How to Measure a Belt
Master Leathersmith Bill Keitel of the Buffalo Billfold Company gives complete and correct step-by-step instructions on how a belt should be measured.
Measure twice, order once. If unsure, always order 1 size up.

Caveats
Not everyone measures a belt the same. There are many resellers of belts who source their belts from China, Mexico, India, and elsewhere. These resellers don't always know the correct way to measure a belt as a true leathersmith does.
Therefore, it's important to double check how each company measures their belt. Don't assume you know how to measure a belt, and don't assume it's your waist size.
If you're interested in getting a real leather belt, made by a real leathersmith with over half a century experience, we recommend you check out these belts.