Online BTU calculators don't agree with each other

28 Jul 2007
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United Kingdom
I need to replace an existing radiator and I'm trying to get a figure for the required BTU.

It’s a bedroom measuring 3m x 4m x 2.3m. The floor is wooden with a heated living room below. The ceiling is below a pitched roof with at least 100mm of insulation. There are two outside walls with an insulated brick cavity. The window is about 3m square with sealed double glazing.

The old radiator is a single non finned panel measuring approx. 1600mm x 500mm.

I've tried various online BTU calculators but the suggested required BTU varies wildly. I've had figures from just below 2000 BTUs to over 5000 BTUs and I don't know what to believe.

The lowest figure came from the calculator at This is the only calculator I’ve found that asks questions about the insulation of the walls, floor and ceiling.

I've just found another BTU calculator at that also asks about the room insulation which also gives a figure of just under 2000 BTU.
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I've got a new 2794 BTU radiator I was bought for another room and never got round to fitting. Hopefully it'll be just about big enough for this job.

Ooops, I just re-read the last post and noticed the figure was in kW. That would appear to be 10242 BTU!
Exactly! I was using the whole of house method - admittedly 3M*4M single story is a small house. I also tried playing around with some of the U values to see what effect it would have - it didn't have much.
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All the calculators do seem a bit erratic yer right .......

Its all very scientific...... I suggest this approach ...... Go to the plumbers merchants & see whats on special ...... Its as gooder system as any other

For a bedroom temp of 18C Mears calculator gives 1150 watts which then is divided by 0.7 for the heat gain from the living room below. This equates to 805 Watts/2747 btu's, so your rad is about right.

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