For demonstration purposes we have used a size 30 inch belt strap.
Start your measurement from the folded end.
Follow the line of the belt to the tip.
You determine your correct size by two indicators.
1. Center hole on a three-hole strap, notice the tape measure reads 30 inches to the third hole.
2. When measuring an existing belt strap measure to the hole you are using now.
Hint: it will be the hole that shows the most wear.