Utilization (util) = amount owed / credit limit.
So use the balance (amount owed) that shows on your credit reports or on your statement and divide it by the credit limit, and that is the percentage of your available credit that you have utilized. After you do the division, multiply the answer by 100 to turn it into a percentage. So, 230 / 1000 = 0.23. Multiply 0.23 by 100 to get 23%.
Around here, we talk about individual util, as in the example above, and overall util, as also seen on Timothy's example. Both are important. To get overall util, add up all your posted balances, and divide by the total of all your credit limits.
Installment loan util is important, but it's calculated separately from revolving (credit card) util, so make sure your separate out your accounts by type before working this out. Revolving util is the one that we mostly obsess about!
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007