Radiator BTU Calculator
Work out how much heat a room needs and what size radiator will heat it. A 4 m by 3.5 m living room with one outside wall needs about 2,400 BTU, which a 500 mm double panel radiator covers with a boiler. With a heat pump at 50°C you'd need one rated at about 5,950 BTU, because it runs the water cooler.
1. The room
Heated to 21°C, the usual design temperature for a living room.
2. Outside walls and windows
3. Above, below and where you live
Older homes tend to be draughtier, so they lose more heat through air leaks.
4. What heats your radiators?
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How to use the result
The figure is the heat the room loses on a cold winter day: the outside temperature heating engineers design for where you live, from −1.5°C in Plymouth to −5.9°C in Glasgow. A radiator with at least that output keeps the room at the right temperature. Radiators are listed in watts and BTU per hour: 1 watt is 3.412 BTU per hour.
If the room has two radiators, split the figure between them. If you're between two sizes, pick the bigger one. The thermostatic valve stops the room getting too hot, and a slightly bigger radiator warms the room faster after the heating has been off.
Fitting a radiator costs about £150 to £250 with a plumber. See our radiator cost guide for prices by type and size.
Why heat pumps need bigger radiators
Makers rate radiators at ΔT50: the water is on average 50°C warmer than the room, about 70°C. A condensing boiler wastes less gas with cooler water, and a heat pump sends water at about 45 to 50°C. MCS, which certifies heat pump installers, suggests sizing radiators for a flow of about 45°C. A radiator's output drops faster than the temperature does, so at 45°C it gives under a third of its rated heat.
| Running at | Typical system | Share of rated output |
|---|---|---|
| ΔT50 | Boiler with water at about 75°C (the rating point) | 100% |
| ΔT40 | Boiler with water at about 65°C | 75% |
| ΔT30 | Boiler turned down to 60°C, or a heat pump at 55°C | 51% |
| ΔT25 | Heat pump at about 50°C | 41% |
| ΔT20 | Heat pump at about 45°C | 30% |
Output at other temperatures is the ΔT50 rating × (ΔT ÷ 50)1.3, which matches the outputs radiator makers publish for steel panel radiators. ΔT is the average water temperature minus the room temperature, so 45°C flow and 40°C return in a 21°C room is about ΔT20.
Thinking about a heat pump? Our heat pump cost calculator includes the cost of bigger radiators.
Radiator types and how much heat they give
Most radiators in UK homes are steel panels. More panels and fins give more heat from the same wall space. These are the published ΔT50 outputs of one popular range (Stelrad Compact) for a radiator 600 mm high and 1,000 mm long. Output goes up roughly in line with length, and other makes are close but not identical.
| Type | Watts | BTU |
|---|---|---|
| Single panel, single convector (Type 11) | 980 | 3,350 |
| Double panel, single convector (Type 21) | 1,345 | 4,600 |
| Double panel, double convector (Type 22) | 1,732 | 5,900 |
| Triple panel (Type 33) | 2,389 | 8,150 |
How the calculator works
It follows the room-by-room heat loss method that UK heating engineers use, based on BS EN 12831. For each outside wall, window, ceiling and floor, heat loss is the U-value (how easily heat passes through) × the area × the difference between the room and outside. Air leaking in and out adds 0.33 × air changes per hour × the room's volume × the same temperature difference.
Room temperatures are the ones MCS uses for heat pump design: living room 21°C, dining room 21°C, kitchen 18°C, bedroom 18°C, bathroom 22°C, hall or landing 18°C, study 21°C, toilet 18°C. The outside temperature is the MCS figure for the nearest of nine cities, such as −3°C for London. Walls between heated rooms don't count, as no heat is lost through them.
For air leakage we use 0.5 air changes an hour in homes built since 2000, the minimum MCS uses, and 1.0 in older homes, our allowance for their extra draughts. A room with no outside walls gets none. U-values come from RdSAP, the method used for energy performance certificates; floor values are for a typical house, as they depend on the floor's size and shape.
| Part of the room | U-value (W/m²K) |
|---|---|
| Solid brick, no insulation (most homes before the 1930s) | 1.7 |
| Cavity wall, not filled | 1.5 |
| Cavity wall, filled with insulation | 0.7 |
| Modern insulated wall (built since 2007) | 0.3 |
| Single glazing | 4.8 |
| Double glazing fitted before 2002 | 2.8 |
| Double glazing fitted 2002 to 2021 | 2 |
| Double or triple glazing fitted since 2022 | 1.4 |
| Loft with thick insulation (about 270 mm) | 0.16 |
| Loft with some insulation (about 100 mm) | 0.4 |
| Loft with no insulation | 2.3 |
| Ground floor, solid concrete | 0.65 |
| Ground floor, suspended timber | 0.7 |
| Ground floor, insulated | 0.22 |
It's a good guide for choosing a radiator. For a heat pump, the installer has to do a full heat loss survey of every room under the MCS standards, and that survey should decide the final sizes.
How to need less heat
The calculator's "where the heat goes" bars show what to fix first. In our cold example room (two solid brick walls, single glazing and a suspended floor) the room needs 5,100 BTU, against 2,400 BTU with a filled cavity, double glazing and one outside wall.
- Filling an empty cavity wall cuts the heat lost through it by more than half. See our insulation calculator.
- Swapping single glazing for new double glazing cuts window heat loss by about 70%. See double glazing costs.
- Topping up loft insulation to about 270 mm makes a big difference to upstairs rooms.
- Sealing draughts around doors, windows and floorboards cuts the air leakage part.
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Frequently asked questions
How many BTU do I need for my room?+
It depends on the room's size, how many walls face outside, the windows and the insulation. As a guide, a 4 m by 3.5 m living room with one outside wall, a filled cavity and modern double glazing needs about 2,400 BTU (710 watts) on a cold day. The same room with two solid brick outside walls, single glazing, a bay window and a suspended timber floor needs about 5,100 BTU.
What size radiator do I need for a 3 m by 3 m bedroom?+
About 1,150 BTU (340 watts) for an upstairs bedroom with one outside wall, a filled cavity, double glazing and a well insulated loft above. That's a 400 mm single panel or a 400 mm double panel radiator, 600 mm high, with a boiler.
Do I need bigger radiators with a heat pump?+
Usually, yes. Radiators are sold on their output at ΔT50, which assumes water at about 70°C. A heat pump runs the water at about 45 to 50°C, so a radiator gives only 30% to 41% of its rated output. In our example living room that means a double panel radiator 1,100 mm long at 50°C, instead of 500 mm with a hot boiler.
What does ΔT50 mean on a radiator?+
It's the difference between the average water temperature in the radiator and the room: 50°C, so water at about 70°C in a 20°C room. Radiator makers test and quote their outputs at ΔT50 under the European standard EN 442, so you can compare like with like.
How many BTU is a 600 x 1000 double radiator?+
A 600 mm by 1000 mm double panel, double convector radiator (Type 22) gives about 1,732 watts, or about 5,900 BTU, at ΔT50. Makes differ a little, so check the figure for the one you buy.
Is it better to have a bigger radiator?+
A little bigger is fine. The thermostatic valve stops the room overheating, and a bigger radiator can warm the room with cooler water, which helps a condensing boiler or heat pump run more efficiently. Too small and the room won't reach temperature on cold days.
How do I convert watts to BTU?+
Multiply watts by 3.412. So 1,000 watts is about 3,412 BTU per hour. To go the other way, divide BTU by 3.412.
Sources
- MCS: MIS 3005-D, the heat pump design standard (room and outside design temperatures)
- MCS heat load calculator: ventilation rates
- MCS: domestic heat pump best practice guide (ΔT50 ratings and 45°C sizing)
- BRE: RdSAP 10 specification (U-values)
- BRE: SAP 10.2 (ventilation heat loss formula)
- GOV.UK: Approved Document L, conservation of fuel and power
Written by Chris Kervin, Co-Founder, Less.co.uk
Last updated: