Credit utilization — how much of your available credit you're actually using — is the second-biggest factor in most credit scoring models, right behind payment history. It typically makes up about 30% of a FICO score, which means paying every bill on time still isn't enough to guarantee a great score if your balances are running high relative to your limits. * The math behind it is simple: divide your total revolving balances (mainly credit cards) by your total available credit limits. Max out a $5,000 limit card at $4,500, and your utilization on that card alone is 90% — a number that scoring models treat as a serious red flag regardless of how reliably you've paid in the past. * Both FICO and VantageScore models generally recommend staying under 30% utilization, but the people with the very highest scores tend to run considerably lower than that — often under 10%. Getting your utilization below that 10% threshold can boost your score by an estimated 10 to 50 points compared to running in the 30%-plus range. * The damage isn't linear, either. Utilization between 10% and 30% is generally treated as fine, with little noticeable impact. Cross above 30%, and the hit becomes more visible. Cross above 50%, and scoring models can treat it as a real red flag, potentially dropping a score by 50 to 100 points or more. * One detail that catches people off guard: some scoring models look not just at your overall utilization across all cards, but at the utilization on your single highest-balance card individually — meaning maxing out one card can hurt your score even if your total utilization across all your cards combined looks reasonable.