Every saw, no matter how good, will become dull once in a while -- especially if it is used a lot. You can resharpen a dull chain many times, but eventually it will have to be replaced with a new chain. Though there are standard lengths from 10 inches to 42 inches, chains come in several styles and link types that affect length, so it is important that you know how to correctly identify the chain for your saw.
Place the end of a measuring tape against the front of the chainsaw housing where the blade emerges from the saw.
Stretch it out to the tip of the saw at its furthest extent. That measurement is the "cutting length" of the bar, and determines the size chain you will need to buy. (Round up to the nearest even number. For example, if the bar measures 15 7/8 inches, purchase a chain for a 16-inch bar.)
Count three rivets in the chain and measure the distance from the centre of the first rivet to the centre of the third rivet. Divide the resulting number by 2 to get the number for the pitch of the chain.
Count the number of drive lengths in your chain. (The drive links are those that point downward like teeth and fit into the groove on the bar to "drive" the chain around when the saw is turned on.) Drive lengths come in different sizes, so the number of links affects the final measurement.
If you do not feel secure about your bar measurement, or plan to purchase a chain and new bar of a different size, take the pitch number and the number of drive lengths to a chainsaw supply store. Chainsaw experts can check those numbers against a chart to determine the chain size.
Stihl saw chains will have the pitch and gauge numbers on the side of the chain teeth, saving you the bother of measuring and counting them.
Chainsaw chains are very sharp. Always wear heavy gloves when handling chains!