Find the exact BTU output for your radiator — or calculate how much BTU your room needs. Supports panel, column, cast iron & electric with ΔT correction.
Two modes — find your radiator's actual output, or calculate exactly what your room needs.
Enter radiator dimensions and type to calculate heat output at any system temperature.
Calculate total heat output required for your room and get a recommended radiator size.
Each radiator type uses a different BTU formula. Each calculator uses specialised formulas for accurate sizing.
Single panel (Type 11), double panel (Type 21), and double panel double convector (Type 22/K2). The most common radiator type in UK and European homes.
2, 3, and 4-column radiators. BTU calculated from column count × height × width. Popular for period properties and vertical spaces.
Per-section BTU calculation for antique and reproduction cast iron radiators. Higher thermal mass than steel — slower heat-up, longer heat retention.
Converts rated wattage to BTU output. 1 watt = 3.412 BTU/hr. Compare electric vs. hydronic radiators for the same room size.
Enter height and width in mm — no model needed. Uses EN 442 standard output coefficients for a fast estimate directly from radiator dimensions.
Linear BTU output per metre or foot for hydronic baseboard radiators. Common in North American homes and renovated UK properties.
The BTU your room needs depends on volume, insulation, windows, and external walls. Start with this formula:
| Insulation Level | BTU per ft³ | W per m³ |
|---|---|---|
| Poor (solid walls, single glazing) | 5–6 | 58–70 |
| Average (cavity walls, double glazing) | 3–4 | 35–46 |
| Good (insulated, double glazing) | 2–3 | 23–35 |
| Excellent (new build, triple glazing) | 1.5–2 | 17–23 |
UK radiator manufacturers publish output in watts at ΔT50. Use watts when browsing radiator catalogues or comparing products.
If your system runs at a different temperature from the rated ΔT50, multiply the radiator's rated output by the correction factor:
Example: A radiator rated 4,000 BTU at ΔT50 will only output 2,072 BTU at ΔT30 when connected to a heat pump.
Type 22 (K2) double panel double convector at ΔT50 — the most popular radiator type in UK homes.
| Radiator Size (H × W) | Watts (ΔT50) | BTU/hr (ΔT50) | Watts at ΔT40 | Watts at ΔT30 | Room Coverage |
|---|---|---|---|---|---|
| 300 × 400 mm | 349 W | 1,191 BTU | 257 W | 181 W | ≤ 4 m² |
| 300 × 800 mm | 698 W | 2,381 BTU | 514 W | 361 W | ≤ 8 m² |
| 400 × 600 mm | 668 W | 2,280 BTU | 492 W | 346 W | ≤ 8 m² |
| 500 × 600 mm | 851 W | 2,904 BTU | 627 W | 441 W | ≤ 10 m² |
| 600 × 600 mm | 1,022 W | 3,488 BTU | 753 W | 529 W | ≤ 12 m² |
| 600 × 800 mm | 1,362 W | 4,649 BTU | 1,004 W | 706 W | ≤ 16 m² |
| 600 × 1000 mm | 1,703 W | 5,811 BTU | 1,255 W | 882 W | ≤ 20 m² |
| 600 × 1200 mm | 2,044 W | 6,974 BTU | 1,506 W | 1,059 W | ≤ 24 m² |
| 600 × 1600 mm | 2,725 W | 9,299 BTU | 2,008 W | 1,411 W | ≤ 32 m² |
| 700 × 1400 mm | 2,828 W | 9,650 BTU | 2,084 W | 1,465 W | ≤ 33 m² |
* Type 22 at ΔT50 (mean water temp 70°C, room 20°C). Room coverage assumes 60 W/m² average insulation.
| Column Type / Height | BTU per Section | Watts per Section | 10-Section Total |
|---|---|---|---|
| 4-column, 660 mm | ~580 BTU | ~170 W | 5,800 BTU / 1,700 W |
| 4-column, 760 mm | ~665 BTU | ~195 W | 6,650 BTU / 1,950 W |
| 3-column, 760 mm | ~512 BTU | ~150 W | 5,120 BTU / 1,500 W |
| 2-column, 960 mm | ~450 BTU | ~132 W | 4,500 BTU / 1,320 W |
Direct answers to the most common radiator sizing questions.
For panel radiators, measure height and width in mm and reference the manufacturer's output table at ΔT50. A 600×1000 mm Type 22 panel radiator produces approximately 5,800 BTU/hr (1,703 W) at ΔT50. Use our calculator above for instant results from any dimensions.
ΔT50 (Delta T 50) means the mean water temperature in the radiator is 50°C above room temperature — typically a flow/return of 75°C/65°C with a room at 20°C. This is the standard EN 442 test condition for all UK and European radiator BTU ratings.
Yes. Heat pumps run at lower flow temperatures (ΔT30–ΔT35), which reduces radiator output significantly. A radiator rated 6,000 BTU at ΔT50 only delivers around 3,108 BTU at ΔT30 (×0.518 factor). Plan to use approximately 2× the radiator size compared to a gas boiler system.
A standard 4-column 760 mm cast iron radiator produces approximately 665 BTU (195 W) per section at ΔT50. Multiply by your total section count. Cast iron has higher thermal mass than steel — it heats more slowly but retains heat longer after the boiler cycles off.
Both measure heat output. 1 watt = 3.412 BTU/hr. UK manufacturers publish in watts; US manufacturers in BTU/hr. To convert: BTU/hr ÷ 3.412 = Watts, or Watts × 3.412 = BTU/hr.
The UK standard is 60 W (205 BTU) per m² for average insulation at ΔT50. For poorly insulated older homes use 80–100 W/m². New builds with excellent insulation typically need 40–55 W/m². Always add 10–15% extra for bathrooms.
For a 15 m² room with average insulation at ΔT50, you need approximately 900 W (3,070 BTU). A 600×600 mm Type 22 (1,022 W) covers this comfortably. Use the room calculator above for a precise result based on your ceiling height and insulation type.