Radiator sizing software?

[Harnser]

After this cold spell I'm trying to work out what size radiators I need in this house as I think the current ones are undersized.

I've been using https://app.starsapp.co.uk/ from Steelrad but it's clunky and slow to use.
And it gives 2 heat loss figures for each room that are a factor of 2 out from each other.

Is there something better that the pros use? I am willing to pay for software that makes my life easier.

 

[Harnser]

I'd be doing the calculation based on expected U values .. for the era of house you have (earlier post) which can be 3x worse on an 80's property versus 2010's
then https://www.diydata.com/planning/ch_design/example1_imperial.php

The Steelrad calculator is good like that; you have to select the materials for each wall, type of door etc:

Edit: The ceiling choices miss an option for downstairs room with heated room above.

I have had to guess the type as the house is rendered. The 1920s part has cinder block inside, the extension some kind of cavity fill so assuming this one above is right for the new part.