Choosing the right radiator size means a room that heats up quickly without wasting energy. Enter your room details below for an instant estimate in BTU per hour and watts, including the larger size you need if you have, or plan to get, a heat pump.
Radiator BTU calculator
An estimate for choosing radiator size. Radiator outputs are listed at ΔT50; heat pumps run cooler, so radiators need roughly double the ΔT50 rating. For a new system, ask your installer for a room-by-room heat loss calculation.
How to use the result
- Radiator outputs are quoted at "ΔT50" (Delta T 50): the standard test for a gas boiler system running at typical temperatures. Choose a radiator rated at or slightly above the figure shown.
- Heat pumps run cooler (often around ΔT30), so a radiator gives out roughly half its rated output. Use the heat pump figure, which is about double.
- Big room, one radiator not enough? Split the total between two radiators, ideally under windows or on external walls.
- Double panel radiators (Type 22) give much more heat than single panels of the same size, so they're useful where wall space is limited.
How the calculator works
It starts from about 40 watts per cubic metre, a typical figure for an averagely insulated UK room heated to 21°C, then adjusts for:
| Factor | Adjustment |
|---|---|
| Room type (target temperature) | Bedroom and kitchen ×0.9, bathroom ×1.1, hallway ×0.8 |
| Windows | Single glazed ×1.15, large windows ×1.1, triple glazed ×0.95 |
| Insulation | Poor ×1.25, good ×0.75 |
| External walls | +5% for each extra external wall |
| North-facing | ×1.1 |
1 watt = 3.412 BTU per hour.
Worked example
A 4 m × 3.5 m living room with a 2.4 m ceiling, double glazing, average insulation and two external walls:
- Volume: 4 × 3.5 × 2.4 = 33.6 m³
- 33.6 × 40 W = 1,344 W, × 1.05 for the second external wall = about 1,410 W (4,810 BTU)
- For a heat pump: about 2,760 W at ΔT50 rating
This gives a good estimate for choosing a radiator. If you're installing a whole new heating system or a heat pump, ask the installer for a room-by-room heat loss calculation. It's more accurate and often required for grants.
FAQs
How many BTU do I need for a room?
Roughly 4,000–5,500 BTU for a typical 12–15 m² living room with average insulation. Use the calculator for your exact room.
What does ΔT50 mean on a radiator?
It's the standard test condition for quoted radiator outputs: a 50°C difference between the average water temperature and the room. Lower flow temperatures (like heat pumps) mean less heat output.
Are BTU calculators accurate?
They give a sound estimate for choosing a radiator. A professional heat loss calculation is more precise, especially for whole-house or heat pump installations.
Do I need bigger radiators for a heat pump?
Usually, yes. Heat pumps work most efficiently at lower water temperatures, so radiators typically need to be around twice the ΔT50 size, or swapped for double-panel models.
How do I convert watts to BTU?
Multiply watts by 3.412. For example, 1,500 W × 3.412 = 5,118 BTU per hour.