The info I have read in belt books is basically measure an existing belt, or measure from the inside of the buckle where the tang touches the rim after going thru the hole of the belt to the middle hole of the 5 to be punched on the other end of the belt. Those 5 holes are usually 3/4" on center, then there is an additional 4 to 5 1/2" of belt past the last hole. Also on the buckle end the leather is skived sloted and folded over, I usually allow 2 1/2 to 3" for that area, so 6" total. Visually easier to understand if you visit some web sites that sell belts they will have a sizing section that probably has a drawing of what I am trying to explain. Disclaimer....I do it this way but it is not the only way....just gets me in the ballpark. Never take the customers word for it, most still have the waist they had in high school in their minds eye. So a 42" would need approx 53 1/2" of leather according to my poor mathmatical mind.