The US public-company audit market is only moderately concentrated by client count (Big Four = 51% of registrants, 2025) but extremely concentrated by economic weight (~91-97% of audit fees and market capitalization) — and concentration is easing at the small-cap margin, not the core

Summary

The US audit market shows a sharp split depending on how you measure concentration. By client count, the Big Four (Deloitte, EY, PwC, KPMG) audit a bare majority of SEC registrants — 51% in 2025, up slightly from 48.4% in 2023 — and the ten largest firms together handle 65%, leaving 35% of registrants (mostly small-cap and OTC companies) to 219 smaller firms. But by economic weight — audit fees or market capitalization — the market is far more concentrated: the Big Four have held 91-95% of total US audit fees every year from 2003 to 2023 with essentially no trend, and audited roughly 97% of total US public-company market capitalization as of the most recent public estimate (PCAOB, 2017). Among the largest companies (large accelerated filers), the Big Four alone audit 89.5%, rising to 94.9% once Grant Thornton and BDO are added. So concentration at the core of the market (large, high-fee clients) has been stable and extreme for two decades; the modest erosion in the Big Four's overall client-count share is happening almost entirely at the small-cap/Nasdaq/OTC fringe, where their fee share fell from 93% to 84% (Nasdaq) and 74% to 54% (OTC) between 2003 and 2023.

What changed, and what didn't

• Client-count share is drifting down for the Big Four, then partially recovering: Ideagen Audit Analytics' annual 'Who Audits Public Companies' reports show the Big Four's share of all SEC registrants at roughly 48.4% (2023), 49.7% (2024), and 51% (2025) — Deloitte overtook EY for the top spot in 2024 largely because EY shed over 100 smaller clients while Deloitte's roster barely moved. The number of PCAOB-registered audit firms has been shrinking for two decades (from ~1,500 in 2010 to below 750 by 2023), and the number of SEC registrants overall has fallen ~54% since 2003 (from ~7,200 to ~4,700), so the client-count denominator itself has been shrinking.
• Fee and market-cap concentration has NOT changed materially: an academic working paper covering 2003-2023 (Bourveau, Fluharty-Jaidee, Mao, Ren, Schmalz & White, 'The Landscape of Auditing in U.S. Capital Markets,' SSRN 2024) finds the Big Four's share of total audit fees stayed in a narrow 91-95% band for the full 20-year window — 'remarkable stability.' The PCAOB's own 2017 estimate put the Big Four's share of total US market capitalization at approximately 97%, with even higher single-firm concentration in specific sectors (e.g., one firm audits ~92% of S&P 500 telecom-sector market cap; two firms audit ~75% of energy, materials, and IT sector market cap).
• Where the erosion is happening: Big Four fee share among Nasdaq-listed companies fell from 93% (2003) to 84% (2023); among OTC-quoted issuers it fell from 74% to 54%, with smaller audit firms now auditing nearly 90% of OTC issuers. This is a shift at the small end of the market, not among the large-cap, high-revenue clients that generate the bulk of audit fees — which is why the overall fee-share number barely moved despite the visible client-count erosion.
• Client stickiness reinforces concentration: Big Four clients switch auditors at only a 3-4% annual rate (down from 8-9% in the early 2000s), versus 8% for mid-size-firm clients and 12-18% for small-firm clients — and 89% of small-firm clients that do switch stay with another small firm. Concentration at the top is self-reinforcing partly through very low churn among the largest clients.

A rough concentration-index illustration

Using the 2025 client-count shares reported by Ideagen Audit Analytics (Deloitte 15%, EY 13%, PwC 12%, KPMG 11%, remaining ~14 percentage points split across five more Top-10 firms, and 35% spread across 219 smaller firms), a back-of-envelope Herfindahl-Hirschman Index (HHI) contribution from the Big Four alone is approximately 15²+13²+12²+11² = 659 — nowhere near the DOJ/FTC's 2,500 threshold for a 'highly concentrated' market, because the long tail of 200+ small firms dilutes an HHI computed on client counts. But an HHI computed on audit fees or market-cap share — where four firms hold roughly a quarter each of ~91-97% of the total, and the remaining several hundred firms split the rest — would land well above 2,000, solidly in 'highly concentrated' territory. This HHI figure is an illustrative calculation performed for this answer from the reported percentages, not a number published by any of the cited sources, and it does not account for shares below the Top 10 (each individually estimated as small, so their contribution to the total HHI is modest but not zero).

Data and methodology notes

The AskAmerica warehouse (askengine/sec schema) does not carry auditor-of-record data for SEC filings — a catalog search for 'auditor' returned no matching table or column, and find_recipe returned no worked recipe specific to audit-market concentration — so this answer relies entirely on external primary/secondary sources fetched live: Ideagen Audit Analytics' annual market-share reports (as summarized and linked via auditupdate.com and Accounting Today, since the underlying IAA whitepapers themselves sit behind a registration wall), the working-paper analysis by Bourveau et al. (SSRN, 2024) covering 2003-2023, and a 2017 PCAOB Board speech for the market-cap concentration figure — the most recent public estimate of that specific statistic found in this research pass. No more recent (2020s) market-cap concentration estimate was located; if one exists, it was not surfaced by this search.