What the Room BTU Calculator does
This calculator works out the heating or cooling a single room needs, in BTU per hour, watts and kilowatts, from its size, how much of it faces the outside, its windows, how well it is insulated and the temperatures you want to hold. It uses a steady-state heat balance - heat lost or gained through each surface plus the air that leaks in - and shows every term so you can see what drives the result.
Use it to choose a radiator, an electric heater, a heat pump head or a room air conditioner, and add rooms to a table to total a whole floor.
How to use it
- Choose heating or cooling, name the room and enter its length, width and ceiling height in metres.
- Enter the length of wall that faces outside air - not walls to other heated rooms - and the total window area.
- Tick the boxes if there is a roof or cold loft above, or a ground floor or cold space below.
- Pick the construction preset closest to your room. It fills typical U-values (heat loss per m² per °C); replace them with real figures from a survey or the building's energy certificate if you have them.
- Set the air change rate (about 0.5 for a new, airtight room, 1 for a typical one, 2 or more for a draughty old one) and the indoor and design outdoor temperatures.
- For cooling, open Heat gains and enter sun on the windows, people and equipment. Use Add to rooms table to build a list and total.
Reading the results
The hero figure is the steady load at your design temperatures: the output a heater or cooler must deliver to hold the room there on that day. Choose a unit rated at or above it.
The assumption trace shows each surface's share. If windows or ventilation dominate, fixing those is usually cheaper than a bigger heater.
The 10% margin for heating is a common allowance so a room warms up in reasonable time after the heating has been off; it is a convention, not a rule.
Worked example: heating a bedroom
A 4 x 3.5 m bedroom with a 2.5 m ceiling has two outside walls totalling 7.5 m and 2 m² of double glazing. Holding 21 °C when it is -3 °C outside is a 24 °C difference.
The outside walls are 7.5 x 2.5 = 18.75 m², minus 2 m² of window, so 16.75 m² at U = 1.0 lose 16.75 x 1.0 x 24 = 402 W. The window loses 2 x 2.8 x 24 = 134.4 W. At one air change an hour the 35 m³ of air costs 0.33 x 1 x 35 x 24 = 277.2 W.
The total is 813.6 W, which is 813.6 x 3.412 = 2,776 BTU/h. With a 10% margin, a heater or radiator of about 900 W (3,070 BTU/h) at design conditions is enough.
Formulas and scoring rules
- Fabric loss or gain
Q = U x A x dT for each wall, window, roof and floor (W)Wall area is external wall length x height minus the windows.- Ventilation
Q = 0.33 x ACH x volume (m³) x dT (W)0.33 Wh/m³K is the heat capacity of air.- Cooling gains
solar = W/m² x window area; people = 75 W sensible each; appliances as entered- Units
BTU/h = W x 3.412142; kW = W / 1000; tons = BTU/h / 12,000- Presets (W/m²K)
walls 2.1 / 1.0 / 0.3; windows 5.0 / 2.8 / 1.4; roof 2.3 / 0.7 / 0.16; floor 0.9 / 0.5 / 0.22 (poor / average / good)Typical indicative values, not measurements.
Limitations: what the result does not prove
- U-value presets are typical, not your walls. Real values depend on materials, thicknesses and workmanship; an energy survey gives better figures.
- Ground floors lose heat in a more complex way than a single U-value suggests, and thermal bridges at joints are ignored.
- Cooling is simplified: solar gain varies with orientation and time of day, and latent (humidity) load is not included. Air conditioner sizing charts such as ENERGY STAR's are a useful cross-check.
- It sizes one room's emitter or unit. A whole heating system - boiler or heat pump capacity, pipework, flow temperatures - needs a full calculation by a heating engineer to your local standard.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- ENERGY STAR - Recommended insulation R-values
- ENERGY STAR - Room air conditioner sizing (cooling cross-check)
Frequently asked questions
How many BTU do I need to heat a room?
It depends on heat loss, not only floor area. A well-insulated modern room may need around 30-50 W per m² on a cold day, while an uninsulated room with single glazing can need 100 W per m² or more. Enter your room's walls, windows and temperatures to get your figure.
What is a U-value?
It is the rate of heat flow through one square metre of a building element for each degree of temperature difference, in W/m²K. Lower is better: an uninsulated solid brick wall is around 2, a modern insulated wall 0.3 or less, and modern double glazing about 1.4.
How do I convert BTU to kW?
Divide BTU per hour by 3,412 to get kilowatts, or multiply kW by 3,412 to get BTU/h. 10,000 BTU/h is about 2.93 kW. The calculator shows both, so you can compare radiators rated in watts with air conditioners rated in BTU.
What outdoor temperature should I design for?
Use the design temperature for your area - a cold day that is exceeded only a few times a year, not the coldest ever recorded. Heating standards and installers publish regional figures; -3 °C is a common choice in mild temperate climates and much lower elsewhere.
Why does ventilation add so much heat loss?
Every air change replaces the whole room's warm air with outside air that must be heated. In a draughty room with two air changes an hour, ventilation can be the biggest single loss, which is why draught-proofing is often the cheapest improvement.
Can I use this to size a radiator?
Yes, for one room. Radiator outputs are usually quoted at a standard temperature difference between the water and the room (often 50 °C). If your system runs cooler, such as with a heat pump, the radiator delivers less than its label, so check the maker's correction factors.
Last reviewed by the A2Z.Tools team against the sources listed above.
