Radiator Sizing

[Alan51]

How accurate are the online guides for sizing a radiator and are there any which are more accurate.
I've moved into an ex-rental house which has a relatively new Main boiler but the rads are older than God and take up so much wall space, I want to replace them with new more efficient radiators.
Thanks.

 

[muggles]

Use https://www.heat-engineer.com/ it'll cost you a small fee but it's super accurate

 

[Harry Bloomfield]

I understand over sized allows your system to run cooler and save fuel.

 

[Alan51]

I understand over sized allows your system to run cooler and save fuel.

Yes I'll still get larger heat output than needed but these are old school rads that are enormous. Some type 22 rads would be half the size.