To determine the size of belt for the mower deck, you need to know the brand, the model and year of the tractor. The different manufactures use different belts on the same size mower deck ie John Deere 38" deck is different that a Cub Cadet 38"deck. The belt size varies from Model to Model ie John Deere LT150 is different from an John Deere LA 100.

IF your lawn tractor is a John Deere STX-38 (which I say because many of the STX's are 12 HP with 38" deck), the manual says that the mower deck belt is 95.5 inches, plus or minus 0.4 inch. If your mower is anything else, I can't help you.

Well, I guess Mr. Gumpster didn't care enough to check back on his post. Guys like that are the reason that I'm about to give up on this forum.