Radiator BTU Calculator

It applies the UK industry standard: room volume (length × width × height) multiplied by a watts-per-cubic-metre figure for the room type, then adjusted for glazing, window size, external wall exposure, insulation level, and aspect. Outputs are quoted at delta T 50, the convention manufacturers use to spec radiator outputs. The result is the heating power your radiators (or underfloor heating) need to deliver to keep the room at 21°C when it's freezing outside.

Because they're both standard but used in different places. Radiator product pages on UK retailer sites usually quote BTU (3,000 BTU, 5,000 BTU). Manufacturer technical specs and Part L heat loss calculations work in watts. 1 watt equals 3.412 BTU. Most calculators only give one; ours gives both so you can compare like-for-like with whatever spec sheet is in front of you.

Usually 10% on top is sensible. The base calculation assumes 21°C indoor and a -3°C design outdoor temperature with the room's insulation working properly. Cold spots, ceiling heights over 2.7m, or bi-fold doors added after the building was insulated all justify the margin. It's easier to throttle a slightly oversized radiator at the thermostatic valve than to live with one that can't keep up in January.

Yes — they need 1.5 to 1.8 times the calculated output, or low-temperature radiators specifically rated for heat pumps. Heat pumps run at 35 to 45°C flow temperature versus 70°C for a condensing boiler. Lower flow temperature means each radiator delivers less heat for the same physical size, so they need to be bigger. The calculator shows a heat pump sizing note in the results to flag this.

For rooms over 15m², open-plan kitchen-diners, kitchen extensions with bi-fold doors, and any new build or substantial refurb where the floor is coming up anyway. UFH delivers around 70 W per m² of floor area, runs at lower flow temperatures (so it pairs naturally with heat pumps), and frees up wall space. The calculator flags rooms over 15m² with a UFH alternative figure so you can compare directly with the radiator-based number.

No, it's a starting point. For a heat pump installation, a deep retrofit, or any project where you need MCS certification or a Boiler Upgrade Scheme grant, you'll need a full Part L room-by-room heat loss calculation done by a qualified installer. The BTU calculator is what you use when ordering replacement radiators for a refurb, scoping a new extension's heating, or sense-checking a quote.