Creditors use a calculation called a credit utilization ratio, which makes up 30% of your FICO score. Next to your payment history (which makes up 35%), it has the second-largest impact on your credit score. A credit utilization ratio measures how much credit you’re using compared to how much you have available. Your credit score decreases as your balance increases in relation to your total available credit. The more unused credit you have, the better your score because it shows creditors that you’re responsible. This makes you a better credit risk. It's best to utilize no more than 10% to 30% of your available credit.