What Size Radiator for a 3 × 3 m Room?

A 3 × 3 m living room with a 2.4 m ceiling, double glazing and one external wall needs a radiator giving about 1,188 W (4,053 BTU/h) at ΔT50. As a bedroom, heated to 18°C, it needs 1,039 W. On a heat pump, look for about 2,308 W at ΔT50.

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Example job shown. Change the numbers to match yours.

Heat the room needs

1,188 W

1.19 kW, for a room of 21.6 m³

In BTU per hourRadiators are often sold by BTU: 4,053 BTU/h
Radiator output to look for (boiler, ΔT50)The output shown at ΔT50, for a flow of about 75°C. Choose the next size up: 1,188 W
Same room on a low-temperature heat pump (ΔT30)Output as shown at ΔT50 on the radiator label, since it gives only about 51% of that at ΔT30: 2,308 W
Heat per cubic metre: 55.0 W/m³
Room volume: 21.6 m³

This is a quick estimate from the volume, with adjustments for the room temperature you want, the glazing, the number of external walls and the insulation. A room-by-room heat loss calculation (as an installer or a heat pump designer does) is more accurate, especially for a heat pump. Radiators are rated at ΔT50 (the radiator 50°C above the room); at a lower flow temperature the output falls steeply, so heat pump systems need larger radiators.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Radiator size for a 3 × 3 m room, by type of room

A 3 × 3 m room is a single bedroom, a study or a small dining room. The floor area is 9 m² and, with a 2.4 m ceiling, the volume is 21.6 m³. These figures assume double glazing, one external wall and filled cavity walls. The last column is the ΔT50 rating to look for if a heat pump supplies the radiators.

Room	Heat needed	BTU	Rating for a heat pump (ΔT50)
Living room (21°C)	1,188 W	4,053 BTU/h	2,308 W
Bedroom (18°C)	1,039 W	3,547 BTU/h	2,019 W
Kitchen (18°C)	1,039 W	3,547 BTU/h	2,019 W
Hall or landing (16°C)	940 W	3,209 BTU/h	1,827 W
Bathroom (22°C)	1,237 W	4,222 BTU/h	2,404 W

External walls and insulation

Every outside wall loses heat, and solid walls lose more than insulated ones. Living room at 21°C, double glazing, 2.4 m ceiling.

External walls	Solid walls, not insulated	Cavity walls, filled	Well insulated
None (an inside room)	1,212 W	1,010 W	858 W
One	1,426 W	1,188 W	1,010 W
Two (a corner room)	1,639 W	1,366 W	1,161 W
Three or more	1,782 W	1,485 W	1,262 W

Taller ceilings

A taller room holds more air to heat. Living room, double glazing, one external wall.

Ceiling height	Room volume	Heat needed	BTU
2.4 m	21.6 m³	1,188 W	4,053 BTU/h
2.7 m	24.3 m³	1,337 W	4,560 BTU/h
3.0 m	27.0 m³	1,485 W	5,067 BTU/h

One radiator or two?

1,188 W is comfortably within the range of a single double-panel radiator, usually placed under the window, where it counters the cold air falling from the glass. Pick the next size up from the figure, at ΔT50.

These are quick estimates from the room volume. For a heat pump, or a room with large windows, ask an installer for a room-by-room heat loss calculation. The radiator size guide explains ΔT ratings, and the boiler size calculator sizes the whole system.

Want to change anything else? The calculator above is the full Radiator Size Calculator (UK, Watts and BTU): edit any number and the results update instantly.

Tools & materials for this project

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Radiator valve (thermostatic)Radiator bleed key Reflective radiator panels Draught excluder strip Room thermometer

Frequently asked questions

What size radiator do I need for a 3 × 3 m room?

About 1,188 W (4,053 BTU/h) for a living room with double glazing, one external wall, cavity wall insulation and a 2.4 m ceiling. The same room as a bedroom, heated to 18°C, needs 1,039 W.

How many BTU to heat a 3 × 3 m room?

4,053 BTU/h for a living room, or 3,547 BTU/h for a bedroom. A cold corner room with solid walls and single glazing needs 6,713 BTU/h.

What radiator size for a 3 × 3 m room with a heat pump?

A heat pump runs at a lower flow temperature, so the radiators give less heat. Choose a radiator rated about 2,308 W at ΔT50 for a living room this size (it gives about 1,188 W at ΔT30), or 2,019 W for a bedroom.

Related sizes and guides

Related sizes

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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