You have to measure. You can't convert from your Western size to an English size.
Place the saddle, with pad, on the horse in the correct position.
Measure from the 2nd set of holes on the offside to the 2nd set of holes on the nearside. It's best if someone is pushing down on the saddle while you measure. If no one is pushing down, you may want to measure from the 2nd set of holes to the first set. When the saddle if fully girthed up and tightened, you want to be using right around the 3rd set of holes.
If you don't have a flexible tape to measure with, use a piece of string, then measure the length of string when you're done.