I bought a new alternator, a 100 amp, to replace the stock 51 amp.
The new one is a bit larger from front to rear and the rear cover hits the motor mount bracket that’s attached to the port stringer. With it hitting, all I have is about 2” of adjustment travel. I don’t know if there is anything under the cover in the area where it hits or I could “modify” it to fit. I really don’t want to cut or grind the mount to gain clearance.

So I need a shorter drive belt.
What is the best way to measure so I can hopefully get the correct size without running back and forth to the parts store?

I cut the belt then overlap and clamp the two ends together with vice grips to get a reasonable fit. Then deduct the amount of overlap from the original belts length. Be sure and get the same type/series of belt when doing it this way.

If available get one half inch longer and shorter to save yourself a trip to the parts store.

I keep the belt wrapper and write it's use and keep it in my tool box.

I had that Idea, but I thought there may be some better way.
I'll try it.
Thanks.

just put the adjustment at mid way and wrap a tape measure around the pullies.... thats your belt length. if you look at your belt, youll notice the part number has the belt length in inches and decimal built into it... the new belt will done the same way so you can actually predict the part number... or closest

Run a piece of string around the pulleys, and measure it.