Chainsaw chain size: pitch, gauge and drive links
A chainsaw chain is described by three numbers, and all three must match your saw. The bar length in inches is not one of them.

Pitch
Pitch is the spacing of the links: the distance between any three rivets, divided by two. It must match the drive sprocket and the bar nose.
Gauge
Gauge is the thickness of the drive links where they ride in the bar groove.
Drive links
Drive links are the tabs under the chain. Their count is the chain's length. Two chains for the same bar length can have different counts if the pitch differs.
Drive links by bar length and pitch
| Bar | Pitch: valid drive links |
|---|---|
| 6" bar | 1/4"
3/8" LP
|
| 8" bar | 3/8" LP
|
| 10" bar | 3/8" LP
|
| 12" bar | 3/8" LP
|
| 14" bar | 3/8" LP
|
| 15" bar | .325"
|
| 16" bar | .325"
3/8"
3/8" LP
|
| 18" bar | .325"
3/8"
3/8" LP
|
| 20" bar | .325"
3/8"
|
| 21" bar | 3/8"
|
| 24" bar | 3/8"
|