Measure 13 inches from the crotch down the inside seam of the leg. Wear a belt or rope around the place whee you want your jeans to rest so that it will also help you to measure your length and crotch.

Women S Jeans Zara United Kingdom
Two body measurements that are required when buying jeans. 2628 2830 etc womens jeans are usually assigned a number based on their waistinseam measurements ie. The waist measurement of a pair of jeans is in most cases not the same as your natural waist at your belly bottom. Measure the circumference at the place where you will actually wear your jeans. So if you want a low mid or high waist jeans you will measure the low mid or high waist circumference and not your natural waist. There are two ways to measure your jeans size. 1 by measuring an existing pair of pants and 2 by taking body measurements.
From this point 13 inches down from the crotch measure from one side of the pant leg to the other the width of the knee. While measuring position the tape very close to the body without tightening. To measure your jeans size using an existing pair of pants use a measuring tape. So if you want a low mid or high waist jeans you will measure the low mid or high waist circumference and not your natural waist. Your waist measurement will be the entire circumference of the area where your jeans will rest. Check the brands sizing chart beforehand to determine which number corresponds to your waistinseam measurements.
The width of the jeans is measured at the upper corner of the pants waistband from left to right. Although mens jeans are usually ordered by waistinseam ie. Place the jeans flat on the surface as shown in the image. While the two measurements used to be more or less identical back in the days when only cowboys and workers wore jeans the waistline has been creeping down over the decades.