Credit Utilization
Credit utilization is the percentage of your available credit that you're currently using — your card balances divided by your credit limits. It makes up about 30% of a FICO score, and keeping it below 30% (ideally under 10%) is one of the fastest ways to improve your credit.
Utilization is the fastest lever on a credit score, because it is recalculated every time balances are reported and carries no memory — last year's maxed-out card stops mattering the moment the balance is gone. It is measured both per card and across all cards, so one card at 95% can hurt even when the overall figure looks healthy. The detail that catches out careful people: issuers report the balance on the statement date, not after the due date, so paying in full every month can still report high utilization unless you pay down before the statement closes. Raising a limit works the same way as repaying — $3,000 on a $10,000 limit is 30%, on $15,000 it is 20%.