How do I work out what size radiators I need?

[Deep86]

I'm planning my refurb and for budgeting reasons I want to start looking at what size radiators I need:
There are so many calculators online but they all give me very different results!

All my external walls are 220mm solid brick and all new walls will be brick insulated cavity block walls (can't wait for that)
Are there any calculators that people can recommend? I've been using the Stelrad one and for the living room it's telling me I need a 11,500 BTU radiator, and for bedroom 4 it's 10,500 which is weird as it's got a heated room below and above it.

Spoiler: Floor 1

Spoiler: Floor 2

Spoiler: Floor 3

Any guides people can recommend? I don't want to hire an expert yet as no work has begun yet.

 

[motorbiking]

U value of the new and old are vastly different which is why the two hugely different areas requirements are so similar. A lot of plumbers go on 100w per m2, rather than calculate the specific U value. In the old part that will leave you under spec'd. You also have to take account of how quickly you want things to heat up. Personally I would over specify and use TRVs. You wont waste heat over specifying it will just heat up quicker.