BTU Calculator UK | BTU, Watts & kW for Any Room

How to use the btu calculator

Pick the Room type. This sets the temperature we aim for (for example 21 °C in a living room, 18 °C in a bedroom, 22 °C in a bathroom) and how often the air changes.
Type in the Room length, Room width and Ceiling height. Each box has its own unit, so you can use metres, centimetres, millimetres, feet or inches.
Say how many External walls the room has, then pick the External wall type, the Number of windows, the Size of each window and the Glazing.
Choose What is above the room? and What is below the room? A heated room above or below loses no heat. A cold loft or solid ground floor does.
Pick Where are you? for the coldest day we plan for, and How draughty is the house?
Read BTU per hour needed, then tap Share link to send the figures to your heating engineer.

Why use this calculator

It adds up where the heat really goes. Heat escapes through the walls, windows, roof, floor and the air that leaks in and out. The calculator works each one out separately, so a corner room with old windows gets a very different answer from a small inner room.

It shows BTU/h, watts and kW together. Radiators and heaters are sold in BTU/h or watts depending on the shop, so you get both without doing the sum yourself.

It shows its working. Under How we worked it out you can see every step with your own numbers, and the results list the loss from each part of the room so you can see what a better window or loft insulation would help with.

It is for heating, not air conditioning. Cooling depends on different things, such as sun through the glass and heat from people and appliances, so don't use this figure to pick an air conditioner.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

Room: 4.5 m × 3.5 m × 2.4 m high = 37.8 m³ (floor 15.75 m²).
Temperature difference (ΔT): we aim for 21 °C in a living room and plan for -3 °C outside in a typical UK location, so ΔT = 21 − (-3) = 24 K.
External wall area: (4.5 + 3.5) m × 2.4 m = 19.2 m². Minus 2.4 m² of windows leaves 16.8 m² of wall.
Walls: U 1.5 W/m²K (cavity wall, not insulated) × 16.8 m² × 24 K = 605 W.
Windows: U 2 W/m²K (double glazing, 2002 to 2021) × 2.4 m² × 24 K = 115 W.
Ceiling: the room above is heated, so we count 0 W.
Floor: U 0.8 W/m²K (solid ground floor) × 15.75 m² × 24 K = 302 W.
Air: 0.33 × 1.5 air changes per hour × 37.8 m³ × 24 K = 449 W.
Total heat loss = 605 + 115 + 0 + 302 + 449 = 1,471 W (1.47 kW).
Convert to BTU/h: 1,471 W × 3.412 = 5,021 BTU/h, rounded to the nearest 50 = 5,000 BTU/h.

The formula

We use a room heat-loss method. The heat a room loses on the coldest day is the heat its heating has to put back.

Heat loss (W) = fabric loss + air loss

Fabric loss is worked out for each wall, window, ceiling and floor: U-value × area × temperature difference. The U-value says how easily heat passes through (lower is better).
Air loss is 0.33 × air changes per hour × room volume × temperature difference. It is the heat carried out by the air that leaks out and gets replaced with cold air.
Temperature difference is the room temperature we aim for minus the coldest outside temperature we plan for.

Then BTU/h = watts × 3.412, rounded to the nearest 50.

Why not just multiply the room volume by a number?

Some quick methods take the room’s volume and multiply it by one figure. That gives the same answer for a room with three outside walls and single glazing as for an inner room with none. Heat loss does not work like that. It depends on what the room is made of, how many outside walls it has, what is above and below it, and how much air it swaps with the outside. Adding those up gives a result that fits your room.

BTU and watts

A BTU is a unit of heat. Heating output is given in BTU per hour (BTU/h), and the same output is also given in watts. 1 W is 3.412 BTU/h, so 1,000 BTU/h is about 293 W. Radiator catalogues often show both.

What we assume

Room temperatures: 21 °C in living and dining rooms, 22 °C in bathrooms, and 18 °C in bedrooms, kitchens, halls, landings, WCs and utility rooms. These are the CIBSE figures for older homes. Newer, well-insulated homes are designed to 21 °C in most rooms, so a bedroom in a new build may need more than we show.
Outside temperature: -3 °C for a typical UK location. If you pick a city, we use its design temperature, for example -3.0 °C in London, -4.5 °C in Manchester and -5.6 °C in Glasgow.
Air changes per hour: 1.5 in living and dining rooms, 1.0 in bedrooms, 2.0 in kitchens, halls, landings, WCs and utility rooms, and 3.0 in bathrooms. “Well sealed, modern” uses 0.5. These older figures are on the cautious side, so they can overstate the loss in a tight house.
Walls (U-value, W/m²K): solid brick 1.7, cavity wall not insulated 1.5, cavity wall with insulation added 0.7, modern insulated wall 0.28.
Windows: single glazing 4.8, double glazing before 2002 2.8, double glazing 2002 to 2021 2.0, modern low-energy double glazing 1.4, triple glazing 1.3.
Roof or ceiling: no loft insulation 2.3, 100 mm 0.40, 150 mm 0.30, 270 mm 0.16. A heated room above counts as zero.
Floor: 0.8 for a solid ground floor or a suspended timber floor. A heated room below counts as zero.
Window size: we start with 1.2 m² for each window, about a 1,190 × 1,040 mm casement, and you can change it. If the windows add up to more than 90% of the outside wall, we limit them to 90%.

A guide, not a survey

This is a guide. A heating engineer does a full room-by-room heat-loss survey before fitting a new boiler, heat pump or radiators. We ignore doors, chimneys, thermal bridges and heat gained from neighbouring rooms. Use this to get a sensible figure and to check a quote, not to replace the survey.

Tips

Measure the glass and frame of a typical window rather than guessing. Windows usually lose more heat per square metre than walls, so the size matters.
If you aren't sure about a wall, loft or floor, pick the less insulated option. You get a bigger number, which is safer than a room that never warms up.
Work out each room on its own. Don't add the rooms together to choose a boiler. A heating engineer sizes the whole system.
Pick a radiator or heater with at least the BTU/h shown, not less. If you are buying radiators for a boiler or heat pump that runs cooler, use the radiator size calculator instead, because they give out less heat than their catalogue figure.
Cutting heat loss can mean you need smaller radiators. Try changing the loft or glazing in the calculator to see how much it saves, then use the insulation calculator to plan the work.

FAQs

How many BTU do I need to heat a room?

Our default living room (4.5 m by 3.5 m, two outside walls, double glazing from 2002 to 2021) needs about 5,000 BTU/h. Your own room will differ, so enter your sizes, walls and windows to get your number.

How many BTU per m² does a room need?

There is no single figure. The default living room works out at about 317 BTU/h per m² of floor, while a 3.5 m by 3 m bedroom with one outside wall and heated rooms above and below needs about 150. The calculator shows BTU per m² for your room, but use the total, not the per m² figure, to choose a radiator.

How do I work out what size radiator I need in BTU?

Work out the room's BTU/h with this calculator, then pick a radiator with at least that output. That is close enough for a boiler running at 75/65. For a cooler boiler or a heat pump, use the radiator size calculator, which corrects for the lower water temperature.

How many BTU do I need for a 20 m² living room?

A 5 m by 4 m living room built like our default one (two outside walls, two windows, solid floor, heated room above) needs about 6,000 BTU/h. A room with fewer outside walls needs less, and one with single glazing or no loft insulation needs more.

How do I convert BTU to watts or kW?

Divide BTU/h by 3.412 to get watts, so 5,000 BTU/h is about 1,465 W or 1.47 kW. Going the other way, 1 W is 3.412 BTU/h and 1 kW is 3,412 BTU/h.

How many BTU does a bedroom need compared with a bathroom?

A bathroom is kept warmer (22 °C against 18 °C for a bedroom) and the air changes more often, 3 times an hour against once. So a bathroom loses more heat for its size, even though it is usually smaller.

What BTU air conditioner do I need for my room size?

This calculator is for heating, so don't use it to size an air conditioner. Cooling depends on sun, people and equipment as well as the room, so ask an installer.