Radiator BTU Calculator

A radiator that's too small leaves a room cold however high you turn the thermostat. One that's too big costs more to buy and can make a room uncomfortable. The answer is to match the radiator's heat output to the room, measured in BTU (British Thermal Units) per hour or in watts.

Our Radiator BTU Calculator estimates the output your room needs. This guide explains how it works, what the adjustments mean and what to check before you buy.

BTU and watts

Radiator outputs are given in BTU per hour, watts or both. They measure the same thing:

1 watt = 3.412 BTU per hour

So a 1,500 W radiator gives out about 5,118 BTU/h.

How the calculation works

The calculator uses a widely used rule of thumb:

1. Work out the room's volume: length × width × ceiling height, in cubic metres.
2. Multiply by a heat rate for the room. Living rooms need around 40 watts per cubic metre, bedrooms around 35, kitchens around 30 because cooking adds heat, and bathrooms around 45 because people like them warmer.
3. Adjust for heat loss. Poor insulation, external walls, large windows and a north-facing aspect all increase the heat needed.
4. Add a 15% margin so the room warms up in reasonable time on cold days.
5. Convert watts to BTU by multiplying by 3.412.

How to use the calculator (Radiator BTU Calculator)

1. Enter the room's length, width and ceiling height in metres.
2. Choose the room type.
3. Choose how well the room is insulated.
4. Enter the number of external walls and whether there are large windows or patio doors.
5. Say whether the room is north-facing.
6. If the room will have more than one radiator, enter how many.

The calculator shows the heat output in BTU/h, watts and kilowatts, and splits it between radiators if you have more than one.

A worked example

With the calculator's default settings, a 5 m × 4 m living room with a 2.4 m ceiling, average insulation and one external wall:

• Volume: 5 × 4 × 2.4 = 48 m³
• Base heat: 48 × 40 = 1,920 W
• Adjustments: average insulation (× 1.2) and one external wall (× 1.1) = × 1.32
• With a 15% margin: about 2,915 W, or roughly 9,950 BTU/h

That's around 10,000 BTU/h, so you might choose one large double-panel radiator or two smaller ones on different walls.

Choosing a radiator

Manufacturers list each radiator's output, usually at ΔT50. This means the difference between the average water temperature in the radiator and the room is 50°C, which suits a traditional boiler running hot.

• Pick a radiator at or slightly above the output you need.
• Double-panel radiators with fins (type 22) give much more heat for their size than single panels.
• Towel rails give out less heat than radiators of the same size, so check the figure if you're relying on one to heat a bathroom.
• Under windows is the traditional spot, because it counters cold air coming off the glass.

Heat pumps and low flow temperatures

Heat pumps, and condensing boilers set to low flow temperatures to save gas, run radiators cooler. At ΔT30, a radiator gives out only around half its ΔT50 rating. If you have a heat pump or plan to get one, choose radiators by their output at your system's temperature, which manufacturers usually publish, or ask your installer.

If you're getting a heat pump through the Boiler Upgrade Scheme, your MCS-certified installer must carry out a room-by-room heat loss calculation, which is more accurate than any rule of thumb.