The outseam measurements of a pair of pants are located along the outside leg seams. One of the purposes of the outseam measurement is to provide men and women the correct length needed for a proper hemline. Inseam measurements, which are also used to measure hems, often cause some embarrassment for the person being measured, since it means measuring from the crotch and private area as opposed to the less intrusive outseam measurements. Since doing either measurement requires two people, outseam measuring is preferable to make both parties comfortable.
Put on the pair of pants that need hemming. Measure the pants from the top of the hip bone to the bottom of the ankle bone. This is the length of your outseam without any modifications. Another way is to measure from the natural waist of the pants to the ankle bone. This is a good way to get the proper measurement if you wear pants higher on your waist.
Wear the shoes you’ll be wearing with these pants. For men, measure outseams from the tip of the hip bone to the top of the shoe welt. If your pants are dress pants, wear the dress shoes that you will be wearing with the pants so that the front of the pant can be measured against the top of the shoe. If they are jeans, wear the shoes you’d normally wear. The hem should brush the top without creasing when hemmed and doesn’t bunch around the shoe.
For women, measure the outseam from the tip of the hip bone to the floor. If you want a longer leg look, wear the highest shoes you’ll wear with these pants so that the measurements can be made to the floor and ignore the ankle bone stopping point, unless you are wearing flats. Since women’s shoes are slanted, the hem break will basically hide the heel and break gently across the top if the outseam is measured and sewn correctly.
Mark the outseam measurement at the ankle, or floor, level. This measurement then becomes the benchmark for adjusting the hem of the pants. In most cases, men’s trousers are three-quarters of an inch shorter in the front, where it breaks over the shoe welt, than in the back. For women, this adjustment to the front and back lengths are dependent upon the height of the heels, but usually the back is longer than the front in relation to the length of the outseam measurement.
Chalk mark the demarcation points in the hemlines and pin them into place. You can take off the pants now and press the hems into place. Using needle and thread, hem the pants; since you’ve measured along the outseam, they should be the perfect length front and back.
Make sure you stand still and straight. Don't slouch or fidget or the person measuring your outseam will get a false reading.