OP is asking about a measurement. There are old ways and there are new ways, but one I've seen a lot is as follows.
Stand barefoot on a hard floor with your back against a wall. Don't wear bulky trousers like blue jeans: compression shorts, cycling shorts, yoga shorts or ... be creative. Pull a large book up between your legs as hard as possible so it presses against your sit bones. Press the book against the wall and have an assistant measure carefully (to the sixteenth of an inch or a millimeter would not be bad) from the top of the book to the floor. Write it down. Measure several more times and use the largest value. That is your cycling inseam length, a best-estimate of the distance from your sit bones to the hard bottom of your heels with straight, locked knees.
Many bike shops have a few apparatus around for helping with this measurement. You can do it that way, or at home using the above procedure and a family member, friend, or significant other as an assistant. Cats and dogs tend not to contribute very well.
This is just one way to measure your cycling inseam. It isn't a fit calculation method or a fitting system. You need a bunch of numbers for those, and this gives you only one of them. But, it is essential.
Last edited by Road Fan; 02-06-20 at 10:54 AM.