The first thing we need you do is work out how long your chain is. The length of your chain is determined by counting the number of drive links in you chain (this is because chains stretch during their lifetime).
The diagram above shows the drive link, it is the tooth on the inside of the chain loop; it sits inside the groove of the guide bar. The easiest way to count them is to take the chain off the saw. Sometimes these drive links may have a code stamped into them, this will vary by chain manufacturer but it will identify the chain type for you.
If this marking does not exist then we need to next work out the pitch of the chain. The pitch is the distance between three of the chain 'rivets' divided by two.
Chain is made in several pitches - 1/4" is the smallest, 3/8" is the most popular, 3/4” is the largest. Pitch is important because the drive sprocket must be the same pitch as the chain
Chain gauge is the drive links thickness where it fits into the bar groove you may need to use vernier callipers to make this measurement.
There are a number of common gauges for chain saws - such as, .043” (1,1mm), .050” (1,3mm), .058” (1,5mm) and .063” (1,6mm). (Chains are still measured in imperial measurements).