Boiler capacity

We're about to start an extension & other work on the house and will be adding 4 new radiators. I need to find out if our boiler has enough spare capacity to cope with these extra rads. Can anyone tell me how to work out the capacity required and how to figure out te existing capacity of the boiler.

I have looked through other threads but could not find anything along the lines of a formula for working this out.

Thanks

It depends on your requirement for energy. How high is that?
You can't ask someone else which shoe might fit you. Even in the shoe shop you have say a size, roughly at least.
You haven't given any details(size of the existing building, size of the old boiler, state of insulation, occupant's behavior , holes in the walls like open fires, size of the extension, is the boiler also responsible to heat the DHW etc.). Honestly, Your question isn't one.
But to make things easier for you and us: How long does the old boiler stop between firing up sequences? Meaning how long does it stand idle before it fires up again when in the heating modus? And then let us know how large the extension is compared to the old part. This is the the min. you would have to tell us (smiley).
A new modulating combi condensing boiler isn't that expensive b.t.w.. And worth it.