Radiator BTU Calculator (UK)
Enter room details for precise BTU output at ΔT50. Supports multiple scenarios.
Steps to Size Radiators
- Measure room in metres: length × width × height for volume.
- Adjust for glazing: single adds 40% more heat loss than triple.
- Account walls/insulation: north walls need 10% extra.
- Multiply base 33 BTU/m³ by factors, round to nearest 50.
- Select radiator exceeding calculated BTU at ΔT50.
BTU Principles
Base formula: volume (m³) × 33 BTU/m³ (UK average) × factors. ΔT50 assumes 75/65/20°C temps per EN442. Part L mandates 55°C flow max, so oversize 15-20% for efficiency. Watts = BTU × 0.293.
FAQ
Why higher for kitchens?
1.4× multiplier for ventilation/equipment heat loss.
Single vs double glazing?
Single: +50% BTU; double: +20%; triple: baseline.
Multiple radiators?
Combine outputs; place under windows for even heat.
New build vs extension?
New: good insulation (1×); extension: average (1.2×).
Heat Loss by Scenario
| 4x4x2.4m Room | BTU/hr | Factor Reason |
|---|---|---|
| Lounge, double glazing, 1 wall | 4,500 | Standard |
| Bathroom, single glazing, 2 walls | 7,200 | High loss + purpose |
| Bedroom, good insulation, north | 3,800 | Low loss offset |
| Conservatory, poor insulation | 9,500 | Glazing heavy |
Common Errors
- Using floor area only: ignores height; volume-based more accurate by 20-30%.
- Ignoring glazing: underestimates by 1,000+ BTU in older homes.
- Wrong ΔT: Δ60 needs 10% less BTU than ΔT50 listings.
- No room factor: lounge vs bedroom differs 20%.
- Undersizing: leads to cold spots; always exceed calculated.
References
- BS EN 442-1:2014. Radiators and convectors. Technical specifications and test methods.
- Building Regulations Part L (2021). Conservation of fuel and power. Volume 1: Dwellings.
- CIBSE Guide B2 (2016). Heating, hot water, ventilation and compressed air systems for non-domestic buildings.
- Stelrad Radiator Sizing Guide (2025). Stelrad Professional.
- Trade Radiators BTU Calculator Methodology (2024).