♨️What size radiator does this room need?

✓ Tested & verified Updated: Aug 9, 2026 How we calculate this

Radiator size calculator

Room length (ft)

Room width (ft)

Ceiling height (ft)

What the room is

Insulation

6,000 BTU/h≈ 1.8 kW

The room works out at 5,779 BTU/h — 6,000 is the nearest size actually sold.

Floor area: 168 sq ft
Room volume: 1,344 cu ft · 38.1 m³
Working rate: 4.3 BTU/h per cu ft

Radiator boxes quote output at delta-T 50. A modern condensing boiler runs cooler than that, so the same radiator gives roughly 20 % less — size up, or run the flow hotter and lose the condensing efficiency.

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What size radiator does a room need?

Work it out from the room’s volume, not its floor area. A radiator heats air, and the amount of air is length × width × ceiling height. The working figure is about 4.3 BTU per cubic foot for a living room held at 21 °C — so a 14 × 12 ft room with an 8 ft ceiling needs roughly 5,800 BTU/h, or 1,700 watts.

What the room is for changes the target temperature and so changes the answer. A bedroom is held at 18 °C rather than 21 °C and needs about 14% less; a hall at 16 °C needs nearly 30% less; a bathroom held at 22 °C needs about 9% more. Insulation moves it again — a draughty single-glazed room needs around 15% more than the same room after new windows.

The catch is the number printed on the radiator. Manufacturers quote output at delta-T 50, meaning the average radiator surface sits 50 °C above the room. A condensing boiler set up properly runs a lower flow temperature, which puts it nearer delta-T 40 — and the same radiator then gives about 80% of its printed output. Size on the honest number, not the box.

Two tables, and the second is the one people find out about after the radiator is on the wall.

Room	Held at	Multiplier
Living room, dining room	21 °C	1.00
Bathroom	22 °C	1.09
Bedroom	18 °C	0.86
Kitchen	18 °C	0.78
Hall, landing	16 °C	0.71

The kitchen sits below the bedroom at the same temperature because appliances and cooking put heat back into the room, and a kitchen with an oven in it is rarely short of warmth.

Flow temperature	Delta-T	Output you actually get
75 °C, traditional boiler	50	100% of the printed figure
65 °C	40	about 80%
55 °C, condensing sweet spot	30	about 60%
45 °C, heat pump	25	about 50%

A radiator that was correct on a 75 °C boiler is half a radiator on a heat pump. That is not a fault in the radiator; it is the same metal moving less heat because there is less temperature difference to move it with.

Frequently asked questions

How many BTU do I need per square foot for a radiator?

Roughly 34 BTU/h per square foot with a standard 8 ft ceiling, but that figure hides the thing that actually varies. Radiators heat air by volume, so the same floor area under a 10 ft Victorian ceiling needs a quarter more output. Work in cubic feet and the number stops moving.

What does delta-T 50 mean on a radiator?

It is the test condition for the printed output: the average water temperature in the radiator is 50 °C above the room temperature — typically 75 °C flow, 65 °C return, 20 °C room. Run the boiler cooler and the output drops: at delta-T 40 you get about 80% of the figure, at delta-T 30 about 60%.

Should I size up if I have a heat pump?

Yes, substantially. A heat pump works best at flow temperatures around 45 °C, which is roughly delta-T 25 — about half the printed output. That is why heat pump retrofits usually replace radiators with larger ones or add fan-assisted units, not because the old ones were wrong but because they were sized for a hotter boiler.

Is one big radiator better than two small ones?

Two are usually better in a room longer than about five metres or with a lot of glass. Output is the same either way, but a single radiator heats the air near it and leaves the far end cool, and a cold outside wall pulls heat out wherever it can. Put output where the heat is being lost — under the window is the traditional answer for exactly that reason.

Does a radiator behind a sofa still work?

Partly. A radiator gives out heat both by radiation and by convection, and a sofa in front of it blocks the radiant half and slows the convection. Expect to lose a noticeable fraction — enough that a room sized correctly on paper feels cold. Leave 15 cm clear and do not put a shelf directly on top.

Do I need to allow extra for a room with big windows?

Yes. The insulation setting in the calculator covers a typical modern room; a room that is mostly glass, or single glazed, loses heat far faster than the wall area suggests. As a rule of thumb, add 10% for a room where glazing is more than a quarter of the outside wall, and pick the poor insulation setting if the glass is single.

How we calculate this

Output is calculated from room volume rather than floor area: volume in cubic feet × 4.3 BTU/h per cubic foot for a living space held at 21 °C. That rate is the imperial equivalent of the widely used metric figure of about 153 watts per cubic metre for a living room. Room-type multipliers adjust for the target temperature normally used for each room — 18 °C for bedrooms and kitchens, 16 °C for halls and landings, 22 °C for bathrooms — expressed relative to the 21 °C living room baseline. The insulation multiplier is the same one used across this site: +15% for a draughty or single-glazed room, −10% for a new build or fully retrofitted one. The result is rounded up to the nearest output radiators are commonly sold in. All printed radiator outputs, and therefore this calculation, assume delta-T 50; the 80% figure quoted for delta-T 40 follows the standard conversion factors published with radiator output tables.

Worked example

With Room length (ft) 14, Room width (ft) 12, Ceiling height (ft) 8, What the room is Living room or dining room — 21 °C, Insulation Average — typical home, this page works out 6,000 BTU/h ≈ 1.8 kW. The room works out at 5,779 BTU/h — 6,000 is the nearest size actually sold.

Floor area168 sq ft
Room volume1,344 cu ft · 38.1 m³
Working rate4.3 BTU/h per cu ft

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