Chainsaws are classified by bar length and the chains are made to fit that size bar. Three factors determine the proper chain for each size saw; these are the bar length, chain length, and chain pitch. You need to know all three numbers to make or buy the proper chain. Pitch defines the size of the chain; there are several chain sizes, with 1/4-inch being the smallest, 3/4-inch the largest and 3/8-inch the most popular. The drive sprocket and nose sprocket must be the same pitch as the chain.
Choose any three consecutive rivets holding the teeth on the chain.
Measure from the center of one rivet, across the second rivet, to the center of the third rivet.
Divide this measurement by two. That number is the pitch of the chain.