If you have a small waist, or you find that your jeans gap at the waist in the back, you can alter the back seam.
The first step is to measure your waist (where the waistband of the jeans sits).
Second, measure the waistband of the jeans. To do this, button the jeans. Pull the waist taut, and measure the waistband from side seam to side seam. Multiply this measurement by two.
Third, subtract the body waist measurement from the jean waistband length. The difference should be divided by two to determine the amount of the alteration.
The jean’s waist measures 16 inches from side seam to side seam. Therefore the waistband measures 32 inches. (16 x 2)
The body measurement at the waist is 30 inches.
The difference is 2 (32 – 30 = 2).
Divide this by two (2 / 2 = 1).
The alteration amount is 1 inch.