Radiator BTU Calculator

The quick “what size radiator do I need” tool – get the BTU and kW output to warm any room.

Part of:Heating & HVAC hub
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How to Use This Calculator

Which calculator should I use? This is the quick, consumer-friendly “what size radiator do I need” version — answer a few simple questions about the room and get a BTU figure to shop with. If you want a full technical breakdown of a room's heat loss (for sizing a whole heating system, boiler or heat pump), use our Heat Loss Calculator instead.

What This Calculator Does

It tells you the heat output — in BTU per hour and kilowatts — that a radiator needs to warm a room comfortably. BTU (British Thermal Units) is how radiators and towel rails are usually rated in shops, so this gives you a figure you can buy against.

How to Use

  1. Choose metric or imperial units.
  2. Enter the room's length, width and ceiling height (2.4 m / 8 ft is typical).
  3. Pick the room type — lounges and bathrooms need more heat per cubic metre than bedrooms and kitchens.
  4. Choose your window glazing, how many outside walls the room has, and whether it faces north.
  5. Read off the recommended BTU and kW output.

Worked Example

For a double-glazed lounge 5 m × 4 m × 2.4 m with one external wall:

  1. Volume = 48 m³.
  2. Base heat = 48 × 30 = 1,440 W, × 1.1 for a lounge = about 1,584 W.
  3. That's roughly 5,400 BTU/h — so you'd look for a radiator (or radiators) totalling around 5,000–6,000 BTU.

Tips

  • If a room needs more BTU than one radiator provides, use two — the outputs simply add up.
  • Radiator makers quote output at “Delta T 50”. If yours quotes Delta T 60 or 70 the real-world output in a UK system is a little lower, so don't undersize.
  • Towel rails give out less heat than a panel radiator of the same size — in a cold bathroom you may need a rail plus a small radiator.
  • It's better to slightly oversize: a bigger radiator run cooler is more efficient, especially with a heat pump.

Common Questions

What size radiator do I need for my room?

Work out the room volume (length × width × height) and multiply by roughly 30 watts per cubic metre, then adjust for the room type, glazing and outside walls. This calculator does that for you and gives the answer in BTU — the figure radiators are sold by.

What is a BTU and how does it relate to kW?

BTU (British Thermal Unit) per hour is a measure of heat output used for radiators. 1 kilowatt equals about 3,412 BTU/h. The calculator shows both so you can match either rating.

How is this different from the Heat Loss Calculator?

This radiator calculator is the quick, consumer version for sizing a single radiator in a room. The Heat Loss Calculator is the technical version that estimates a room's full heat loss for designing a whole heating system, boiler or heat pump. Both use the same underlying physics.

Should I round up or down when choosing a radiator?

Round up. A slightly oversized radiator warms the room faster and can run at a lower temperature, which is more efficient — especially with modern boilers and heat pumps. Undersizing means the room never quite gets warm on the coldest days.

Do bathrooms and lounges need more heat than bedrooms?

Yes. Bathrooms are heated to a higher temperature and lounges are used most in the evening, so both need more output per cubic metre. Bedrooms and kitchens (which gain heat from appliances) need a little less. The calculator adjusts for this automatically.

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