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Radiator size calculator

Work out the radiator size you need for a room, in BTU/h and watts. We work out the room's heat loss, then correct it for how hot your water runs, so you can match the result to the output figure in any radiator catalogue.

Fitter connecting copper pipes to a white panel radiator on a wall

Sets the temperature we aim for and how often the air changes.

2.4 m is typical.

Count walls that face outside, in order: length, width, length, width. We use the length walls first.

windows

A typical casement is about 1.2 m² (1,190 × 1,040 mm). Measure the glass and frame.

The coldest day we plan for (the outside design temperature).

Flow and return are the water temperatures leaving and returning to the boiler or heat pump.

radiators

We split the output equally between them.

Your result

Radiator output needed (BTU/h at ΔT 50)

6,950 BTU/h

Match this to the "BTU" figure in a radiator catalogue. Pick the next size up.

Radiator output needed (watts at ΔT 50)
2,040 WCatalogue watts, rated at ΔT 50
Room heat loss
1,471 W5,000 BTU/h. This is what the room needs once the radiator is running at your water temperature.
Output factor at your water temperature
0.724ΔT 39 K (mean water 60 °C − room 21 °C)
Radiator ΔT
39 K
Walls loss
605 W41% of the total
Windows loss
115 W8% of the total
Roof or ceiling loss
0 W0% of the total
Floor loss
302 W21% of the total
Air changes (ventilation) loss
449 W31% of the total
  • This is a guide, not a full heat-loss survey. A heating engineer should do a proper room-by-room survey before you fit a new system. We ignore doors, chimneys, thermal bridges and heat gained from neighbouring rooms.
  • Compare like with like: check a radiator's output is quoted at ΔT 50. Some catalogues also list lower ΔT figures, which are smaller.

Add this to your project shopping list, then print it or take it to the merchant.

Open shopping list

How to use the radiator size calculator

  1. Pick the Room type, then type in the Room length, Room width and Ceiling height.
  2. Fill in the outside walls, External wall type, windows, Glazing, and what is above and below the room. Add Where are you? and How draughty is the house? to finish the room details.
  3. Choose your Heating system: older boiler (75/65), modern condensing boiler (70/50), heat pump (50/40), low-temperature heat pump (45/35), or My own temperatures if you know your flow and return.
  4. If more than one radiator will heat the room, set Number of radiators in the room. We split the output equally between them.
  5. Read the Radiator output needed, in BTU/h and watts at ΔT 50. This is the figure to match to the catalogue. Pick the next size up.
  6. Tap Add to shopping list to keep it, or Share link to send it to your installer.

Why use this calculator

It corrects for your water temperature. A catalogue radiator is rated at one test condition. If your boiler or heat pump runs cooler, the radiator gives out less heat. This calculator works out how much bigger you need to go, so you don't end up with a room that stays cold.

It starts from your room, not a guess. It uses the same room heat-loss method as our BTU calculator, so walls, windows, roof, floor and draughts all count.

It is neutral. It gives you the output to look for, not a product to buy. You can use the figure to compare radiators from any shop or maker.

It shows the working, including the correction factor, so you can check it or hand it to your installer.

How we worked it out

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

  1. Room: 4.5 m × 3.5 m × 2.4 m high = 37.8 m³ (floor 15.75 m²).
  2. 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.
  3. 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.
  4. Walls: U 1.5 W/m²K (cavity wall, not insulated) × 16.8 m² × 24 K = 605 W.
  5. Windows: U 2 W/m²K (double glazing, 2002 to 2021) × 2.4 m² × 24 K = 115 W.
  6. Ceiling: the room above is heated, so we count 0 W.
  7. Floor: U 0.8 W/m²K (solid ground floor) × 15.75 m² × 24 K = 302 W.
  8. Air: 0.33 × 1.5 air changes per hour × 37.8 m³ × 24 K = 449 W.
  9. Total heat loss = 605 + 115 + 0 + 302 + 449 = 1,471 W (1.47 kW).
  10. Convert to BTU/h: 1,471 W × 3.412 = 5,021 BTU/h, rounded to the nearest 50 = 5,000 BTU/h.
  11. Water temperature: (flow 70 + return 50) ÷ 2 = 60 °C on average.
  12. Radiator ΔT = average water temperature − room temperature = 60 − 21 = 39 K.
  13. Catalogue outputs are quoted at ΔT 50. At ΔT 39 a radiator gives 0.724 of its catalogue output (interpolated from the standard table; based on output ∝ ΔT^1.3).
  14. Catalogue (ΔT 50) output needed = 1,471 W ÷ 0.724 = 2,032 W, which is 6,933 BTU/h. Rounded up: 2,040 W or 6,950 BTU/h.

The formula

First we work out the room’s heat loss in watts, using the same method as the BTU calculator: fabric loss (U-value × area × temperature difference for walls, windows, ceiling and floor) plus air loss (0.33 × air changes per hour × volume × temperature difference).

Then we correct it for the radiator’s water temperature:

  1. Mean water temperature = (flow + return) ÷ 2.
  2. Radiator ΔT = mean water temperature − room temperature.
  3. Output factor for that ΔT, from the table below.
  4. Catalogue output needed = room heat loss ÷ output factor.
  5. Round up to the next 10 W, and the next 50 BTU/h (watts × 3.412).

What is ΔT50, and why does the factor matter?

Radiators in catalogues are rated at ΔT 50. That is the standard test condition: the average water temperature 50 °C above the room. A radiator running cooler gives out less. The output goes up and down with ΔT to the power 1.3, and manufacturers publish the result as a table. We use the same table and fill the gaps in between.

ΔT (K) 20 25 30 35 40 45 50 55 60
Output factor 0.304 0.406 0.515 0.629 0.748 0.872 1.000 1.132 1.267

At ΔT 30 a radiator gives about half its catalogue output, so you need one about twice the size.

What the systems mean for a 21 °C living room

Heating system Flow / return ΔT Output factor
Older boiler 75 / 65 49 K 0.974
Modern condensing boiler 70 / 50 39 K 0.724
Heat pump 50 / 40 24 K 0.386
Low-temperature heat pump 45 / 35 19 K 0.285

For our default living room (about 1,470 W of heat loss), that means a catalogue radiator of about 5,200 BTU/h with an older boiler, 6,950 BTU/h with a condensing boiler, 13,050 BTU/h with a 50/40 heat pump and 17,650 BTU/h with a 45/35 heat pump. A bedroom is kept at 18 °C, so the same system has a bigger ΔT there.

What we assume

  • Room temperatures, outside design temperatures, air changes and U-values are the same as the BTU calculator.
  • The output factor follows the manufacturer table above, and above ΔT 75 we extend it with the same formula (ΔT to the power 1.3).
  • With more than one radiator, we split the output equally between them.
  • This is a guide. Check with your installer or a Gas Safe engineer before changing a heating system. A full heat-loss survey is the proper way to size a new system.

Tips

  • Check the catalogue figure is rated at ΔT 50. Some catalogues also list lower ΔT figures, which are smaller numbers, and comparing the wrong ones makes a radiator look too small or too big.
  • Round up to the next size, but don't go much bigger. A radiator that is a little big is fine. A radiator that is too small never warms the room.
  • If you are fitting a heat pump, expect bigger radiators in most rooms, or underfloor heating in some. A heat pump installer should design the whole system, so use this to check what they propose.
  • Two smaller radiators can suit a big room better than one large one, for example under two windows. Set the number of radiators to see the size of each.
  • Don't change boiler flow temperatures yourself. Ask a Gas Safe engineer to set them.

FAQs

What size radiator do I need for my room?
Our default living room (4.5 m by 3.5 m, two outside walls) loses about 1,470 W. With a modern condensing boiler at 70/50, you need a radiator rated at about 6,950 BTU/h (2,040 W) at ΔT 50. Enter your own room to get your number.
How do I convert radiator BTU to watts?
Divide BTU/h by 3.412 to get watts. A 6,950 BTU/h radiator gives about 2,040 W. Going the other way, multiply watts by 3.412.
What is ΔT50?
ΔT is the average water temperature in the radiator minus the room temperature, and catalogues rate radiators at ΔT 50 (for example flow 80 °C, return 60 °C, room 20 °C). If your water runs cooler, the radiator gives out less than its catalogue figure.
Why do heat pumps and condensing boilers need bigger radiators?
They run the water cooler, so the radiator is not as much hotter than the room. In a 21 °C room, a radiator on a 70/50 condensing boiler gives 72% of its catalogue output, on a 50/40 heat pump 39%, and on a 45/35 heat pump about 29%. So the radiator has to be that much bigger to give the same heat.
Should I go for a bigger radiator than the calculator says?
Go up to the next size, not further. We already add the correction for your water temperature and round up to a size you can find in a catalogue. A much bigger radiator costs more and takes more wall.
How many BTU is a double panel radiator?
There is no single figure, because it depends on the height and length and the maker. Look up the output at ΔT 50 in the catalogue and match it to the figure from this calculator.
Do I need a bigger radiator for a conservatory or a room with lots of windows?
A room with lots of glass loses more heat, and the calculator includes this through the number and size of windows and the glazing type (single glazing loses 4.8 W/m²K, modern double glazing 1.4). A conservatory is a special case, so ask a heating engineer.

Last reviewed: 6 November 2026 · How we calculate · Report an error