Any reliable BTU calculator?

[xyz321]

Hi, as per my other thread I am looking to replace all radiators in house. For replacement as suggested here I should do BTU calculation. I tried a few (B&q, plumb nation, home supply) and all give quitw different output. Difference is over 20%. So I don't know which one to use?

Any suggestions please. Thanks.

 

[SandyLyon]

Building insulation values and ventilation requirements have changed so mouth over the years.
I always did calculations "long hand" using the appropriate "u" values and air changes, most of the other guys I went to college with used a "Mears" calculator (no computers in those dark days). Even back then there would be discrepancies between different methods.
Stelrad, Myson etc also do online or downloadable calculators, you need to know the construction values of your property and any other factors that may effect the calculations.

 

[Terrywookfit]

Better too large than too small....Faster heat up and TRV will turn off.