The Big 4 audit firms are Deloitte, EY, KPMG, and PwC. Together these four accounting firms audit the vast majority of FTSE 100 and FTSE 250 companies and a large slice of the global market for professional services. They are the largest professional services firms in the world by revenue and they dominate the listed and PIE (public interest entity) market.
This guide explains who the Big 4 are, how the four firms came to dominate, what they do beyond accounting and auditing, and where mid-tier alternatives fit. It is written for UK readers thinking about the firms and the choices available to a growing private company. The four are multinational networks rather than single-country businesses.
Who are the Big 4 audit firms?
The Big 4 are:
- Deloitte. The largest of the four by global revenue. The firm traces its name to William Welch Deloitte, who founded the original London practice in 1845. Strong in consulting, financial advisory, risk consulting, and tax.
- EY (Ernst & Young). The result of a 1989 merger between two large American firms. Big in taxation, transaction advisory, and consulting services.
- KPMG. KPMG was founded in 1987 by the merger of Peat Marwick International and Klynveld Main Goerdeler. The full Klynveld Peat Marwick Goerdeler name gives the four-letter brand. Tax and advisory services across most major economies.
- PwC (PricewaterhouseCoopers). Created in 1998 when Price Waterhouse joined Coopers & Lybrand. The partners chose to merge in order to compete on global scale. Tax, consulting, and corporate finance.
Each Big 4 brand is a network of firms rather than a single legal entity. The local member firm in each country is a separate legal partnership, often a UK LLP for the UK office. So the UK and US arms are separate businesses operating under a common brand and shared methodology. The “big four accounting firms” label covers the entire network rather than any one office.
How did the Big 4 become the Big 4?
The path runs from the Big 8 in the 1980s, to the Big 6 in 1989, to the Big 5 in 1998, to the Big 4 in 2002. A series of mergers thinned the field; one collapse finished it.
- 1989: Big 8 to Big 6. Ernst and Whinney joined with Arthur Young to form Ernst & Young. The Deloitte network had earlier merged with Haskins & Sells in 1952; the 1989 step combined those firms with Touche Ross to form Deloitte & Touche.
- 1998: Big 6 to Big 5. Price Waterhouse joined with the other firm to form PricewaterhouseCoopers. KPMG and Andersen had also discussed combining, but those talks fell through.
- 2002: Big 5 to Big 4. Arthur Andersen collapsed after its role in the Enron scandal. The firm’s auditing arm was wound down within months. Most of its non-US offices joined other Big 4 brands. Andersen never recovered.
The accountancy market has been a Big 4 market ever since. Periodic regulatory reviews have looked at whether the concentration is healthy (the Competition and Markets Authority and the FRC in the UK have both run inquiries), but no fifth firm of comparable size has emerged. Smaller firms have grown but none has matched the four in scale.
What do the Big 4 do?
The auditing core is historical, but in terms of revenue each accounting firm now earns more from advisory services and consulting than from the original profession. The typical revenue mix for a Big 4 network is roughly:
- Statutory and assurance work. 30 to 35% of revenue. The original profession, plus other forms of independent opinion.
- Advisory services and management consulting. 30 to 40% of revenue. Strategy, technology and digital transformation, deal advisory, restructuring, risk consulting, regulatory.
- Tax. 20 to 25% of revenue. Corporate tax, transfer pricing, indirect tax, expat tax, M&A tax.
- Other. Legal services in some jurisdictions, transaction services, ESG and sustainability, financial reporting advice.
The shift towards advisory has been one of the big stories in professional services since the early 2000s. Regulators in the UK and elsewhere have pushed back because of independence concerns: a services company that earns more from selling consulting work to its statutory clients has a conflict of interest. The FRC’s operational separation rules require the UK Big 4 to ring-fence their statutory practices from non-audit work for FTSE 350 clients.
Big 4 market share
In the UK, the Big 4 sign off on roughly 97 to 99% of the FTSE 100 and around 95% of the FTSE 250. Outside the listed market, the share is lower. Mid-market private companies are split between Big 4 and the next tier of accounting firms (BDO, Grant Thornton, RSM, Forvis Mazars, Crowe). Private UK firms under £100m turnover are dominated by mid-tier and independent specialists, including auditing firms like Audit Group.
Globally the four firms together earn roughly $200 billion a year in combined revenue. Deloitte and PwC each have over 350,000 people; KPMG and EY are slightly smaller. The Big 4 also engage with Fortune 500 companies at scale; working with Fortune 500 companies is a defining feature of being one of the big four. They are the largest professional services firms in the world by every measure.
Big 4 revenue (most recent year)
| Firm | Global revenue ($bn) | Headcount | UK revenue (£bn) |
|---|---|---|---|
| Deloitte | ~67 | ~457,000 | ~5.6 |
| PwC | ~55 | ~370,000 | ~6.3 |
| EY | ~52 | ~395,000 | ~3.9 |
| KPMG | ~38 | ~275,000 | ~2.9 |
Figures are approximate full-year totals from each firm’s most recently published annual review and were accurate at last update. The Big Four also publish detailed UK transparency reports each year covering quality metrics, partner pay, and the gender and ethnicity pay gap.
Big 4 vs mid-tier accounting firms
The mid-tier firms, sometimes called the “next 6” or the “challenger firms”, include BDO, Grant Thornton, RSM, Forvis Mazars, Crowe, and Saffery. They are smaller but still substantial: BDO globally is the fifth-largest network, and Grant Thornton is in the top eight.
For a private UK company, the choice between a Big 4 firm and a mid-tier alternative comes down to:
- Cost. Big 4 fees are typically 30 to 50% higher than mid-tier for similar-scale work. The premium reflects brand, methodology, and partner-time pricing.
- International coverage. If you need one accounting firm across 20 countries, the Big 4 are usually the easiest choice. Mid-tier networks have global coverage too but with more variable depth.
- Sector specialism. The Big Four also have deep specialism in financial services, FTSE-scale industrials, and complex consolidations. Mid-tier firms often beat them on owner-managed businesses, charities, and specific sectors.
- Service model. Mid-tier and independent firms typically give partners more time on client matters. Big 4 engagements run on a leveraged model with heavy use of juniors.
If you are weighing the choice for a specific engagement, our Big 4 vs mid-tier firms comparison guide goes through it in more detail and includes typical fee benchmarks.
Why did the Big 8 become the Big 4?
Two reasons: a series of mergers and one collapse. The Big 4 also reflect the broader consolidation of accountancy in the late 20th century. Big global clients wanted to deal with fewer, larger advisers. Each step combined complementary geographies and client lists. The collapse (Arthur Andersen in 2002) was a one-off triggered by the Enron scandal and the loss of confidence in the firm’s work.
There has been talk for years of the Big 4 becoming the Big 5 again. BDO has been mentioned as a candidate. So far, no breakthrough has changed the count. The latest serious split came in 2024 when EY’s plan to spin off its consulting business (Project Everest) was abandoned by the partnership.
Is it Big 5 or Big 4?
It’s Big 4. The phrase has been “Big 4 accounting firms” since 2002 when Arthur Andersen collapsed. Before that, “Big 5” referred to the same four plus Andersen. Some commentators occasionally call BDO, Grant Thornton, or RSM a fifth Big 4 firm, but that’s loose usage. Industry definitions still treat the four firms as the closed top tier.
Who audits the Big 4?
The Big 4 sign each other off. Specifically, each firm’s accounts are reviewed by one of the other three. PwC’s accounts are signed by Deloitte, for example. Mutual rotation is the norm because no other accounting firm is large enough to take on the work, and self-review would be a clear independence breach. Each Big 4 network also has internal quality monitoring and is subject to regulatory inspection by the Public Company Accounting Oversight Board in the US, the FRC in the UK, and equivalents elsewhere.
How do you join a Big 4 firm?
Three main routes:
- Graduate scheme. Each Big 4 firm runs a UK graduate intake of 1,500 to 2,500 each year. Most join as associates and study for the ACA, ACCA, or CPA equivalent in the first three years. The internship and summer placement programmes feed the graduate intake.
- Apprenticeship. All four firms run school-leaver and Level 7 apprenticeship programmes. These have grown sharply since 2017 and now form a significant share of the early-career intake. Big 4 social mobility schemes have widened the catchment.
- Experienced hire. Lateral hires from mid-tier firms, industry, and other Big 4 offices. Most senior moves above manager level happen this way. Strong technical skills are a near-universal requirement; the Big 4 also look for analytical and technical depth at senior interview stage.
The graduate and apprentice programmes are highly competitive. Each Big 4 receives ten or more applications for every place. Strong A-levels (or apprenticeship equivalent) and a 2:1 degree (or merit at apprentice level) are common minimum thresholds in 2025, although some have moved away from formal grade requirements in recent years to improve employability for non-traditional candidates.
Big 4 alternatives for UK private companies
You don’t have to use a Big 4 firm. For most UK private companies under £200m turnover, a mid-tier or specialist independent firm is a better fit. The benefits of the smaller-firm route are usually:
- Partner attention. A senior named partner runs the engagement, not just signs the report. Most Big 4 work below FTSE-scale is run day-to-day by managers and seniors.
- Faster turnaround. Specialist firms often complete the work in weeks rather than months because their team isn’t carrying a portfolio of FTSE clients.
- Lower cost. 30 to 50% saving on the Big 4 fee for comparable work is typical.
- Specialism. Smaller firms in statutory-only practice, charity work, academies, or financial services often have deeper hands-on expertise than a Big 4 generalist.
Audit Group is part of Jack Ross Chartered Accountants, established 1948. We are an ICAEW Registered Auditor based in Manchester. We act for private UK companies, charities, academies, and sector specialists. If you are weighing a Big 4 firm against a mid-tier or specialist alternative, we are happy to give a fixed-fee comparison.
Common questions
Who are the Big 4 in audit?
Deloitte, EY, KPMG, and PwC. They are the four largest accounting firms in the world by revenue and sign off on the majority of listed companies in the UK and globally.
Which Big 4 accounting firm is the best?
None of them is universally the best. The four firms compete closely on quality, scale, and specialism. PwC is generally seen as strongest on financial services. The technology and consulting integration leader has shifted between firms over the years. EY has historically been strong on transactions. KPMG is the smallest globally but has deep relationships in many sectors.
What is the difference between Big 4 and Big Four?
Nothing. The two spellings refer to the same group. Both appear in industry coverage, with “Big Four” slightly more common in formal writing and “Big 4” more common in shorthand.
Are Big 4 accounting firms public?
No. All four are private partnerships. They are owned by the partners and don’t have publicly traded shares. The networks publish annual revenue figures voluntarily but they are not subject to the listed-company disclosure regime that their clients face.
Who owns Big 4 accounting firms?
The partners. Each Big 4 network is structured as a partnership of partnerships, with thousands of equity partners across the world. Profits are paid out each year; partners don’t typically hold long-term equity stakes that build value.
Get a fixed-fee proposal from a UK alternative
If your company is in scope for an external review and you’d like a comparable quote against a Big 4 fee, send us last year’s accounts and a short note on what’s changed. We will come back with a fixed fee, a draft timetable, and the team who would lead the engagement. No tender process, no panel pitch.
