The largest customer's share does not describe the whole customer base

Two businesses can each receive 40% of revenue from their largest customer and still have different concentration across the remainder. One may depend on three additional large accounts, while the other has six smaller accounts. The largest-customer percentage cannot distinguish those structures.

A squared-share measure provides another view. The Department of Justice describes the Herfindahl-Hirschman Index for market concentration as the sum of squared shares. The calculation can also be applied descriptively to customer revenue shares, but antitrust thresholds are not customer-risk limits and should not be imported into this analysis.

Define the measure narrowly: it describes how the selected revenue is distributed among the customer groups used in the calculation. It does not estimate whether those customers will default, cancel or reduce their purchases.

Consider invented business A with customer revenue shares of 40%, 30%, 20% and 10%. Expressing shares as decimals, the sum of squares is 0.40 squared plus 0.30 squared plus 0.20 squared plus 0.10 squared, or 0.30.

Business B has a 40% largest customer and six other customers contributing 10% each. Its sum of squares is 0.40 squared plus six times 0.10 squared, or 0.22. Both businesses have the same top share, but A's remaining revenue is more concentrated.

If shares are entered as whole percentages instead, the corresponding figures are 3,000 and 2,200. Those are the same measures on a scale multiplied by 10,000. Label the scale and avoid comparing a decimal-form result with a whole-percentage-form benchmark.

For equal customer shares, the sum of squares equals one divided by the number of customers. Ten equal customers therefore produce 0.10. Taking the reciprocal of a general decimal-form index gives the number of equal-sized customers that would produce the same concentration value.

Business A's reciprocal is about 3.33 and business B's about 4.55. These are descriptive effective counts, not actual customer counts. A still has four customers and B still has seven in the example.

The reciprocal can help explain why adding many tiny accounts may not change concentration much. The arithmetic gives larger shares more influence because they are squared. It should be presented alongside actual customer counts and top-account shares so the summary does not hide the commercial relationships.

A customer may appear under several billing entities, locations or product accounts. Decide which relationships should be treated as one economic group for the management question. A shared parent purchasing decision can make separate ledger accounts less independent than their names suggest.

For example, splitting a 40% customer into two 20% ledger accounts reduces its squared-share contribution from 0.16 to 0.08 if the accounts are incorrectly treated as separate. The reported index falls even though the underlying commercial dependence has not changed.

Keep the grouping map and its rationale with the report. Grouping every account in an industry together would answer a different question about sector exposure. A business may need both customer-group and sector views, but each should retain its own clear definition.

Annual revenue concentration describes sales during that year. It can differ from the concentration of outstanding receivables, next month's expected collections or contribution. A customer with modest annual sales may account for most of a near-term payment requirement.

Calculate separate distributions when those exposures drive the decision. For receivables, use the defined outstanding balance at one cutoff and reconcile credit balances or disputed items. For contribution, explain the cost basis before computing shares, especially if some customers have negative contribution.

The simple share index assumes a meaningful nonnegative population whose shares sum to one. Negative net revenue or contribution values can break that interpretation. Investigate the underlying returns or losses and choose a reporting approach that does not disguise them through an arbitrary adjustment.

Suppose a profitable large customer leaves and the remaining customers become more evenly balanced. The concentration measure may fall because the dominant account is gone. That is not evidence that the company's cash position improved.

Show the absolute revenue and contribution alongside the shares. A diversification effort that adds profitable customers can lower concentration while expanding the business; losing the largest relationship can lower it while shrinking the business. The index alone cannot distinguish those paths.

A change in period can also alter the result. One large project invoice may dominate a quarter but represent a smaller share of annual activity. Use periods appropriate to the contract cycle and retain recent changes that matter for the upcoming cash forecast.

Ten equal customers do not necessarily provide ten independent sources of cash. They might depend on the same industry, funding source or distribution platform. A shared event could affect all of them without changing the historical revenue-share index.

Likewise, a concentrated customer base can contain a long-term commitment with strong payment evidence, while a more dispersed base can have weaker collection behavior. The concentration measure contains no information about those probabilities or contract protections.

Use the distribution as a way to select scenarios for further review. Examine a large account's delay, a sector-wide demand reduction or a common renewal window using the actual revenue, avoidable costs and payment dates. Do not convert the index itself into an unsupported probability of loss.

Report the period, exposure definition, customer grouping, largest shares, index scale and absolute total. When the measure moves, explain which customers or grouping changes caused the movement.

The squared-share calculation is useful because it shows more of the distribution than the top account alone. Its value comes from that limited descriptive role, while the customer records and cash scenarios establish what the concentration means for the next business decision.