Credit Utilization
Also called: utilization ratio, credit utilisation, balance to limit ratio
Credit utilization is the share of your available revolving credit you're currently using, and it drives roughly 30% of a FICO score. Below 30% is the common advice; below 10% scores best. The mechanic that catches people out is timing: most issuers report the statement balance, not the balance after you pay, so paying in full every month can still register 60% utilization. Paying down before the statement date fixes it.
Formula
Utilization = Statement balances ÷ Total credit limits