There are three measurements that every chainsaw user must know ahead of time to match the right size chainsaw chain to their chainsaw:

1. The number of drive links (yes, they must be counted)
Count the number of drive links on your chain. (A drive link is the "tooth" of the chain.)
The number of teeth, or drive links, equals the length of the chain. Write down this number.

2. Pitch (how close together links are on the chain.)
Calculate the pitch of the chain. The pitch is the distance in inches between any three rivets, divided by 2.
The rivets are the "bolts" holding the links of your chain together.
Measure the distance from the center of the first rivet to the center of the third rivet.
Divide this length in half and write down this number.

3. Gauge (the thickness of the drive links )
Measure your chain's gauge in inches and write it down.
The chain gauge is the width of the notch in the drive link where the link fits into the groove of the bar guide.
This will be a very small number, around 0.05 inch.
View the drive link from the end -- not from the rivet side -- to see the notch.