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 calculated both per-card and across all cards. Because issuers report balances on the statement date, paying down a card before that date can lower reported utilization even if you pay in full each month. Closing a paid-off card can hurt your score by reducing total available credit.