Radiator BTU Calculator

Radiator sizing is calculated in BTU/h (British Thermal Units per hour). A BTU is the energy needed to raise 1 lb of water by 1°F, and UK radiators are rated in BTU/hour or in watts, where 1 BTU/hr = 0.293 W and 1 kW = 3,412 BTU/hr. The base calculation multiplies room volume (length × width × height in metres) by a base factor of about 153 BTU per m³. This is then adjusted for room characteristics: external walls, window type and size, insulation level, location in the house and desired temperature. The aim is to work out the heat the room loses, then choose radiators whose combined output matches or slightly exceeds it.

Common correction factors include +15% for each external wall, +20% for single-glazed windows (or −10% for triple glazing), +15% for rooms above a garage or unheated space, and +10–15% for north-facing rooms. Rooms with high ceilings of 3m or more need about 20% extra, solid-wall rooms 10%, and rooms with double doors or large windows 15%. Bathrooms typically require a higher target temperature (22°C versus 21°C for living rooms). For a 5 m × 4 m × 2.4 m living room, the volume is 48 m³, the base figure is 48 × 153 = 7,344 BTU/h, and two external walls multiply that by 1.30 to give 9,547 BTU/h. Double glazing needs no adjustment, so radiators totalling about 10,000 BTU/h would suit.

Enter the room dimensions and characteristics and the calculator shows the total BTU/h requirement and suggests appropriate radiator sizes. Most UK radiator manufacturers publish BTU outputs at Delta T50, the standard comparison condition of 50°C ΔT. Modern boilers run at lower flow temperatures of 50-60°C, so compare radiators on their Delta T 50 rating rather than the legacy 75°C ΔT figure, which overstates what you will get at real boiler temperatures. If your system runs at a lower flow temperature still (for example with a heat pump), you will need larger radiators.

Radiator type matters as much as size. A single panel (Type 11) has a low output and a slim profile, a double panel with a single convector (Type 21) sits mid-range, and a double panel with double convector (Type 22) gives the highest output but is deeper. At 1,000mm × 600mm and 50°C ΔT, a Type 11 produces 920W (3,140 BTU/hr) while a Type 22 produces 1,700W (5,800 BTU/hr), so the same wall space can deliver close to double the heat. Fit a thermostatic radiator valve (TRV) on every radiator so that each room can be controlled individually.

Heat pumps change the sums. They deliver heat at a 35-45°C flow temperature against 70-80°C from a gas boiler, so a radiator sized for gas at 50°C ΔT will not deliver enough heat, and the surface area needs to be 2-3× larger. A standard double panel putting out 1,500W at gas temperatures drops to roughly 500W at heat pump temperatures. A retrofit therefore means replacing radiators, adding fan-coil units or installing underfloor heating. The Boiler Upgrade Scheme grant of £7,500 requires sufficient heat emission, and the installer's survey assesses radiator sizing as part of that.

For quick estimates, a 4 m² bathroom needs about 2,500 BTU from a heated towel rail, a 12 m² kitchen 5,000-6,000 BTU, an 18 m² lounge 6,000-8,000 BTU and a 14 m² double bedroom 5,000-6,000 BTU. Getting it slightly wrong is only forgiving in one direction. A shortfall of 10-20% can be recovered by turning the TRV up and allowing a longer warm-up, whereas a badly under-specified radiator never gets the room to its target temperature. Over-specifying wastes capital and makes the radiator overshoot the target, cycling on and off.