hr needed to heat a room from its size, type, insulation and external walls, with an optional radiator cost estimate.
Optional: estimate radiator cost
Related tool: This calculator gives the BTU/hr a room or space needs and an optional radiator cost. If you just want a quick radiator size with glazing and orientation options, try our Radiator BTU Calculator.
BTU (British Thermal Unit) is a measure of heat output. The BTU/hr a space needs depends on its volume, insulation, external walls and what the space actually is — a snug bedroom and a glass-walled conservatory of the same size have very different needs. This calculator works out the room volume, then adjusts it for the space type, insulation level and number of external walls to give a figure you can shop with. Radiators, towel rails and electric heaters are all rated in BTU/hr (and kW).
Pick your space at the top. “Standard room” lets you choose the room type (lounge, bedroom, kitchen and so on). The other presets — garage conversion, loft conversion, open-plan or double-height, rooms with large windows, conservatory, classroom and church or village hall — automatically load sensible starting values for height, insulation and external walls, and apply a heat-demand factor suited to that kind of space. Adjust any of them to match your actual space.
Conversions are the classic “why is this room always cold” culprits. A garage conversion starts life with a cold slab floor, a big door opening and thin walls, so it needs more output until you insulate it properly. A loft conversion loses heat through the sloping roof and gables — count each sloped surface as an external wall. Insulate well and the requirement drops sharply, so set the insulation level honestly.
Glazing is the weak link: a wall of large windows or bi-fold doors loses far more heat than an insulated wall, so those rooms need extra output. Tall open-plan, double-height or stairwell spaces suffer the stack effect — warm air rises and gathers up high — so size for the full volume. Conservatories combine both problems and top the table. Big public rooms like a school classroom or church or village hall add high ceilings, heavy ventilation and stop-start heating into the mix; the presets give a sound starting estimate, but a formal design should come from a heating engineer.
Whether a house is semi-detached or detached mostly changes one thing per room: how many walls face the outside. A mid-terrace room might have just one external wall; a semi-detached room often has two; a detached room can have three. Each external wall adds roughly 10% to the heat loss, so choose the “Number of external walls” to match — that is how this tool reflects the difference between property types.
A metric bedroom 4.5m (14.8ft) × 3.5m (11.5ft) × 2.4m (8ft) with good insulation and 2 external walls:
Switch between metric (metres) and imperial (feet) at the top. The calculator converts imperial measurements automatically and gives the result in BTU/hr and kW, the two units used worldwide for heating output.
Measure the room length, width and height, choose the space or room type and insulation level, then this calculator estimates the required heat output. It multiplies the room volume by a base figure and adjusts for the space type, insulation and number of external walls, giving the answer in both BTU/hr and kW.
A garage conversion usually needs more heat than a normal room of the same size because it starts with a cold concrete floor, a large door opening and little insulation. Select the 'Garage conversion' preset and enter your measurements; if you have since insulated the floor, walls and door reveal, change the insulation level to Average or Good for a lower, more accurate figure.
Loft conversions lose most of their heat through the sloping roof and gable ends, so count each sloped surface as an external wall. Choose the 'Loft / attic conversion' preset, enter the floor size and the average ceiling height, and the calculator gives the BTU/hr to look for. Well-insulated rafters and dormers reduce the requirement noticeably.
Glass loses far more heat than an insulated wall, even when double-glazed, so rooms with big windows or bi-fold doors need extra output. Use the 'Room with large windows' preset. If the glazing is triple-glazed or the room faces south and gets plenty of sun, you can size towards the lower end of the suggested range.
Divide the BTU/hr figure by 3,412 to get kilowatts (kW), or multiply kW by 3,412 to get BTU/hr. For example, 5,000 BTU/hr is about 1.47 kW. This calculator shows both figures automatically so you can shop for radiators or heaters rated in either unit.
