Depends if you have Long or short points on your saddle
borrow a friends girth if you can and see if you need to go up or down in size
Measure from side of the gullet of your saddle around your horses forth to the other side of the gullet.
It depends a lot on the size of the horse. My 15.2h mare needs a 48in girth, while my 17.2h gelding needs a 56in. However, one mare at my barn is only 16h and needs a 52in. (She's chubby) Also, if the girth has elastic on both sides, it will stretch more than a girth that only has it on one side.