Knowing how to shorten trousers is a useful skill. Men's dress trousers frequently come with unfinished hems that need to be sewn. It is also beneficial to be able to lengthen trousers, especially when children have a growth spurt and their pant legs suddenly become too short, but the trousers still fit around the waist and are in good condition. Sometimes trousers shrink in the wash and become too short in the leg for comfort. Fortunately, most trousers have a large enough hem with additional fabric hidden inside to lengthen the trousers by taking down the hem.
Insert the pointed end of the seam ripper under the loop of a stitch from the seam that holds the hem in place on one of the pant legs. Apply pressure to break the thread. Repeat every 4 or 5 stitches. Pinch a length of the thread between your forefinger and thumb from this stitch line, on the opposite side of the fabic where the thread is unbroken, and pull. The thread will unravel through the torn stitches. Unfold the hem and press with a steam iron or iron with a damp cloth. Repeat for the opposite hem.
Put on the trousers and wear the shoes you will most often wear with the trousers. Have an assistant mark the desired finished length of the trousers in the front and back of your foot where the pant should meet your shoe. Have the assistant mark a spot at the front of your foot on your instep and midway down the back of your heel at the back of your foot. Mark these spots on the trousers with pins. Fold the trousers evenly between the two pinned spots, turning the fold toward the wrong side of the trousers.
Press the folded edge in place with the iron. Unfold a length of seam binding that will fit around the circumference of the bottom of the pant leg. If the seam binding is the type that comes folded, unfold it to its maximum width. Iron the binding flat.
Turn the trousers inside out, so the wrong side is facing you. Place the binding on the wrong side of the pant leg. Align the long edge of the seam binding with the raw, unfinished edge of the pant leg with the right side of the binding facing the right side of the fabric of the trousers on the inside. The wrong side of the binding will be facing you. Pin in place. Stitch the seam binding to the pant leg with a straight stitch as close to the edge as possible.
Fold the new hem over to the wrong side of the pant leg, along the pressed fold. Pin in place.
Sew the hem in place by hand, stitching close to the open edge of the seam binding. Take small stitches through the turned edge of the hem and even smaller stitches through the pant fabric, checking the right side of the fabric as you take the stitch, to ensure the stitches are not visible from the right side.
Take several tight stitches through the same spot on the hem to bind off the stitching once the circumference of the pant leg has been completed.
Repeat steps 3 through 6 for the other leg. Press the hems in place.
Check that you have at least 1 inch of extra fabric in your hem to make taking the hem down worthwhile.
It's best to lengthen trousers before they have been worn, as the original fold on the hemline might remain visible, especially if the edge of the fabric has been scuffed.