How to Know When Your Chain Needs Replacing
Chain wear is not visible by looking at it. A worn chain feels fine to ride and looks fine on the bike. The problem appears later: worn chains accelerate wear on cassettes and chainrings, which cost significantly more to replace than a chain.
A chain wear indicator tool is the standard way to measure. Mechanics use a gauge that measures whether the chain has stretched beyond acceptable limits. The measurement is taken by inserting the gauge between links. If it drops in at the 0.5% mark, replacement is advisable. If it reaches 0.75%, both chain and cassette may need replacing together.
For everyday urban commuting, chain check intervals of every two to three months are typical. Wet conditions and heavy use shorten that interval. A clean, lubricated chain lasts longer and shifts more precisely.