When One Account Pays the Wages: Measuring and Managing Customer Concentration in UK SMEs

Customer concentration is usually discussed in sales meetings. A founder celebrates a large account, the team builds around the work, and the account becomes proof that the business can win. The finance risk appears later, when a single client pays half the invoices and the payroll calendar depends on that client's approval cycle.

Concentration is a balance-sheet issue

Revenue concentration affects more than the sales pipeline. It shapes receivables, cash forecasting, VAT and PAYE planning, and the credibility of growth forecasts. If one client represents half of revenue and takes 60 days to pay, the business is financing that client's working capital. If the same client also represents most of the gross profit, a lost account is not a sales setback; it is a solvency event.

A useful distinction is between revenue dependence and profit dependence. A large, low-margin contract can flatter turnover while contributing little. A smaller account with high gross margin can be the real dependency. UK founders should measure both.

How to measure dependence

Start with five numbers, updated quarterly:

• Top-customer revenue share: revenue from the largest client divided by total revenue.
• Top-three revenue share: the same calculation for the three largest clients.
• Gross profit share: gross profit from the largest client divided by total gross profit.
• Receivables share: the proportion of trade debtors owed by the largest client.
• Payroll cover: the number of monthly payroll runs the business could fund from unrestricted cash if the largest client disappeared.

There is no universal safe threshold. A share below 20% is usually comfortable; 20% to 35% warrants a watching brief; 35% to 50% is elevated; above 50% is critical for most small employers. The precise line matters less than the trend. A client moving from 18% to 30% over two years is a signal even if the absolute number still looks manageable.

A concentration index can add rigour. The Herfindahl-Hirschman Index, calculated by squaring each client's revenue share and summing the results, rises sharply as dependence grows. It is not necessary for every founder, but it is useful when a board or lender wants a single measure.

The contract is a risk control

Concentration risk is often written into the contract long before it appears in the accounts. Founders should review notice periods, termination rights, payment terms, price review clauses, exclusivity and set-off provisions. A client that can leave with seven days' notice is a different risk from one committed for twelve months. A client that can withhold payment against a disputed invoice is a different risk again.

Three contract questions matter most. First, how much notice must the client give? Second, can the client terminate for convenience, and on what notice? Third, do payment terms give the business enough time to fund payroll and VAT before cash arrives? If the answer to the third question is no, the business is extending credit it may not be able to afford.

Cash reserves and tax timing

A concentration stress test should be run against the tax calendar, not just the profit and loss account. VAT returns, PAYE payments and corporation tax instalments fall due on fixed dates. If the largest client accounts for half of revenue, the founder should ask how many of those dates can be met from reserves if the client leaves or delays payment.

A practical reserve target is three months of fixed costs, including payroll, rent and finance payments. For a business with high concentration, a longer target may be prudent. The reserve should be held in unrestricted cash or near-cash, not in a facility that depends on the same client's invoices.

Diversification targets that are realistic

Diversification does not mean adding ten small clients at once. It means setting a measurable cap and a route to it. For example, a founder might aim to reduce the largest client to 35% of revenue within four quarters, with a new-logo target and a pipeline coverage ratio to support it. The cap should be reviewed alongside gross profit, not revenue alone.

Sector diversification also matters. Two large clients in the same industry can behave like one client if the sector enters a downturn. A useful test is to ask whether the top clients share a common budget cycle, regulator or customer base. If they do, the apparent diversification is thinner than the percentages suggest.

Warning signs in the account

• Payment terms quietly stretch from 30 to 60 days.
• One contact controls approvals and no escalation route exists.
• The client asks for exclusivity or a long notice period without a matching commitment.
• Revenue from the client grows faster than the business's ability to serve it.
• The client's own sector shows signs of stress.
• The contract is verbal, expired or based on an old statement of work.

A quarterly concentration review

A short quarterly review is enough for most small teams. Update the top-customer and top-three shares, review the largest contract's notice and payment terms, run the payroll stress test and check the next two VAT and PAYE dates. Record the result and set one diversification action for the next quarter.

Customer concentration is not a reason to turn away a large client. It is a reason to price the risk, document the contract and hold enough cash to survive a sudden absence. A business that knows its dependence is better placed to negotiate, borrow and plan than one that discovers it when the invoices stop.