Childrens belts have 5 holes spaced 1″ (25mm) apart and the belt sizes are the measurement from the buckle (but not including it) to the tightest (first) and loosest (last) holes. The sizing of the belts allows for maximum growing room so for example if your child's waist is 18″, the belt will fasten on the tightest or next hole out, allowing 3 or 4 more holes of expansion.

Use the size guide below to choose the most suitable size and there is also an easy Metric conversion guide

Childrens belt sizes
Order waist size  Classic belts 
5 holes at
18″ 18″-22″
20″ 20″-24″
22″ 22″-26″
24″ 24″-28″