You should take your waist measurement and order the closest size. In case of doubt between two sizes, always order the biggest size. Note that the length may vary up to 1 cm due to specific raw material properties.
IMPORTANT! If you are measuring in INCHES, please, order one size bigger.
If you would like to measure a belt (one that you already have) instead of your actual waist, please, note that you need to first understand how to correctly measure a belt. See image below: