What size radiator do I need? (BTU and watts by room size and type, UK)
A 4 x 4 m living room with a 2.4 m ceiling and one external wall needs a radiator of 7,500 BTU/h (2,186 W) on the UK merchants' rule (room volume in ft³ x a factor for the room type, plus percentage adjustments); the same room as a bedroom is 6,000 BTU/h (1,749 W) and as a bathroom 9,000 BTU/h (2,623 W). Radiators are sold by watts and BTU/h at the same time, and 1 W is 3.412 BTU/h. The tables below give five room sizes by room type, what each adjustment adds, and the flat rule some retailers use beside it.
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1. Radiator BTU and watts by room size and room type
The rule (Turnbull & Co, BTU Calculator for Radiators, rule of thumb): length x width x height in feet, times a factor for the room type, 3 for a kitchen, 4 for a bedroom, 5 for a living room, dining room or office, 5 for a hallway, 6 for a bathroom, 6.5 for a conservatory, plus the percentage adjustments. Every room here has a 2.4 m ceiling, one external wall (+10 %) and double glazing, the calculator's starting room. Watts = BTU/h / 3.412.
| Room (m) | Volume (m³) | Bedroom | Living room | Kitchen | Bathroom |
|---|---|---|---|---|---|
| 3 x 3 | 21.6 | 3,400 BTU/h (984 W) | 4,200 BTU/h (1,230 W) | 2,500 BTU/h (738 W) | 5,000 BTU/h (1,475 W) |
| 3 x 4 | 28.8 | 4,500 BTU/h (1,312 W) | 5,600 BTU/h (1,639 W) | 3,400 BTU/h (984 W) | 6,700 BTU/h (1,967 W) |
| 4 x 4 | 38.4 | 6,000 BTU/h (1,749 W) | 7,500 BTU/h (2,186 W) | 4,500 BTU/h (1,312 W) | 9,000 BTU/h (2,623 W) |
| 4 x 5 | 48 | 7,500 BTU/h (2,186 W) | 9,300 BTU/h (2,732 W) | 5,600 BTU/h (1,639 W) | 11,200 BTU/h (3,279 W) |
| 5 x 6 | 72 | 11,200 BTU/h (3,279 W) | 14,000 BTU/h (4,098 W) | 8,400 BTU/h (2,459 W) | 16,800 BTU/h (4,918 W) |
A 3 x 3 m box room needs 3,400 BTU/h (984 W) as a bedroom; a 5 x 6 m living room, 14,000 BTU/h (4,098 W). The boiler size calculator takes any room, in metres, with its windows, walls and insulation.
2. What the adjustments add
The 4 x 4 x 2.4 m living room (1,356 ft³ x 5 = 6,780 BTU/h before adjustments) with each adjustment on its own, then the worst case. The percentages add up and are applied once (Turnbull & Co, BTU Calculator for Radiators, feature adjustments); double glazing adds nothing.
| Case | Adjustment | BTU/h | Watts |
|---|---|---|---|
| No adjustments (internal room, double glazed) | +0 % | 6,780 | 1,987 |
| One external wall | +10 % | 7,458 | 2,186 |
| Two external walls | +20 % | 8,136 | 2,385 |
| North-facing | +10 % | 7,458 | 2,186 |
| One single-glazed window | +10 % | 7,458 | 2,186 |
| Two single-glazed windows | +20 % | 8,136 | 2,385 |
| Poor insulation | +15 % | 7,797 | 2,285 |
| French doors or a large glass area | +15 % | 7,797 | 2,285 |
| Worst case: two external walls, north-facing, two single-glazed windows, poor insulation | +65 % | 11,188 | 3,279 |
Poor insulation (+15 %) and French doors (+15 %) are the largest single items; a north-facing room with two external walls and two single-glazed windows in an uninsulated house is 11,200 BTU/h (3,279 W), 1.65 times the internal, double-glazed room.
3. The merchants' rule against the flat x4 rule
A second published rule of thumb multiplies the room's ft³ by 4 whatever the room is used for (BestHeating, BTU Calculator, a second rule of thumb). The calculator shows both figures for the same room; here they are for the five room sizes as living rooms with one external wall.
| Room (m) | ft³ | Factor 5 + wall | Flat x4 | Difference |
|---|---|---|---|---|
| 3 x 3 | 763 | 4,195 | 3,051 | +1,144 |
| 3 x 4 | 1,017 | 5,594 | 4,068 | +1,526 |
| 4 x 4 | 1,356 | 7,458 | 5,424 | +2,034 |
| 4 x 5 | 1,695 | 9,323 | 6,780 | +2,543 |
| 5 x 6 | 2,543 | 13,985 | 10,171 | +3,814 |
For a living room with one external wall the merchants' rule is 1.38 times the flat rule; for a bedroom the two are the same factor (4) before the wall is added. Neither is a heat loss calculation: a room-by-room heat loss survey by the installer is the figure to buy on, and the boiler size guide has the combi and system kW that sit above the radiators.
Frequently asked questions
- What size radiator do I need for a 4 x 4 m room?
- 7,500 BTU/h (2,186 W) as a living room with one external wall and double glazing: 1,356 ft³ x factor 5 = 6,780 BTU/h, plus 10 % for the wall (Turnbull & Co, BTU Calculator for Radiators, rule of thumb). As a bedroom (factor 4) it is 6,000 BTU/h (1,749 W); as a kitchen (factor 3), 4,500 BTU/h (1,312 W).
- How many BTU does a radiator need for a 12 x 12 ft bedroom?
- 5,100 BTU/h (1,486 W) with an 8 ft ceiling and one external wall: 1,152 ft³ x factor 4 = 4,608 BTU/h, plus 10 %. The boiler calculator takes the room in metres; 12 ft is 3.66 m.
- How do I convert radiator BTU to watts?
- Divide BTU/h by 3.412 (Turnbull & Co, BTU Calculator for Radiators, feature adjustments): 7,458 BTU/h is 2,186 W. Radiator listings quote both; the figure here is the room's requirement, and the radiator's rated output should meet or exceed it.
- What do north-facing, single glazing and poor insulation add?
- 10 % for a north-facing room, 10 % per single-glazed window, 10 % per external wall, 15 % for poor insulation and 15 % for French doors or a large glass area, all added together before multiplying (Turnbull & Co, BTU Calculator for Radiators, feature adjustments). On the 4 x 4 m living room that runs from 6,800 BTU/h (1,987 W) with none of them to 11,200 BTU/h (3,279 W) with two external walls, north-facing, two single-glazed windows and poor insulation (+65 %). Table 2 has each on its own.
- Why do two calculators give different radiator sizes?
- Because there are two published rules of thumb. The merchants' rule uses a factor by room type (3 for a kitchen, 4 for a bedroom, 5 for a living room, 6 for a bathroom) plus adjustments; a second rule multiplies the ft³ by 4 for every room (BestHeating, BTU Calculator, a second rule of thumb). For the 4 x 4 m living room that is 7,500 BTU/h against 5,400 BTU/h. The calculator shows both; table 3 has them side by side.
- Do I add up the radiators to size the boiler?
- Not on the UK installers' tables: a combi is picked by bedrooms and bathrooms (24-27 kW to 35-42 kW), and the number of radiators is a cross-check, 24 kW for up to 10, 28 kW for 10 to 15, 32 kW for 15 to 20 (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). The boiler guide has the tables; the sum of the radiator outputs in kW is the heat the boiler must deliver for space heating, and the calculator's W/m² heat loss figure is the whole-house version of it.
More guides
- What size boiler do I need? (UK combi and system kW by bedrooms and bathrooms)
Combi and system boiler kW for every bedrooms and bathrooms pair from the UK installers' tables, how many radiators a 24, 28 or 32 kW combi serves, and heat loss vs kW.
- How many BTU do I need for 1,000 sq ft? (200 to 1,200 sq ft, cooling and heating)
1,000 sq ft is 21,000 BTU/h on the ENERGY STAR chart (1.75 tons) and 32,500-55,000 BTU/h to heat by zone. 200 to 1,200 sq ft, with the adjustments.
- What size air conditioner do I need for a 1,500 sq ft house? (tons by home size)
The chart reads 1,500 sq ft as 30,000 BTU/h (2.5 tons); room by room the units add to 44,000. Both readings, 1,000-3,000 sq ft, and a seven-room example.
- How many BTU do I need for a bedroom? (12x12 and other room sizes)
A 12 x 12 bedroom is 5,000 BTU/h on the ENERGY STAR chart. Eight bedroom sizes with the sun, people and ceiling adjustments, m² to kW, and the unit class for each.
Last updated 2026-09-22. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.