JJ Denim demonstrates how to measure the waist size of your jeans. If you are unsure about measuring the waist size of your jeans, the information can be found in a few easy steps. To measure the waist of a jean, first lay the jean flat on the ground with the back pockets facing up. Be sure that the jean is buttoned. Next, with a tape measure, measure the distance from one side of the jean to the other. With these steps, you can be assured that the waist size of the jean that you are shopping for and that you order online or over the phone will be perfect when you receive it.