WASHINGTON — If you're thinking of buying a house, there's one number that's more important than all the others.

It's not your salary. It's not your savings account balance. It's not even the price of the house.

It's your credit score.

In just three digits, that score tells lenders just about everything they say they need to know about how likely a person is to pay back a mortgage loan in a timely manner. The more risk potential home buyers pose, the less likely they are to get a loan with the lowest possible interest rate and the best terms. It's the rare lender who looks only at a credit score, but a low score will put you in a bad position.

What people don't know is that they can goose their credit scores relatively quickly with a few small steps, lending and credit experts say. But first they must find out what their scores are and understand what they mean.

Most consumers neglect to do that, surveys have shown. About 97 percent of people have no idea what their credit scores are, and 86 percent did not check their credit reports last year even though doing so is free, according to an informal survey by Credit.com Educational Services in San Francisco.

"Many people don't know where they stand and they don't know that they can improve their standing," said John Ulzheimer, the group's president. "Some think their credit is good and it's not, and others think their credit is bad and it's not."

Ryan and Colleen Kelly fell into the latter category. With $30,000 in credit card debt, they figured their chances of getting the mortgage they needed to finance the home they wanted would be slim.

But this week they closed on a new home that they could buy with an interest-only mortgage that requires a very low monthly payment. In a few weeks, they'll be out of an apartment and into a townhouse in Olney, Md. Their plan is to use the money they save each month on their mortgage to pay down their credit cards.

"I definitely thought our credit is worse than it is," said Ryan Kelly, 29, an administrative assistant at the National Institutes of Health. "We don't get late-payment calls or creditors calling. But we still carry a balance on our credit cards, and I was concerned that would affect our credit score."

It did. But not as much as they expected because the couple had taken steps in previous years, perhaps unwittingly, that offset the potential damage.

For starters, their credit cards had been open several years with an excellent record of on-time payments, which helps assure lenders that the couple will not fall behind on house payments.

Because many home buyers do not understand that concept, they often rush to close longtime accounts in good standing before applying for a mortgage. In effect, that weakens their credit scores. Open credit cards do not hurt your credit score. What hurts is having credit cards that are nearly maxed out.

The nation's most widely used scoring formula, called FICO was adopted widely by mortgage lenders in the late 1990s after Fannie Mae and Freddie Mac endorsed it.

The nation's three largest credit-reporting agencies — Equifax, TransUnion and Experian — use FICO software to calculate credit scores. They then sell the scores to lenders that underwrite car loans, credit cards and mortgages.

In FICO's estimation, whether someone pays their bills on time is such a good predictor of creditworthiness that payment history makes up 35 percent of a FICO score. That's followed by the amount owed, the length of credit history, the types of credit involved and new credit opened.

Catherine Cavanaugh took a hit on all those fronts when she got into financial trouble a few years ago, stopped paying her bills and ruined her credit rating.

In 2000, she filed for personal bankruptcy protection, and her Silver Spring, Md., townhouse was foreclosed on later that year. Department stores stopped extending her credit. Landlords refused to rent her the apartments she wanted. She could not secure a cell phone account.

Then two months ago, Cavanaugh attended a home-buyers education workshop and began working with a credit counselor.

Weeks later, Cavanaugh's FICO score was up to 662 from 408, she said. Now she has one closed credit card left to pay off.