BTU Calculator

How a radiator is sized (and why we show a range)

The method every UK radiator retailer uses online — BestHeating, Toolstation, Drench, B&Q, Stelrad's basic tool — is the same rule of thumb: room volume (m³) × a base heat rate in watts per cubic metre, by room type × adjustment factors for glazing, walls, insulation age, orientation and position × a 10–15% safety margin, then converted to BTU/h. It's a widely-converged industry convention (corroborated independently by more than one merchant), not a single number published by a standards body — the actual W/m³ table isn't in BS EN 442 or a public CIBSE table, so we show it as a range rather than a falsely precise single figure.

For the 48.0 m³ lounge above, the base heat rate for a living room runs 34–48 W/m³ (typical 40), so the output needed spans 1,877 W–2,650 W (typical 2,208 W) before you even factor in your specific glazing, walls or orientation.

Delta-T50 vs Delta-T60 — the trap on manufacturer spec sheets

Since 2014, UK radiators have been tested and rated under BS EN 442 at Delta-T50 (flow 75°C, return 65°C, room 20°C — an average water temperature 50°C above room temperature). Older radiator catalogues, and some re-published spec sheets, still quote the pre-2014 Delta-T60 figure, which reads about 26.7% higher for the exact same radiator.

If a product page only shows a Delta-T60 output, multiply it by 0.789 to get the comparable Delta-T50 figure before matching it against this calculator's result — e.g. a radiator rated 1,000 W at DT60 is really only about 789 W at DT50. Mixing the two scales is the single most common radiator-undersizing mistake.

Flow temperature also changes the picture for heat pumps: a gas boiler typically runs at 75°C flow (DT50, the 100% baseline), while an air source heat pump runs nearer 45°C flow (roughly DT25, ~41% of DT50 output). A radiator sized for a gas boiler at DT50 needs to be roughly 2.5× larger for the same room on a heat-pump retrofit — get a proper heat-loss survey before switching.

BTU and watts — the same heat, two units

BTU/h (British Thermal Units per hour) and watts both measure the same thing — a rate of heat output — just in different units. The exact conversion is 1 W = 3.412142 BTU/h (and 1 BTU/h = 0.29307107 W). Radiator packaging in the UK is usually labelled in BTU/h; electric heaters and heat pump specs are usually in W or kW. For the lounge example, 2,208 W (2.21 kW) typical is the same amount of heat as 7,534 BTU/h.

What the adjustment factors do

Glazing: single glazing loses more heat than double (×1.4 vs the double-glazed baseline), triple glazing a little less (×0.85). External walls: a room with no external wall needs less (×0.85); each additional external wall adds roughly 10–25%, up to a conservatory-style room with walls on every side (×1.3). Building age: a post-2010 new build with modern insulation needs notably less (×0.7) than a Victorian solid-wall property (×1.45), which loses heat much faster. Orientation: south-facing rooms get some passive solar gain (×0.95); north-facing rooms lose more (×1.1). Room position: a room with unheated space above or below — a loft or garage — needs extra output (×1.15) to compensate.

A worked contrast: a 12.0 m³ Victorian bathroom, single glazed, two external walls, north-facing, needs a typical 5,323 BTU/h (1,560 W) — noticeably more per cubic metre than the modern lounge above, purely from the adjustment factors.

When you need a real heat-loss calculation instead

This calculator — like every other online BTU tool — is a sizing estimate, not a certified heat-loss survey. It doesn't model your actual wall/window/floor/roof U-values, room-by-room air changes, or party walls shared with a heated neighbour. Get a professional BS EN 12831 heat-loss calculation (the CIBSE Domestic Heating Design Guide method) before final purchase for a whole-house system, and always before sizing radiators for a heat pump retrofit or for Building Regulations compliance — both require the real room-by-room figures, not a rule-of-thumb estimate.