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.
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Radiator Sizing Guide
What's the key thing to understand about 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.
What should I know about 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.
What should I know about 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.
What should I know about 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 payback period on insulation measures can be reduced to near-zero, since the scheme covers some or all of the installation cost for qualifying low-income and vulnerable households — check eligibility on GOV.UK before paying for insulation work out of pocket.