ere are some typical questions that are asked about credit:

"Have I applied for too much credit?"

"Why can't I get credit even though I always pay my bills?"

"Why does my girlfriend have a better credit record than I do even though she has 10 credit cards and I have only one?"

Questions like these reveal that most Americans don't understand how credit works. You know that no one who grants you credit today is just going to "take your word for it" that you'll pay back the loan. But what you need to understand are what credit issuers consider "scientific methods" for determining whether you qualify for a credit card, a car loan or a mortgage. Otherwise, you may not get the money.

Credit issuers rely on computers to compile profiles of good credit risks and bad risks. They look at what you have in common with each group. And that determines, to a large extent, whether you get the loan. One of the most important factors is the debt-to-income ratio.

As credit expert Gerri Detweiler explains in her book, The Ultimate Credit Handbook, it's not just a matter of whether you can afford to pay back a loan. The lender wants to measure how deeply you're in debt. The way he does that is by measuring the ratio of your income to your debt.

For example, mortgage lenders generally won't approve your loan if your mortgage debt would exceed 28 percent of your income. Your total debt ù including all other debts ù should not exceed 36 percent to qualify for a mortgage. That doesn't leave you much room for the rest of your bills.

These debts don't include food, utilities or taxes that you pay. For these calculations, mortgage lenders look at items like credit card bills, student loans and car loans and how your mortgage would affect your overall ability to pay.

Detweiler offers specific steps you can take to see how a credit issuer views your debt and income. Gather up your most recent credit card statements. Call the bank that holds your car loan and get your current balance. "This is very important," Detweiler says. "Your rough idea of your balance is not good enough."

Detweiler suggests that you list all your bills in one column. In the second column list your monthly payments. In the third column list your balance due.

Of course, credit cards don't have a monthly payment. Detweiler suggests that you ask your credit card company how the balance is computed. Or she provides this option: Estimate your monthly payments as 4 percent of the balance. In other words, if you owe $1,000, list your monthly payment as $40 ($1,000 times .04.) "That's a little high," Detweiler says, "so it puts you on the safe side."

Now you must figure your monthly income. Start with your gross annual income, which is your income before taxes. Add to that ù also on an annual basis ù any other income such as investment earnings, child support, alimony, Social Security benefits and free-lance income from consulting. Detweiler says you should not include any overtime or bonus money unless it's guaranteed. If your income is based on an hourly wage, multiply your average weekly paycheck by 52 weeks to come up with an estimated annual income. Be sure to use the gross income figure, which is your weekly income before tax.

Now divide your monthly debt payments by your total monthly income. So, if your total monthly income is $2,500 and your total monthly debt payments are $850, your debt-to-income ratio is $850 divided by 2,500 or .34, which is 34 percent.

What does it mean? Detweiler provides these guidelines:

10 percent or less: You probably won't be surprised to know you're in great shape. Because lenders view you so favorably, make certain you have good, low-rate cards.

11 percent to 20 percent: You shouldn't have trouble getting loans. But as you approach 20 percent, Detweiler says you've probably taken on too much debt. Scale back, particularly if buying a house is on your agenda

21 percent to 35 percent: Stop charging! Although you probably aren't having trouble getting new credit cards, you're spending too much of your monthly income on debt repayment. You're probably having trouble saving money.

36 percent to 50 percent. Cut up your cards and develop a plan to get out of debt. Debt Counselors of America can be reached through its Web site or by e-mailing counselor@mail.dca.org. But don't panic. You're not alone. Get started whittling down your debt.