The deposit market share is a way of measuring the size and performance of a bank in the United States based on the banks total amount of deposits.
[1] In practice however, the term is used to refer to the deposit market share of commercial banks and savings and loan organizations calculated by the Federal Deposit Insurance Corporation (FDIC).
The FDIC calculations do not include money deposited at credit unions, and the cash accounts at brokerage firms.
This metric is considered by many to be an important banking performance measure although many regard it as is misleading for the following reasons: When two banks merge, a survey is done to ensure that the combined deposit market shares will be no larger than 25% in a particular state, or 10% nationally.
If one or both of those percentages are higher than allowed, the banks can elect to still do the merger but they would need to divest (i.e. sell off branches and customer accounts) enough branches to get them within the guidelines.