Which btu calculator?

Have measured my room(s) dimensions, window sizes etc and have had quite varying results. For a start, none seem to accommodate a 'normally' (1900's) built stone terrace house with a cavity (they have options for brick etc) I mean how thick would the stone be anyway?
One site quoted 1.1 Kw and another 1.9Kw, so quite a difference in rad size there. Even if I went for the 1.9kw the quote didn't say wether that was 'just enough' or over sized for a lower boiler flow temperature. Looking at the room in general, it's not massive and I'm sure if I had a 2kw electric fire on (for comparison) it would be roasting. As a guide, the front room is L 5m, W 3.3m, H 3m, with a plastic double glazed bay window totalling 4.8 m2, 3 internal walls (mid terraced stone), heated upper room, suspended wooden floor, closed chimney breast.
Btw one site said I'd need 2 radiators ! Where would we sit?
Any good guides that people use?
Thanks

We use Stelrad stars pro version its capable of almost any type of structure. But you do have to have a fair understanding of how heat calculations work to get best from it.
I've looked at most 'diy' heat calculations on line most I've seen tend to be oversized.

Johnytuono said:

Btw one site said I'd need 2 radiators ! Where would we sit?

Click to expand...

Between the two should be a nice warm location

But I used 3 different sites and just took the room worst case from each site to size my boiler and accepted that these sites are not an exact science and left it to my TRVs to fine tune the actual heating of the rooms

Sample output of spreadsheet showing differences