Radiator BTU Calculator UK — Sizing Guide
Under-sized radiators = cold room even at full boiler output. Here is the proper UK BTU calculation used by heating engineers.
Step-by-step method
1. Measure the room in metres
Length × Width × Height = volume in m³.
2. Apply the multiplier
Lounge: 153 BTU/m³. Bedroom: 135 BTU/m³. Kitchen: 108 BTU/m³. Bathroom: 172 BTU/m³. Common areas 135 BTU/m³.
3. Adjust for glazing and walls
Double glazing: no change. Single glazing: +15%. North-facing: +10%. French doors: +20%.
4. Pick the radiator
Match total BTU to radiator spec. Single (K1), double convector (K2), or triple (K3). K2 delivers ~1.7× K1 output for same length.
5. Delta T rating
Manufacturer BTUs use ΔT50 — but modern condensing boilers run at ΔT30. Multiply spec BTU by 0.6 for real-world output on a modern combi.
Frequently asked questions
What size radiator for a 4×3m lounge?+
Volume 4×3×2.4 = 28.8m³ × 153 = 4,406 BTU. A 600×1000mm K2 delivers ~4,800 BTU at ΔT50, ~2,900 at ΔT30. So you'd need a 600×1400mm K2.
Can radiators be oversized?+
Yes — modern boilers modulate down. Oversized rads run cooler = more comfort. Undersized fail permanently.