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What size radiator do I need?

Enter your room's measurements and we'll work out the heat output it needs, in BTU and watts, using manufacturers' published figures.

Radiator size neededΔT50

Enter your room's measurements.

How it works

  1. Work out the room's volume: length × width × height, in metres. Viessmann
  2. Multiply by Viessmann's figure for the room: 108 for a double-glazed bedroom, 132 for a single-glazed bedroom, 135 for a double-glazed living room. That gives the heat the room needs, in BTU per hour. Viessmann
  3. Radiator catalogues quote output at ΔT50. If your heating runs cooler, a radiator gives out less heat, so you need a bigger one. We divide by Stelrad's correction factor: 0.747 at ΔT40 and 0.513 at ΔT30. Stelrad, Worcester Bosch
  4. Watts = BTU per hour × 0.2931. NIST, Stelrad

An estimate, not a design

Manufacturers are clear that a rule of thumb like this is a starting point. Stelrad says to round up to the next available radiator size and to consult a heating engineer if you're unsure. A proper heat loss calculation takes into account walls, windows, insulation and how exposed the room is.

Viessmann doesn't publish a figure for kitchens, bathrooms or single-glazed living rooms. For a bathroom, which is designed to be warmer, we use the living room figure as the closest published guide.

Common questions

How many BTU do I need for a room?

Viessmann's rule of thumb is the room's volume in cubic metres multiplied by 108 for a double-glazed bedroom, 132 for a single-glazed bedroom or 135 for a double-glazed living room. A 4.8 × 4 × 2.4 m double-glazed bedroom needs about 4,977 BTU per hour.

What does ΔT50 mean on a radiator?

It's the standard condition radiator outputs are quoted at. If your heating runs cooler, for example with a heat pump or a lowered flow temperature, the radiator gives out less. Stelrad's factor at ΔT30 is 0.513, so a radiator gives about half its ΔT50 output.

How do I convert BTU to watts?

Multiply BTU per hour by 0.2931. To go the other way, multiply watts by 3.412.

Sources