Use Google - you'll find several. The Barlo rads one isn't too bad. There WAS a Myson one which was better. IDHE do the dfinitive one but you have to buy it for a few pounds.
The B&Q one is cr*p - it doesn't even take into account how many external walls there are.
They're ALL krap! none of them want to know the wind speed, humidity, sunshine index........ and then stupid gits called heating engineers add the figures up, add a bit for luck throw it away and fit a 30kW boiler with 22mm pipes.
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