The best way to measure girth size, if measuring tape isn't available, is to try a couple of girths on after putting on the saddle. Avoid buying additional girths by measuring the first one correctly with help from an equestrian and horse riding coach in this free video on horse care and equine basics.
To measure a girth size, the easiest way to do it without a measuring tape is just to try different... a couple of girths on. We'll start by putting the saddle on. And one way you can judges without even putting the girth on is just to hold the girth up to the billicks and the end of this girth should reach to about where her fat lock is. If it doesn't reach. If it only comes to here, you know it's going to be too small. If it goes all the way down to here, you know it's going to be too long. So, I'll begin by just putting the girth on loosely. Putting it on the first hole. And what you want to find is that the girth comes to about the middle of the billicks or a little less than that on both sides. That way you know if should your horse lose or gain a few pounds, you have some adjustability and you won't end up having to get another girth. So this one's perfect and we have to put it on the third hole on this side. And up to the third hole on this side.