Radiator BTU Calculator

Who this is for

Homeowners replacing an old radiator or sizing one for a new room, and heating engineers who want a quick sanity check before a full survey. This is a guide figure using a widely-used watts-per-cubic-metre rule of thumb — enough to shop with confidence, not a substitute for a proper survey.

What is a BTU?

BTU stands for British Thermal Unit — the traditional way UK radiators are rated for heat output, even though the underlying physics is measured in watts. This calculator works out the watts your room needs first, then converts to BTU/hr using watts × 3.412, and shows both so you can compare against any radiator's spec sheet.

What does ΔT50 mean on radiator listings?

Radiator BTU/hr and watt outputs are quoted at Delta T50 — meaning the water inside is 50°C hotter than the room. Modern condensing boilers, and especially heat pumps, run at lower flow temperatures than that, so the real-world output is lower than the number on the box. If your system runs cooler, size up and check the manufacturer's output at ΔT30 or ΔT35.

Is it better to oversize a radiator?

A modest amount of oversizing paired with a thermostatic radiator valve (TRV) is fine, and actually helps with low-temperature running on heat pumps and modern boilers. Going hugely oversized just wastes money on the radiator itself and eats into wall and floor space for no real benefit.

Is this as good as a proper heat-loss calculation?

No, and we'd rather tell you that than let you assume otherwise. The watts-per-cubic-metre method here is a rule of thumb — quick, reasonable for shopping, but it doesn't account for wall area, window size, floor type or the specific building fabric room by room. A proper heat-loss calculation (to BS EN 12831) does, and any good heating engineer will carry one out before designing or resizing a whole system.