How many BTU do I need per square metre?

As a rough guide, poorly insulated rooms need 150-200 BTU per m², average insulation needs 100-150 BTU per m², and well-insulated modern rooms need 70-100 BTU per m². However, room height, window area, external walls, and ventilation all affect the actual requirement.

What is the difference between single and double panel radiators?

Single panel radiators (Type 11 or 21) have one water channel panel. Double panel radiators (Type 22) have two panels and produce roughly 50-70% more heat from the same wall space. Double panels are better for larger rooms but project further from the wall.

Should I oversize or undersize radiators?

It is better to slightly oversize. An oversized radiator with a TRV (thermostatic radiator valve) will reach temperature faster and then the valve throttles back to maintain comfort. An undersized radiator will run flat out and never reach the desired temperature on cold days.

Do I need a bigger radiator for a bathroom?

Bathrooms are typically heated to 22-23C compared to 21C for living rooms, so they need more heat output per m². This calculator applies a +1C adjustment for bathrooms. Also consider a heated towel rail which provides both heating and towel warming.

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Radiator BTU Calculator — Room Heating Requirements

Calculate the BTU output needed to heat any room. Get radiator size and quantity recommendations.

Room Length

Longest dimension of the room

Room Width

Shortest dimension of the room

Room Height

Standard UK ceiling height is 2.4m

External Walls

Walls facing outside

Window Area

Total glazed area on external walls

Insulation Level

Affects heat loss rate through walls and windows

Room Type

Adjusts target temperature for room use

How We Calculate This

This calculator estimates total heat loss from the room and converts it to BTU/h. Heat escapes through walls (fabric loss), windows, and ventilation (air changes). The total heat loss determines the radiator output needed.

Heat loss components

Fabric loss: Heat conducted through external walls, based on U-value and area
Window loss: Heat conducted through glazing, based on window U-value
Ventilation loss: Heat lost to air changes, based on room volume and air change rate

Conversion

1 Watt = 3.412 BTU/h. The total watt heat loss is converted to BTU for radiator selection.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.