What is a radiator BTU calculator?

It tells you how much heat output (measured in BTU per hour, or watts) a room needs to reach a comfortable temperature. Size the radiator to the BTU figure — not the other way around.

Does this calculator use Δt 50?

Yes. All manufacturer output figures assume a mean water temperature 50 °C above room air (Δt 50). If you are sizing for a heat pump flowing at 45-50 °C, derate the radiator output by roughly 17% (Δt 40) or 30% (Δt 30).

Why does insulation matter so much?

A poorly insulated room loses heat faster than it can be replaced. Our poor/average/good multipliers (1.25 / 1.00 / 0.85) approximate the SAP insulation bands. If in doubt, assume average.

Why does a corner room need more output?

External walls lose more heat than internal walls, and windows lose more still. We apply a multiplier from 0.92 (fully internal) up to 1.20 (three external walls) to reflect the fabric loss.

Bathrooms come out higher than I expect — why?

Bathrooms typically need to be 5 °C warmer than other rooms (around 22 °C), so the output factor is the highest in the table. Towel rails alone rarely provide enough — check the BTU figure against the towel rail spec.

Can I use this figure for heat loss reports?

No. This is a quick per-room sizing estimate. A full MCS heat loss calculation models U-values, air changes, thermal bridging and design temperatures room by room. Use our heat loss tool for that.

Is the calculation stored anywhere?

No. The maths runs entirely in your browser. Nothing is sent to our server unless you choose to email yourself the result.

What if the required output is bigger than any single radiator?

We'll suggest splitting the load across multiple radiators. In large rooms, two properly-sized radiators on opposite walls also give better temperature distribution than one oversized unit.

Free calculator for engineers

Radiator BTU Calculator

The right output, every room. Dimensions, insulation, glazing — we give you BTU, watts, and a short list of suitable radiators from Type 11 through Type 33.

Length (m)

Width (m)

Height (m)

Room type

Insulation

External walls

Double glazingNorth-facing windowHeated room aboveHeated room below

Required output

4,913BTU

(1,440 W · 28.8 m³ volume)

Suitable radiators

Type 22 double 600 × 1000
1,830 W · 6,244 BTU(+27%)
Type 22 double 600 × 1400
2,565 W · 8,752 BTU(+78%)
Type 22 double 600 × 1800
3,295 W · 11,243 BTU(+129%)

Outputs based on Δt 50 °C. Derate by ~17% for Δt 40 °C (heat pump flow) or ~30% for Δt 30 °C.

How it works

1
Measure the room
Length, width and ceiling height in metres. Tape-measure accuracy is fine.
2
Describe the fabric
Room type, insulation level, number of external walls, glazing and whether neighbouring rooms are heated.
3
Read the required output
The tool gives you BTU and watts instantly, plus suitable Type-11, 21, 22 and 33 radiator sizes.

How we calculate the output

The starting point is the room volume (length × width × height) multiplied by a room-type factor in watts per cubic metre. Living rooms, dining rooms and bathrooms need more heat per m³ than bedrooms, kitchens or hallways because of comfort set-points and dwell time.

From there we apply four correction factors:

Insulation. Poor insulation pushes the output up by 25%; good insulation brings it down by 15%.
External walls. A fully internal room gets a small discount; a three-external-wall corner room gets a 20% penalty.
Glazing. Single glazing adds 15%. North-facing adds a further 10%.
Heated neighbours. Heated rooms above and/or below each knock 5% off the load.

The result is a Δt 50 design load. If the room is served by a heat pump flowing at lower temperatures, derate the radiator output by 17% for Δt 40 or 30% for Δt 30. In other words: a radiator rated 1,830 W at Δt 50 gives roughly 1,520 W at Δt 40 and 1,280 W at Δt 30.

Common sizing mistakes

Sizing for the boiler, not the room. A radiator needs to replace the room's heat loss. Boiler kW is a totally separate calculation — see our boiler size calculator.

Using towel rail output for the whole bathroom. Towel rails often provide less than half the required output on their own. Add a compact radiator below the window where possible.

Oversizing "just in case". Radiators much larger than the load cycle on and off aggressively, especially on modulating combi boilers — the room overheats, the flow temperature doesn't stabilise, and condensing mode is lost. Size for the load, not for peace of mind.

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Frequently asked questions

Heat loss calculator →

Whole-house heat loss for heat pump and boiler sizing conversations.

Boiler size calculator →

Work out the right kW output for a replacement combi or system boiler.