What it means
The classic version is the market concentration ratio, written CR4 or CR8, which adds up the market share of the four or eight largest firms. A CR4 of 80% says four companies control four fifths of the market, which tells you competition is limited and pricing power sits with a small group.
Investors and managers borrow the same arithmetic for internal risk. Customer concentration, the share of revenue coming from your top few clients, is one of the first numbers a buyer or a lender looks at, because a business where three customers make up 70% of sales is far more fragile than one where they make up 15%.
Calculating it is simple, but defining the denominator is where judgement enters. Market share needs an agreed market definition, and customer concentration needs a decision about whether to use revenue, gross profit or contribution, since a large but low-margin customer is a different risk from a large and highly profitable one.
The ratio has a known weakness: it ignores everything outside the top few. Two markets can both show a CR4 of 75% while one has four equal players and the other has one dominant firm with three small rivals, which is why competition authorities usually pair it with the Herfindahl-Hirschman Index that squares every share.
In practice the ratio is most useful as a trend rather than a snapshot. A customer concentration figure that has climbed from 28% to 51% over two years is a clear signal about where sales effort needs to go, regardless of what the absolute level is.
In practice
Real-world examples.
Example
A software founder preparing for a funding round calculates that her top three customers generate $4,200,000 of $7,000,000 revenue, a 60% concentration. Investors price the risk into the valuation, so she spends the next year deliberately winning smaller accounts to bring the figure down.
Example
A packaging manufacturer discovers that 72% of its raw board comes from two mills. When one mill has an unplanned outage, the supply concentration ratio it had been reporting quietly for years suddenly becomes the most important number in the business.
Example
A competition regulator reviewing a supermarket merger publishes a CR4 of 68% before the deal and 74% after it. That six point increase, combined with regional data, becomes the basis for requiring store disposals in specific towns.
Formula
Calculation
CR4 = (revenue of the four largest firms) / (total market revenue) x 100.
A national market for commercial laundry services is worth $2,500,000,000 a year. The four largest operators report annual revenues of $800,000,000, $600,000,000, $375,000,000 and $225,000,000.
Sum of the top four: $800,000,000 + $600,000,000 + $375,000,000 + $225,000,000 = $2,000,000,000.
CR4 = $2,000,000,000 / $2,500,000,000 = 0.80, or 80%.
Four firms hold 80% of the market, leaving 20% spread across everyone else. A regulator reviewing a proposed merger between the third and fourth firms would note that the combined entity would hold $600,000,000 of revenue, equal to $600,000,000 / $2,500,000,000 = 24% of the market, matching the current number two and reducing the field of large competitors from four to three.Case study
Seen in the real world.
This is an illustrative and fictional example. Kesterline Components, an invented precision engineering firm, grew rapidly by serving one large aerospace customer that valued its short lead times. By its fourth year that customer accounted for 64% of revenue, and the founders regarded the relationship as the company's greatest strength.
When the founders approached a bank for a $2,000,000 expansion loan, the credit team calculated the customer concentration ratio and declined the facility at the requested size. The bank's fictional position was simple: the loan would be repaid from a cash flow that a single procurement decision by a single customer could remove.
Kesterline responded by building a diversification plan with a measurable target, taking top-customer concentration from 64% to below 40% within two years by entering medical device and rail markets. In this illustrative outcome the bank agreed a smaller facility immediately and the balance once the ratio fell below 45%, which arrived seven months later.
Watch out
Common mistakes.
- Reporting concentration on revenue alone. A customer that is 30% of sales but 12% of gross profit is a very different exposure from one that is 30% of both, and margin-based ratios often tell the more useful story.
- Treating a low concentration ratio as proof of a competitive market. A CR4 can look reassuring while the market is in fact segmented, so that each firm is effectively dominant in its own region or niche.
- Forgetting to group related customers. Three subsidiaries of the same parent are one credit risk and one purchasing decision, so they must be aggregated before the ratio means anything.
Questions
People also ask.
What counts as a dangerous customer concentration level?
There is no universal threshold, but many lenders and acquirers start asking hard questions once a single customer passes roughly 20% of revenue and treat 40% or more as a material risk.
How does the concentration ratio differ from the Herfindahl-Hirschman Index?
The concentration ratio simply adds the top few shares, while the index squares every firm's share, so it reflects the whole distribution and reacts strongly to one dominant player.
Can concentration ever be a good thing?
Yes, in the short term, since serving a few large accounts is efficient and can fund fast growth, but it should be a chosen and monitored position rather than an accident.
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