The S&P 500 sits at the centre of the US equity market. S&P Dow Jones Indices describes it as comprising 500 leading companies and covering approximately 80% of available US market capitalisation.
There is a small numerical wrinkle. The index currently contains 503 securities, because Alphabet, Fox and News Corp each have two share classes in the benchmark. For this analysis, Big4News counts each issuer once, producing a company-level universe of 500 businesses.
The September quarterly rebalance brought Bloom Energy, Everpure and Illumina into the index, replacing Molson Coors Beverage, The Trade Desk and Builders FirstSource. Those changes took effect on 21 September 2026 and are reflected in the dataset.
For each company, Big4News identified the external auditor, the first financial year of the current audit relationship where this could be established precisely, and the latest disclosed audit fee.
Final verification completed 30 September 2026. Audit fees are shown in US dollars.
S&P 500 Audit Market Share
The Big Four audit 494 of the 500 companies
At S&P 500 level, the US audit market is effectively a four-firm market.
EY has the largest number of mandates, auditing 154 companies, or 30.8% of the S&P 500.
PwC follows with 144 companies, or 28.8%, while Deloitte audits 121, or 24.2%. KPMG has a smaller—but still substantial—share with 75 companies, or 15%.
Combined, the Big Four audit 494 of the 500 companies: 98.8% of the index.
The numbers illustrate how little of the large-company US audit market remains outside the four global networks.
That concentration has deep historical roots. The disappearance of Arthur Andersen in 2002 reduced what had been the Big Five to four firms, adding to longstanding concerns about how many audit firms have the scale, geographic reach and specialist expertise needed to audit the world’s largest corporations.
Only six companies use an auditor outside the Big Four
The entire non-Big Four share of the S&P 500 consists of six companies and two audit firms.
Grant Thornton audits four: Carvana, Diamondback Energy, Federal Realty Investment Trust and Insulet.
BDO audits two: Henry Schein and Supermicro.
There is therefore no sizeable mid-tier segment comparable with the position of Forvis Mazars in France’s CAC 40 or the broader collection of local firms represented in India’s Nifty 50.
For an S&P 500 company looking outside the Big Four, the pool of firms already demonstrating experience at this level of the US market is extraordinarily small.
EY leads overall—but the picture changes by sector
EY’s overall lead does not translate into dominance across every part of the index.
Among the 83 industrial companies, EY audits 31 and Deloitte 29.
In financial services, the work is more evenly distributed: Deloitte audits 22 of the 76 companies, PwC 21, EY 18 and KPMG 15.
PwC has a particularly strong position in healthcare, auditing 24 of 60 companies, and in materials, where it holds 12 of the 25 mandates.
Deloitte stands out in utilities, auditing 18 of the sector’s 31 companies.
Technology is highly competitive between the two largest firms by mandate count: EY audits 25 of the 74 information-technology companies and PwC 24, with KPMG and Deloitte each holding 12. BDO’s Supermicro mandate is the sector’s only non-Big Four audit.
The aggregate market-share figures therefore conceal substantial differences in industry position and specialisation.
Some auditor relationships pre-date the SEC itself
The most unusual feature of the US data may be the extraordinary length of some audit relationships.
Deloitte’s reported audit lineage at Procter & Gamble dates to 1890.
Its relationship with Dow dates to 1905, while EY’s tenure at Sherwin-Williams dates to 1908 and Deloitte’s at Deere & Company to 1910.
Other relationships stretch back almost as far. EY’s tenure at Coca-Cola dates to 1921; PwC’s at Goldman Sachs to 1922 and IBM to 1923; PwC has audited Caterpillar since 1925; and KPMG’s relationship with General Mills dates to 1928.
Among the 489 companies for which an exact starting year was established, 42 have an audit relationship dating to 1950 or earlier.
These dates do not necessarily mean that today’s legal accounting firm existed under its current name for the entire period. US auditor-tenure rules specifically allow continuity through predecessor firms following mergers and reorganisations.
The PCAOB’s auditing standard requires the audit report to state the year in which the auditor began serving consecutively as the company’s auditor and says that predecessor firms acquired by or merged into the current firm are included in determining that tenure.
That is particularly important when interpreting century-long relationships in an industry that has itself gone through repeated consolidation.
The US rotates audit partners, not audit firms
The longevity is possible because US public companies are not generally required to replace their audit firm after a fixed number of years.
Instead, federal independence rules focus on rotating the partners responsible for the engagement.
The SEC requires lead and concurring audit partners to rotate off an issuer engagement after a maximum of five years and remain off it for five years. Other covered audit partners are generally limited to seven years, followed by a two-year cooling-off period.
That creates an important distinction between auditor tenure and audit-partner tenure.
A company can therefore retain the same accounting firm for decades—and in some S&P 500 cases for more than a century—while the individuals leading and reviewing the audit change periodically.
This contrasts with markets such as the European Union, where public-interest entities are subject to mandatory rotation of the audit firm itself.
Recent appointments are the exception rather than the rule
Against this backdrop, genuinely recent engagements form a small minority.
Only 11 of the exact tenure dates in the dataset begin in FY2025 or later.
They include KPMG’s FY2026 engagement at Lam Research, Deloitte at Dexcom, PwC at Fastenal and KPMG at Trimble from FY2025.
Not every recent starting date represents a conventional auditor switch. The dataset also contains newly separated or standalone businesses, where the first year reflects the audit relationship of the new entity.
Lam Research provides a clearer example of an actual change: KPMG replaced EY and audited FY2026, with the company’s filing separately reporting KPMG’s FY2026 audit fee and EY’s predecessor-year fee.
US companies disclose audit tenure unusually clearly
One reason the S&P 500 tenure data is more complete than for several other indices in this series is the US reporting regime itself.
Under PCAOB standard AS 3101, an issuer’s independent auditor’s report must include the year in which the accounting firm began serving consecutively as the company’s auditor.
This produces a comparatively strong primary-source record of audit relationships.
Big4News was consequently able to establish an exact first financial year for 489 of the 500 companies. Only 11 tenure fields remain unresolved because the evidence did not establish the precise first year to the standard applied in this research.
That does not mean the auditor is unknown. The external auditor has been identified for all 500 companies.
The largest audit bills belong to America’s financial giants
A current audit-fee figure was verified for 466 of the 500 companies.
The median among those disclosed fees is approximately $8.1 million.
The largest figure belongs to JPMorgan Chase, at $91 million, followed closely by Citigroup at $88.8 million and Goldman Sachs at $86.8 million.
Other major audit bills include:
Morgan Stanley — $66.2 million
Bank of America — $65.8 million
Berkshire Hathaway — $61.8 million
Prudential Financial — $57.0 million
Microsoft — $53.88 million
MetLife — $53.3 million
Wells Fargo — $52.9 million
IBM — $49.0 million
Amazon — $47.26 million
US fee disclosure has one advantage for comparison: proxy statements commonly separate Audit Fees, Audit-Related Fees, tax fees and other fees.
Big4News uses the amount classified by the company as Audit Fees. That category can still encompass more than the annual financial-statement opinion: companies may include the audit of internal control over financial reporting, quarterly reviews, subsidiary statutory audits, regulatory filing work and other services ordinarily associated with the audit.
The figures should therefore be treated as the issuer’s disclosed Audit Fees category rather than as a measure of hours spent solely on the year-end financial-statement audit.
About the data
The dataset is constructed at company level. Although the S&P 500 currently contains 503 securities, Alphabet, Fox and News Corp are counted once each because their multiple share classes do not represent separate audit clients.
Auditor identifies the external audit firm. Deloitte, EY, KPMG and PwC are standardised at network level for consistency across the Big4News Who Audits? series.
Since is the first financial year for which the current firm held the external audit engagement. It is not the year shareholders most recently ratified the auditor’s appointment.
Where an SEC filing or auditor’s report gives a precise continuous tenure, that evidence is used. Predecessor-firm tenure is carried forward only where the source establishes continuity. Ambiguous historical lower bounds are not turned into exact years.
Audit fee is the latest amount specifically disclosed as Audit Fees in the company’s annual report or proxy filing. Audit-related, tax and other fees are excluded as separate categories.
Most amounts relate to FY2025, although a later FY2026 disclosure is used where one was already available at the research cutoff.
Where an auditor changed and the most recent fee belonged to the predecessor, Big4News does not attribute the predecessor’s fee to the incoming firm.
The research relies primarily on SEC proxy statements, annual reports, independent auditor reports and company investor-relations filings.
All 500 auditor identities have been populated. Exact tenure is available for 489 companies, while 466 audit-fee figures were verified. Remaining blanks are deliberate and represent fields that could not be supported to the required standard rather than estimates.
Underlying data snapshot: 30 September 2026.
Who audits the world’s largest companies?
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