How Concentrated Are Local US Deposit Markets? Herfindahl Identities, Booking Artefacts and Merger Screens from 76,097 Bank Branches, June 2025

Bank merger review in the United States starts from the Herfindahl-Hirschman index (HHI) of deposits in local markets. I compute it for every county and metropolitan area from the FDIC Summary of Deposits for 30 June 2025: 76,097 branches of 4,431 institutions held by 4,195 owners, with 18.1 trillion dollars of deposits. I prove three exact facts used in the analysis: the HHI equals one over the number of firms plus the number of firms times the variance of shares; a merger raises it by twice the product of the merging shares, before any competitive response; and the HHI lies between the square of the largest share and the largest share. Local markets are concentrated: across 3,194 counties the median HHI is 2,889 and 78.0% exceed 1,800, with 237 one-owner counties. The deposit-weighted mean is misleading, because 77 branches above the 99.9th percentile hold 37.8% of all deposits, mostly balances booked at head offices; capping them lowers the deposit-weighted county HHI from 2,807 to 1,716. Non-metropolitan counties are far more concentrated (median 3,376) than metropolitan ones (2,044). Among hypothetical mergers between two of the eight largest branch-network owners, the 1,800 and 200 point screen would flag 821 county overlaps and the 1,800 and 100 point screen 1,027, a 25% increase.