If you want it as accurate as possible, then inflate your front tire to your normal riding pressure, sit on the bike, and measure how far it rolls in one wheel rotation while you're sitting on it.

The usual method is to make a mark on the tire, align the mark to a starting mark on the floor, then roll forward until the mark on the tire is pointing at the floor again, jump off, and mark the floor with your ending point. Now measure the distance between the starting and ending marks.