First up- chainsaws do not have blades- they have bars.
Chains are ordered to fit a system- ie: they must match the drive sprocket of the saw and the specifications of the bar.
You must know and match, pitch, gauge and drive link count.
If still legible- all this information should be stamped or etched into the bar near the end that mounts to the saw.
A 20 inch chain refers to the length of bar that loop is made to fit- so a 20 inch chain fits a 20 inch bar.
In 3/8 this is one of the few that will interchange between Husqvarna and Stihl bars and will be 72 drive links.
In .325 gauge it will be 76 drive links.
So in order to order a chain to suit a bar, you need to know-
1: Bar length
2: Pitch size (3/8, .325, 3/8LP etc)
3: Gauge (thickness of the drive links to suit the gap in the bar rails) 0.050, 0.058 or 0.063
4: Drive link count (number of drive links required to complete a circuit of the bar and drive sprocket.
If you have an old chain that suits the bar in question- you can count the drive links of it and order the same. It is also possible to measure the pitch and gauge off of an old chain- but easier to find that information on the bar stampings/etchings.
