Identify the drivetrain
Record the number of rear sprockets and the model markings on the shifter, rear derailleur, and cassette or freewheel. Speed count is a useful starting point, but the exact product family can still change compatibility.
Verify chain and connector
Use the drivetrain maker's compatibility information to confirm the chain model and approved joining method. Some chains have a directional orientation or require a specific connecting pin or reusable or single-use quick link.
Size by the approved method
Do not copy the old link count automatically. A previous chain may have been sized incorrectly, and drivetrain changes can affect length. Follow the bicycle or drivetrain manufacturer's sizing method.
Inspect the rest of the system
A worn cassette or chainring may not mesh cleanly with a new chain. If shifting skips, binds, or makes unusual noise after installation, stop riding and have the drivetrain checked.