Credit utilization, which accounts for a large chunk of your overall FICO score, measures how much of your available revolving credit is being used each month. The higher the ratio, the more it hurts your credit score.
You can easily do the math by dividing the sum of your account balances by the sum of your account credit limits. But don't forget about your closed credit card accounts too! Some of them may still need to be included in your calculation.
In fact, closed credit card accounts with balances and credit limits that are still reported to the credit bureaus will be considered by FICO in the calculation of credit utilization ratios. So, if you recently chose to opt out of an interest rate hike, close your account, and pay off your credit card balance at the old rate, there's a good chance that card is still affecting your overall credit utilization.
Once a closed account has reached a zero balance or the creditor no longer reports your credit limit to the credit bureaus, you can then expect the card to completely drop out of your credit utilization ratio.
