Usually you do a lot of counting! There are ways/helps to do this.
This seems to be a nice breakdown:
https://johannamaskoquilts.blogspot....uilts-aka.html
Usually you figure out how big a square you need to cut your hex and then multiply how many you need to determine your yardage. This is actually one of my favorite quilting sites, it has a variety of graphs, here's one on hexes, you fill in the size you need (they are using finished edge as the size, so for cutting you want the finished size plus seam allowance)
https://www.incompetech.com/graphpaper/hexagonal/