How do I measure for new inner tubes on ancient MB Terrain?

[Grumpla]

I'm not sure how to measure them - is it the diameter of the wheel hub, of the outer rim of the tyre, or wot?

I'm feeling very sleep-deprived and confused. Can any of you folk help me out? Any recommendations of where to order them from that might deliver fairly quick?

It is an old Terrain (at least six or seven I think, I bought it third-hand) if that helps. Red frame and dark green covers.

Bump

I had a landrover pushchair and used to take it to my local bike repair shop.
They would replace the inner tubes for me.
You could simply do that and not have too worry about finding out the size!

Yup that's what I did! Kind lady scraped mud off wheel and pointed out where it says 12 1/2" too

Yes, just like a car tyre the sizes are written on the sidewall of the tyre.

Its only really the first number that is important for inner tubes. There is some confusion between various size nomenclatures as well 12.5 x 2.25 is the same as 47-203.