How are wheels measured?

houndlegs

Afternoon all
How are wheels measured?
Is it the actual rim diameter,or diameter of tyre?
I'm trying to figure out what size wheels I need to replace some knackered ones on an old bike I'm doing up.
The wheels measure 605mm(just under 2ft) across the rim diameter.
So,are they 24 inch wheels?

Many thanks

ugo.santalucia

http://sheldonbrown.com/rim-sizing.html

cycleclinic

Sheldon knows!

houndlegs

Many thanks