For reference, our simple manual calculation: Volume (m³) × 50 (for average insulation) = Watts, then Watts × 3.412 = BTUs. Adjust based on factors above.
| Room Size (m²) | Height (m) | Watts Needed | BTUs Needed |
|---|---|---|---|
| 8–10 | 2.4 | 1,000–1,200 | 3,400–4,100 |
| 12–15 | 2.4 | 1,400–1,800 | 4,785–6,140 |
| 16–20 | 2.4 | 2,000–2,500 | 6,825–8,530 |
| 25–30 | 2.4 | 3,000–3,600 | 10,230–12,280 |
Add 15–20% for open-plan or poorly insulated rooms. For new builds, use a lower factor (35–45 W/m³).
Manual BTU & Watt Calculator
- Calculate your room volume:
Length × Width × Height = Volume (m³) - For average insulation in the UK, multiply volume by 50:
Required Watts = Volume (m³) × 50 - Convert to BTU:
1 Watt = 3.412 BTU(multiply your watts by 3.412)
Example: A 4m × 4m × 2.4m room = 38.4m³
38.4 × 50 = 1,920 Watts
1,920 × 3.412 = 6,563 BTUs
| Room Size (m²) | Height (m) | Watts Needed | BTUs Needed |
|---|---|---|---|
| 8–10 | 2.4 | 1,000–1,200 | 3,400–4,100 |
| 12–15 | 2.4 | 1,400–1,800 | 4,785–6,140 |
| 16–20 | 2.4 | 2,000–2,500 | 6,825–8,530 |
| 25–30 | 2.4 | 3,000–3,600 | 10,230–12,280 |
Add 15–20% for open-plan or poorly insulated rooms. For new builds, you may use a lower factor (35–45 W/m³).
Need More Help?
If you require personalised guidance to select the perfect radiator type and size for your home or project, please contact our expert team. We’ll help you find the best heating solution tailored to your needs.