Aubier belt sizes generally reflect the size of the pants you are wearing. However, it is common to see significant differences in sizing from one brand of pants to another. For example, a 32 from Frank and Oak will not be the same size as a 32 from H&M.
When choosing belt sizes, it is best to take your actual measurements. As you would a belt, thread a flexible tape measure through the waistband of your pants. Keeping the tape tight but not too tight, note the circumference obtained and refer to the table below. If you do not have a flexible tape measure, you can use a string and then measure its length.
In cases where your actual measurement is close to a minimum or maximum, it is best to choose a size that allows a bit of wiggle room. If your actual measurement is 35, opt for a 32 belt rather than a 34 belt.