Originally Posted by GrayJay

Dont forget 507mm for 24" kid MTB's, 571mm for 650c (26" narrow road tires) both are fairly common. There are also a couple of other 20" wheel sizes that all use different rims. I
nterestingly, the outer diameter of the tire for both 507mm and 571mm are very close. I converted a 24" mtb frame into a road bike for my daughter by switching it to 650c wheels and fork and drilling the frame bridge for a road brake on the rear tire.