A credit score is a number based on a statistical analysis of a person's credit files, that in theory represents the creditworthiness of that person, which is the likelihood that the person will pay their bills. A credit score is primarily based on credit report information, typically from one of the three major credit bureaus: Experian, TransUnion, and Equifax. Income is not considered by the major credit bureaus when calculating a credit score.

There are different methods of calculating credit scores. FICO, the most widely known type of credit score, is a credit score developed by Fair Isaac Corporation. It is used by many mortgage lenders that use a risk-based system to determine the possibility that the borrower may default on financial obligations to the mortgage lender. The credit bureaus all have their own credit scores: Equifax's ScorePower, Experian's PLUS score, and TransUnion's credit score, and each also sells the VantageScore credit score. In addition, many large lenders, including the major credit card issuers, have developed their own proprietary scoring models.

Americans are entitled to one free credit report within a 12-month period from each of the three credit bureaus but are not entitled to receive a free credit score. The three credit bureaus run Annualcreditreport.com, where users can get their free credit reports. Credit scores are available as an add-on feature of the report for a fee. Obtaining a free credit report or buying a credit score from annualcreditreport.com has some disadvantages relative to alternative ways to obtain your report or score. If the consumer disputes an item on a credit report pulled using the free system, the credit bureaus, under the Fair Credit Reporting Act (FCRA) now have 45 days to investigate, rather than 30.

Alternatively, consumers wishing to obtain their credit scores can in some cases purchase them separately from the credit bureaus or can purchase their FICO score directly from Fair Isaac. Credit scores (including FICO scores) are also made available for "free" through subscription to one of the many credit report monitoring services available from the credit bureaus or other third parties, although to actually get the scores for free from most such services, one must use their credit card to sign up for a free trial subscription of the service and then cancel before the first monthly charge. Credit Karma reportedly offers truly free scores and is supported by advertising instead of user fees. Until March 2009, holders of credit cards issued by Washington Mutual have offered a free FICO score each month through the bank's Web site. (Chase, which took over Washington Mutual in 2008, announced that this feature would be discontinued in March, 2009.)

Under the FCRA, a consumer is entitled to a free credit report (but not a free credit score) within 60 days of any adverse action (e.g. being denied credit, or receiving substandard credit terms from a lender) taken as a result of their credit rating.

The FICO credit score ranges between 300 and 850. The VantageScore score ranges from 501-990.