The Inseam is the classic measurement used by tailors everywhere to hem a pant. It runs along the inside of the leg, from the crotch to the welt of the shoe, and has been the source of a lot of humor about pant fittings. Because of that, many of us use the, ahem, “less personal” Outseam measurement….
You will need someone else to take this measurement for you. The Inseam is measured from the crotch of the pant to the welt of the shoe. In a proper men’s shoe, the welt is the top of the sole. Many modern shoes, especially women’s shoes, but that’s another story. The general idea is that you’re measuring to where you want your pants to stop, which is generally about level with the bottom of the heel in men, or the actual floor in women. (I’m not trying to be a sexist here. Most women have a goal of making their legs look as long and lean as possible, and making your pants go clear to the ground without a cuff is a great way to do it. Men seem less concerned, and generally wear flatter shoes that mess up the line of the pant in front if it’s too long.) It’s really important to take this measurement wearing the shoes you’ll be wearing the finished pant in. Stand with the feet shoulder width apart, in a normal posture with the weight evenly distributed on both feet. Don’t look down, don’t flinch, don’t fuss, and warn the person taking the measurement if you’re particularly ticklish.
The Outseam measurement is taken from the level of the pant crotch to the welt of the shoe (or preferred target length for the pant), along the outside of the leg. It’s technically less accurate because you’re sighting round abouts where you believe the level of the crotch is rather than having your hand right up in there, and men are decidedly particular about how much space they need in a pant so the crotch isn’t always at the anatomical crotch like you’d expect it to be. On the plus side, your hand is not in anyone’s crotch. (I’ve only ever had one actor complain that he felt short-changed by this. Usually they’re pretty ok with it.)