Radiator BTU Chart: Room Sizes, Radiator Outputs and the ΔT Catch
A radiator BTU chart answers two questions in one place: how much heat does this room need, and which radiator puts that much out? Below are both charts, plus the one correction most people miss: the figure on the box assumes your heating runs hotter than it probably does.
Radiator BTU chart by room size
This chart uses the same model as our radiator BTU calculator: room volume × 141.3 BTU per m³, for a living room with average insulation and a 2.4 m (8 ft) ceiling. Treat it as a starting point, not gospel.
| Floor area | Example room | Room volume | Heat needed (BTU/hr) | Heat needed (watts) |
|---|---|---|---|---|
| 9 m² (97 ft²) | 3 × 3 m small bedroom | 21.6 m³ | ≈ 3,050 | ≈ 890 |
| 12 m² (129 ft²) | 4 × 3 m bedroom | 28.8 m³ | ≈ 4,070 | ≈ 1,190 |
| 16 m² (172 ft²) | 4 × 4 m lounge | 38.4 m³ | ≈ 5,430 | ≈ 1,590 |
| 20 m² (215 ft²) | 5 × 4 m living room | 48 m³ | ≈ 6,780 | ≈ 1,990 |
| 25 m² (269 ft²) | 5 × 5 m open-plan | 60 m³ | ≈ 8,480 | ≈ 2,490 |
| 30 m² (323 ft²) | 6 × 5 m kitchen-diner | 72 m³ | ≈ 10,170 | ≈ 2,980 |
To convert: watts × 3.412 = BTU/hr, and BTU/hr ÷ 3.412 = watts.
Adjust for your room
- Bedrooms are usually kept cooler, so you can often go 10–15% lower.
- Bathrooms run warmer, so add 10–20%.
- Big windows, two or more outside walls, or high ceilings all push the figure up. A north-facing room with patio doors is a different animal from an internal box room.
- Poor insulation (solid walls, single glazing) can add 25–50%. Good modern insulation can take a similar chunk off.
Sizing radiators from a chart alone is a bit like buying shoes by age: right on average, wrong for plenty of feet. The calculator lets you put in your actual room and insulation.
Radiator output chart: what each size puts out
Panel radiators are described by type (how many panels and fins) and size. These are typical outputs at ΔT50 (the standard test condition) for 600 mm (24 in) high radiators. Real figures vary by brand, so always check the manufacturer's data.
| Width | Type 11 (single panel, single fin) | Type 21 (double panel, single fin) | Type 22 (double panel, double fin) |
|---|---|---|---|
| 400 mm (16 in) | ≈ 1,200–1,400 BTU | ≈ 1,800–2,000 BTU | ≈ 2,300–2,500 BTU |
| 600 mm (24 in) | ≈ 1,800–2,100 BTU | ≈ 2,700–3,000 BTU | ≈ 3,400–3,700 BTU |
| 800 mm (32 in) | ≈ 2,400–2,800 BTU | ≈ 3,600–4,000 BTU | ≈ 4,600–4,900 BTU |
| 1,000 mm (40 in) | ≈ 3,000–3,500 BTU | ≈ 4,500–5,000 BTU | ≈ 5,700–6,100 BTU |
| 1,200 mm (48 in) | ≈ 3,600–4,200 BTU | ≈ 5,400–6,000 BTU | ≈ 6,800–7,300 BTU |
| 1,400 mm (55 in) | ≈ 4,200–4,900 BTU | ≈ 6,300–7,000 BTU | ≈ 8,000–8,500 BTU |
Output scales roughly in line with width, so a 1,200 mm radiator gives about twice what a 600 mm one does of the same type and height.
The ΔT correction (why heat pumps need bigger radiators)
ΔT ("delta T") is the difference between the average radiator temperature and the room. Catalogue figures are usually at ΔT50, which means water at about 75°C in a 20°C room. Many boilers today run cooler for efficiency, and heat pumps run much cooler. Output drops sharply as ΔT falls:
| ΔT | Typical system | Multiply ΔT50 output by |
|---|---|---|
| ΔT50 | Older boiler, high flow temperature | 1.00 |
| ΔT40 | Condensing boiler, turned down | ≈ 0.75 |
| ΔT30 | Heat pump, mid flow temperature | ≈ 0.51 |
| ΔT25 | Heat pump, low flow temperature | ≈ 0.40 |
So a radiator rated at 6,000 BTU at ΔT50 gives only about 3,000 BTU at ΔT30. That's why heat pump installs often swap in larger, double-finned radiators. It's not the radiator getting worse; it's the water arriving lukewarm, like trying to warm your hands on a cup of tea you forgot about.
Worked example: sizing a living room radiator
A 5 × 4 m (16 ft 5 in × 13 ft 1 in) living room, 2.4 m ceiling, average insulation:
- From the chart: ≈ 6,780 BTU (1,990 W).
- On a boiler running at ΔT50: one Type 22 at 600 × 1,200 mm (≈ 6,800–7,300 BTU) does the job.
- On a heat pump at ΔT30: you'd need about 6,780 ÷ 0.51 ≈ 13,300 BTU of ΔT50-rated radiator. That's two Type 22s at 600 × 1,000 mm or more, or taller models.
Frequently asked questions
How many BTU do I need per square metre?
Using a 2.4 m ceiling and average insulation, around 340 BTU per m² (about 32 BTU per ft²) for a living room. Adjust for room type, glazing and insulation.
Is a bigger radiator better?
A slightly oversized radiator is fine. Thermostatic valves stop it overheating the room, and it'll still cope on cold days or at lower flow temperatures. Wildly oversized just wastes money and wall space.
What is ΔT50?
The standard test condition for radiator output: the radiator averages 50°C above room temperature (about 70°C in a 20°C room). Figures at ΔT50 look bigger than what most modern systems actually deliver.
Do towel radiators give out much heat?
Usually far less than a panel radiator of the same size, often 1,000–2,500 BTU. Check the rated output if the towel rail is the only heat in a bathroom.
How do I convert watts to BTU?
Multiply watts by 3.412. For example, 1,500 W ≈ 5,120 BTU/hr.
Size every room properly
Put your actual rooms through these and you'll know exactly which radiators to buy:
- Radiator BTU Calculator: the BTU and watts for your room, including windows, walls and insulation.
- Room BTU Requirement Calculator: presets for loft and garage conversions, conservatories and open-plan spaces.
- Heat Loss Calculator: a fuller look at heat lost through walls, windows and floors.