Radiator BTU / Output Calculator
Calculate the heat output needed for any room to select the correct radiator. Based on room volume, insulation quality, and heat loss factors.
Radiator Sizing Guide
BTU Calculation
Heat required = room volume (m³) × heat loss factor. Base factor: 40–50W/m³ for well-insulated rooms, 60–70W/m³ for poorly insulated. Add 10% for each external wall, 20% for single-glazed windows, 10% for north-facing rooms.
Panel Types
Type 11 (single panel, single convector): suitable for small rooms under 1,500 BTU. Type 21 (double panel, single convector): common standard size. Type 22 (double panel, double convector): highest output for a given physical size — best for larger rooms or poor insulation.
Flow Temperature
Radiator output ratings assume 75°C flow / 65°C return. If you have a heat pump or low-temperature system (35–45°C), the actual output is significantly lower and you need larger or more radiators to compensate.
Energy Efficiency Priorities
Home energy improvements should be prioritised by payback period. Quickest payback: draught proofing (£50-200 cost, saves £60-100/year — under 2 years). Loft insulation (£300-500 DIY, saves £150-200/year — 2-3 years). Switching to LED lighting (under £100, saves £40-60/year — under 2 years). Longer payback: cavity wall insulation (3-5 years). New efficient boiler (5-10 years). Solar panels (7-15 years). Heat pump (10-25 years at current energy prices). For homes eligible for ECO4 funding, the pa
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