Customer concentration risk: how to measure it and what counts as high

Updated October 7, 2026. 14 min read. Rules and thresholds are checked against the sources listed at the end.

Customer concentration risk is the risk that losing one customer, or a small group of them, would cut revenue enough to hurt profit, cash flow or the value of the business. It is measured with the customer concentration ratio (the share of revenue from your largest customer, top five or top ten) and with the Herfindahl-Hirschman Index (HHI), which scores the whole customer list.

Largest customer

Customer revenue ÷ total revenue × 100

Top 5 share

Revenue of the five largest ÷ total revenue × 100

HHI

Sum of every customer’s % share squared (0 to 10,000)

Common warning lines

One customer at 10%+ (disclosure), 15–20%+ (advisers, buyers); top five above 25%

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Revenue by client (last 12 months)

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Concentration risk score

38/ 100High risk

Largest client: 42.9%
Top 3 clients: 81%
HHI (0 to 10,000): 2,773
Effective clients: 3.6
Top 5 clients: 100%
New revenue to get largest under 20%: $480,000

Northside Dental42.9%

Harbor Logistics22.9%

Maple & Co15.2%

Other clients10%

Brightwell Gym9%

Total revenue $420,000 across 5 clients. Bars turn amber at 15% and red at 25% of revenue.

What is customer concentration?

Customer concentration describes how revenue is distributed across customers. A business with forty customers of similar size has low concentration; a business where one customer pays 45% of the bills has high concentration. Agencies and professional firms usually call it client concentration, analysts say revenue concentration; the measurement is the same.

Concentration is not a problem in itself. Large customers can bring steady volume and lower selling costs, and some research finds suppliers with concentrated customer bases earn higher accounting returns (Patatoukas, 2012). The risk is what happens when one of those relationships ends, shrinks or renegotiates.

Customer concentration ratio: formula and worked example

The customer concentration ratio is the percentage of total revenue that comes from one customer, or from a defined group of your largest customers.

Customer concentration ratio = customer revenue ÷ total revenue × 100

Top-N ratio = revenue of your N largest customers ÷ total revenue × 100

Use the same period for both numbers. The trailing twelve months is the usual choice because it smooths seasonality and matches how lenders and buyers read your accounts. Use recognised revenue, not contract value, and leave out pass-through costs you bill at cost.

Worked example: a five-client agency

A design agency earned $400,000 over the last twelve months:

Client	Revenue	Share	Share squared
Northwind Retail	$180,000	45%	2,025
Bright Labs	$92,000	23%	529
Harbor Foods	$64,000	16%	256
Kestrel Legal	$44,000	11%	121
Vale Studio	$20,000	5%	25
Total	$400,000	100%	HHI 2,956

Largest-customer ratio: 180,000 ÷ 400,000 = 45%. Losing Northwind would remove 5.4 months of revenue in a year.
Top-3 ratio: (180,000 + 92,000 + 64,000) ÷ 400,000 = 84%. Five clients on paper, three in practice.
HHI: 2,025 + 529 + 256 + 121 + 25 = 2,956, equivalent to 10,000 ÷ 2,956 = 3.4 equally sized clients.
ClientGuard score: 40 out of 100, high risk (70% weight on the largest client, 30% on the HHI).

In Excel or Google Sheets, with revenue in B2:B6: share =B2/SUM($B$2:$B$6), top-3 share =SUM(LARGE(B2:B6,{1,2,3}))/SUM(B2:B6), HHI =SUMPRODUCT((B2:B6/SUM(B2:B6)*100)^2).

Why the HHI matters as well as the ratio

The largest-customer ratio answers “how bad is the worst single loss?”. The HHI answers “how fragile is the whole book?”. Two businesses can both have a 20% largest client while one has five more clients at 10–19% and the other has sixteen at 5%. Their HHIs are 1,730 and 800. Read the full method in the HHI guide for small businesses.

What counts as high customer concentration?

No law sets a concentration limit for a private company. What exists are disclosure rules for public companies, supervisory guidance for lenders, and rules of thumb used by buyers and advisers. They cluster around the same numbers:

Source	Threshold	What it says	Type
US GAAP, ASC 280-10-50-42	10% of revenue	A public entity must disclose that a single external customer is 10% or more of revenue, and the revenue amount (not necessarily the name).	Accounting standard
IFRS 8, paragraph 34	10% of revenue	Same 10% major-customer disclosure for entities reporting segments under IFRS.	Accounting standard
US GAAP, ASC 275-10-50-16/18	No percentage	Disclose concentrations that make the entity vulnerable to a near-term severe impact. For customers, loss in the near term is “always considered at least reasonably possible”. Applies to private companies reporting under US GAAP too.	Accounting standard
SEC Regulation S-K, Item 101	No percentage since 2020	Registrants describe any material dependence on key customers. The old rule naming any customer at 10%+ of consolidated revenue was replaced in November 2020.	Securities regulation
OCC, Asset-Based Lending handbook	10% of receivables; cap 10–20%	Receivables are concentrated when one account is 10%+ of the portfolio; lenders should limit such accounts to 10–20% of the borrowing base.	Bank supervision guidance
BizBuySell	10% per customer	“No single customer should constitute more than 10% of a company’s revenues.”	Rule of thumb (business sales)
Wall Street Prep	10% single, 25% top 5	One customer above 10%, or the top five above 25%, are potential red flags.	Rule of thumb (finance)
Design Business Association (via Design Week)	15–20% per client	A single client should account for no more than 15% to 20% of an agency’s income.	Rule of thumb (agencies)
FOCUS Investment Banking	20%, 30%, 40%	Over 20% draws detailed buyer review, over 30% more buyers decline, and valuation can be reduced “perhaps 20–35%”.	M&A adviser view
DOJ/FTC 2023 Merger Guidelines	HHI above 1,800	Markets above 1,800 are “highly concentrated”. This is about competitors in a market; applying it to customers is an analogy.	Competition guidance

Putting that together, ClientGuard reads the largest-customer share roughly like this: under 20% low, 20–35% medium, 35–60% high and above 60% critical, adjusted up or down by the HHI. The top-5 share and the trend over time matter as much as any single number. If you want the background on the 20% line in particular, see the 20% client rule.

What high customer concentration means in practice

Valuation and selling the business

Buyers pay for predictable cash flow. Mercer Capital puts it simply: “All else equal, higher risk translates into a lower valuation multiple.” In practice concentration shows up as a lower price, more of the price paid through earn-outs tied to keeping the big customer, larger escrows, or buyers walking away. FOCUS Investment Banking also notes the exceptions: they have sold businesses with very high concentration at premium multiples where the relationship was long and contracted.

Borrowing

Lenders discount concentrated receivables. Under the OCC guidance above, receivables above the concentration limit can be excluded from the borrowing base. Research on bank loans finds the same in pricing: higher customer concentration is associated with higher interest rate spreads, more restrictive covenants and shorter maturities (Campello and Gao, 2017).

Cost of capital

For listed suppliers, Dhaliwal, Judd, Serfling and Shaikh (2016) find a positive association between customer concentration and the cost of equity, stronger where losing a major customer is more likely. For a small business the same logic shows up as a higher discount rate in a valuation.

Day-to-day bargaining power

A customer that pays a third of your bills knows it. Expect pressure on price, payment terms and scope, and less freedom to turn down work or say no to a bad request.

Client concentration risk for accounting firms and other professional services

Accounting firms face the commercial risk every service business has, plus an independence rule for audit work: depending on one audit client for a large share of fees can impair independence.

IESBA Code (international), Section 410: when fees from a public interest entity audit client are more than 15% of the firm’s total fees for two consecutive years, the firm must consider a pre-issuance review by an outside professional and tell those charged with governance. If it continues for five consecutive years, the firm must stop being the auditor, with narrow exceptions. For other audit clients, more than 30% of total fees for five consecutive years triggers a review. Effective for periods beginning on or after December 15, 2022.
UK FRC Revised Ethical Standard 2024: a firm shall not act as auditor where fees from a listed entity or public interest entity will regularly exceed 10% of its annual fee income, or 15% for other entities (paragraphs 4.21–4.22), with disclosure duties below those levels.
AICPA (US): the fee-dependency guidance refers to fees that are “a large proportion” of the firm’s total for five consecutive years, without a fixed percentage.

Outside audit, the commercial thresholds above apply to accounting, legal, consulting and marketing firms alike. Because professional firms usually have high fixed costs (people), losing a 20% client tends to remove far more than 20% of profit.

The second risk: industry concentration

Ten clients in one industry can behave like one client when that industry has a bad year. Tag each client by sector and add up revenue per sector. M&A Source, the IBBA’s M&A affiliate, describes the ideal as no customer above 10% of revenue and no more than one-third of revenue from one industry.

How to reduce customer concentration risk

Set a numeric target. To bring a client under 20% without losing any of its work, total revenue must reach five times that client’s revenue. For Northwind above, that is $900,000, or $500,000 of new revenue. Seeing the number makes it clear that diversification takes years, so the next steps matter in the meantime.
Protect the big account while you diversify. Longer terms, notice periods of 60–90 days, renewal dates you track, and more than one relationship inside the client.
Grow the next tier. Clients at 5–15% are usually the fastest route to a flatter book; expanding work with them is cheaper than winning new logos.
Win new clients in a different industry so the new revenue also lowers sector concentration.
Price the largest account properly. Long-standing large clients often sit on older rates. A price review either raises margin or lowers dependence.
Hold a cash buffer sized to the exposure: enough to cover costs for the months it would take to replace the largest client.
Measure every quarter. Track the largest-customer share, top-5 share and HHI on the trailing twelve months, and watch the direction as much as the level.

Common mistakes when measuring it

Counting entities instead of relationships. Subsidiaries under common control are one customer; ASC 280 and IFRS 8 treat them that way too.
Mixing periods. A client’s twelve months against a nine-month total inflates the ratio.
Using gross billings. Media or materials passed through at cost are not your revenue.
Measuring once. Concentration moves every month as projects start and end.

Frequently asked questions

What is customer concentration risk?

It is the risk that losing one customer, or a small group of customers, would cut revenue enough to damage profit, cash flow or the value of the business. The more revenue sits with a few customers, the higher the risk.

How do you calculate the customer concentration ratio?

Divide a customer’s revenue by total revenue for the same period (usually the last twelve months) and multiply by 100. The top-5 or top-10 ratio adds up the shares of your five or ten largest customers. Example: a $180,000 client in a $400,000 business has a ratio of 45%.

What is considered high customer concentration?

There is no single official cut-off for private companies. US GAAP and IFRS require public companies to disclose any customer at 10% or more of revenue, many buyers and lenders treat a single customer above 10–20% as a red flag, and agency advisers commonly recommend keeping any one client under 15–20%. A top-5 share above 25% is another commonly cited warning sign.

What does high customer concentration mean?

It means a large share of revenue depends on very few customers. In practice it gives those customers more bargaining power, makes revenue less predictable, and tends to lower what buyers will pay for the business and raise what lenders and investors charge.

What does low customer concentration mean?

Revenue is spread across many customers, so no single loss changes the year. A common rule of thumb is no customer above 10% of revenue and the top five below 25%.

Is customer concentration the same as revenue concentration or client concentration?

Yes. Agencies and professional firms usually say client concentration, analysts say customer or revenue concentration. They are measured the same way.

What are debtor concentration limits?

In asset-based lending and invoice finance, a lender caps how much of the borrowing base can come from one customer’s receivables. The US OCC’s Asset-Based Lending handbook treats a single account at 10% or more of receivables as concentrated and suggests limiting such accounts to 10–20% of the borrowing base.

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Sources (last checked October 7, 2026)