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Radiator BTU and size calculator

To size a radiator, work out the room's heat loss in watts (1W is 3.412 BTU an hour) from its size, walls, windows and insulation, then pick a radiator whose rated output at ΔT50 covers it. A 5m × 4m living room in a 1930s house with unfilled cavity walls needs about 1,619W (5,525 BTU). On a heat pump the radiator must be rated about three times higher.

What do you want to do?

Walls to the outside

The rest are internal walls to heated rooms.

About 1.5m² for a typical window.

Detailed settings

Radiator schedule

This room needs

1,619W

It loses about 1,472W (5,022 BTU/h) at -3°C outside. With a 10% margin, look for a radiator rated 1,662W at ΔT50 on a gas or oil boiler, 75/65.

5,525 BTU/h, or 1.6kW.

BTU in watts, as a rough guide

  • 2,000 BTU ≈ 586W: small, well-insulated box room
  • 5,000 BTU ≈ 1,465W: average bedroom
  • 7,000 BTU ≈ 2,052W: typical living room
  • 10,000 BTU ≈ 2,931W: large or poorly insulated living room
Where the heat goes
External walls714W
Windows202W
Floor176W
Ventilation380W
Margin at 10%147W
Heat needed1,619W
Show how we worked this out
  1. External walls18.6m² × 1.6 × 24°C714W
  2. Windows3m² × 2.8 × 24°C202W
  3. Floor20m² × 0.8 × 11°C (ground at 10°C)176W
  4. Ventilation0.33 × 1 air changes × 48m³ × 24°C380W
  5. Margin at 10%1,472W × 1.1147W
  6. Heat needed1,472W lost, plus the margin1,619W

The same room on every system

ΔT50 rating needed by heating system
SystemRating needed at ΔT50
Gas or oil boiler, 75/651,662W
Condensing boiler run low, 60/502,673W
Heat pump, 55/453,287W
Heat pump, 45/404,851W
Heat pump, 40/356,843W

A heat pump runs cooler water, so the same room needs a radiator rated two to three times higher.

Radiators that would do it

  • Type 22, 700mm high, 900mm long (1,765W at ΔT50)
  • Type 22, 600mm high, 1000mm long (1,732W at ΔT50)
  • Type 21, 700mm high, 1100mm long (1,683W at ΔT50)
  • Type 21, 600mm high, 1400mm long (1,883W at ΔT50)
  • Type 22, 450mm high, 1400mm long (1,919W at ΔT50)
  • Type 21, 450mm high, 1600mm long (1,688W at ΔT50)

Sizes from Stelrad Compact outputs. Makers vary by about 10%: check the datasheet. Lengths run 400 to 2000mm.

Radiator schedule

Temperatures, U-values and radiator outputs checked 18 Sept 2026. An estimate, not a heat loss survey.

How to calculate what size radiator you need

  • Work out how much heat the room loses on a cold day, in watts: through the walls, windows, ceiling and floor, and through air leaking in and out.
  • Add a margin, usually 10%, for warm-up and very cold spells.
  • Adjust for your heating system. Radiators are rated at ΔT50, the temperature a gas boiler runs at. Cooler systems get less out of the same radiator.
  • Pick a radiator rated at least that much at ΔT50, in a size that fits the wall.

A worked example. A living room 5m by 4m with a 2.4m ceiling, in a 1930s house with unfilled cavity walls, two outside walls, 3m² of older double glazing, a heated room above and a suspended timber floor. On a -3°C day it loses about 1,472W. With a 10% margin it needs 1,619W.

On a gas boiler at 75/65 that means a radiator rated 1,662W at ΔT50, such as a type 22, 600mm high and 1000mm long (1,732W). On a heat pump at 45/40 it means 4,851W at ΔT50, which is more than one standard radiator: two type 22s at 700 × 1400mm, for example.

How radiator BTU is calculated

Heat lost through each surface is its U-value (how easily heat passes through it) times its area times the temperature difference across it. Heat lost to air is 0.33 × air changes per hour × the room's volume × the temperature difference. Add them up for the room.

That gives watts. BTU per hour is watts × 3.412. So a room needing 1,500W needs 5,118 BTU.

Most radiator retailers' calculators ask the same questions but hide the working. The breakdown under the result shows every line.

The outside temperature is the cold day the room is designed for. The default is -3°C; Detailed settings has the 14 MCS reference locations, from Plymouth, the mildest, to Glasgow, the coldest.

What is ΔT50, and why it matters

ΔT50 means the water in the radiator is, on average, 50°C warmer than the room: flow at 75°C and return at 65°C in a 20°C room. Radiators are rated at ΔT50 under BS EN 442. Run them cooler and they give out less, by (ΔT ÷ 50) to the power of about 1.3.

So a radiator rated 1,500W at ΔT50 gives about 739W on a system running 55/45, and about 501W at 45/40, in a 21°C room.

Radiator sizes for heat pumps

A heat pump is most efficient at low flow temperatures, so the radiators have to be bigger to put the same heat into the room. This table shows what share of its ΔT50 rating a radiator gives at each flow and return, in a 21°C room.

Radiator output as a share of its ΔT50 rating (room at 21°C)
Flow and returnOutputRadiator rating needed
75/65°C97%1 × the room's heat loss
55/45°C49%2 × the room's heat loss
50/40°C39%2.6 × the room's heat loss
45/40°C33%3 × the room's heat loss
40/35°C24%4.2 × the room's heat loss

Most existing radiators need to be two to three times bigger for a heat pump. Swapping a type 11 or 21 for a type 22 of the same size often gets part of the way without moving pipework.

Radiator size chart

Output at ΔT50 for standard panel radiators, by type, height and length. Type 11 is a single panel with one set of fins, type 21 two panels with one set, type 22 two panels with two sets.

Radiator output in watts at ΔT50 (Stelrad Compact)
Type and height600mm800mm1000mm1200mm1400mm1600mm1800mm2000mm
Type 11, 300mm3054075096117138149161,018
Type 11, 450mm4546057569071,0581,2101,3611,512
Type 11, 600mm5887849801,1761,3721,5681,7641,960
Type 11, 700mm6708941,1171,3401,5641,7872,0112,234
Type 21, 300mm4475967458941,0431,1921,3411,490
Type 21, 450mm6338441,0551,2661,4771,6881,8992,110
Type 21, 600mm8071,0761,3451,6141,8832,1522,4212,690
Type 21, 700mm9181,2241,5301,8362,1422,4482,7543,060
Type 22, 300mm5897869821,1781,3751,5711,7681,964
Type 22, 450mm8231,0971,3711,6451,9192,1942,4682,742
Type 22, 600mm1,0391,3861,7322,0782,4252,7713,1183,464
Type 22, 700mm1,1771,5691,9612,3532,7453,1383,5303,922

For BTU, multiply the watts by 3.412. Makers vary by about 10%, so check the datasheet for the radiator you buy.

What is a good BTU for a radiator?

There is no single good figure: it is whatever the room needs. As a rough guide, 2,000 BTU (586W) suits a small well-insulated box room, 5,000 BTU (1,465W) an average bedroom, 7,000 BTU (2,052W) a typical living room, and 10,000 BTU (2,931W) a large or poorly insulated one. Work out your room rather than going by the guide.

Is it better to oversize radiators?

Within reason, yes. A bigger radiator can heat the room at a lower water temperature, which lets a condensing boiler condense more and a heat pump run more efficiently. It is the boiler you should not oversize, not the radiators.

Is it better to have one large radiator or two small?

Two spread the heat better in a long room, or where there are two windows. One is cheaper to buy and fit. Split the room's figure between them: two radiators each rated at half the total do the same job.

Electric radiator sizing

An electric radiator is rated at its power, so watts is watts: there is no ΔT to adjust for. Size it to the room's heat loss plus the margin. Choose "Electric radiator" as the heating system.

Common mistakes when sizing radiators

  • Sizing to the wall space instead of to the room's heat loss.
  • Reading a ΔT60 or ΔT70 rating as if it were ΔT50. Higher ΔT ratings make a radiator look bigger than it is.
  • Forgetting the flow temperature: the same radiator gives about a third as much on a heat pump.
  • No margin for the coldest days.
  • Treating the whole house total as the boiler size, without hot water and pipe losses.

Frequently asked questions

How do I calculate the BTU of my radiator?

Look up its type, height and length, then read its ΔT50 output from the maker's table and multiply watts by 3.412. A type 21 at 600 × 1200mm is about 1,614W (5,507 BTU). Use "Check my radiator" above to see what it gives on your system.

What is a good BTU for radiators?

Whatever the room needs. As a rough guide, about 5,000 BTU (1,465W) for an average bedroom and 7,000 BTU (2,052W) for a typical living room. Older, draughtier and bigger rooms need more. Work out your own room with the calculator.

What room size would a 2000 BTU radiator be suitable for?

2,000 BTU is about 586W, enough for a small, well-insulated box room or a small bathroom in a modern house on a gas boiler. In an older house, or on a heat pump, the same radiator heats much less.

Is a 7000 BTU radiator good?

7,000 BTU is about 2,052W, a typical figure for a living room in a house with some insulation, on a gas boiler at ΔT50. Whether it is enough depends on your room's size, walls and windows, and on your heating system.

Is it better to oversize radiators?

Within reason, yes. A larger radiator heats the room at a lower water temperature, which suits condensing boilers and heat pumps. Oversizing the boiler is the mistake to avoid, not oversizing the radiators.

Which radiator has the highest BTU?

Among standard panel radiators, a type 22 (double panel, double fins) at the tallest and longest size. A type 22 at 700 × 2000mm gives about 3,922W (13,382 BTU) at ΔT50. Triple panel (type 33) and column radiators can go higher.

How to calculate what size radiator you need?

Work out the room's heat loss in watts, add a 10% margin, divide by your system's output factor to get the ΔT50 rating, then choose a radiator rated at least that much. The calculator does each step and lists sizes that fit.

What are common mistakes when sizing radiators?

Sizing to the wall space rather than the heat loss, reading a ΔT60 rating as ΔT50, ignoring the flow temperature, leaving no margin, and using the whole-house total as the boiler size without hot water.

How many radiators should a 3 bedroom house have?

Usually one in each heated room, including the hall, landing and bathroom, and sometimes two in a large living room. Size each one to its room: use "Add this room" to build a schedule for the whole house.

How much water does a 600mm radiator hold?

It depends on the type and the length. The maker's datasheet lists the water content for each size. A double panel holds roughly twice what a single panel of the same size holds, and the content rises with the length.

Is it better to have one large radiator or two small?

Two spread heat more evenly in a long room or one with two windows; one is cheaper to fit. Two radiators each rated at half the room's figure do the same job as one.

Does the calculator work in feet?

Yes. Switch the room size to ft/in and type it as you would say it, such as 16ft 5in. The result still comes out in watts and BTU.

What flow temperature should I use?

Use what your system runs at. A gas boiler on a traditional setting runs about 75/65, condensing boilers can run lower, and heat pumps usually run between 35 and 55°C. If you are unsure, the installer or the boiler settings will say.

Why is my result different from Stelrad's or B&Q's?

Retailer calculators use their own factors and do not show them. This one shows every line, from the U-values to the ventilation, so you can see why, and change any of them under Detailed settings.

Sources

An estimate for choosing radiators. A heat pump installation needs a full room-by-room heat loss survey to MCS standards, and this is not one.

By Janusz Wozniak, Founder & Lead Developer

Built so a heating engineer can size a room, or a whole house, on a phone at the customer's table, with the working shown.

Last updated .

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