Customer concentration: what your biggest customer really earns you

Turnover share hides profit share. A short exercise shows how much profit your biggest customer leaves, and what a 5% discount or a lost account would do.

The problem: turnover concentration hides profit concentration

Most owners know their biggest customer matters. Fewer can say how much profit that customer actually leaves after delivery, rush orders, credit and account management. The average hides the story: a customer who is 25% of turnover can be a very different share of profit, and the profit at stake is what you lose if they leave or ask for a better price.

Why it matters now

FreeAgent's guide to customer concentration cites Forbes in saying that as little as 10% of revenue from one client can put a business at risk, and its own alert starts at 25% from a single customer. That is a rule of thumb about revenue, not profit, but it shows where attention usually stops.

The Bank of England's Agents' summary of September 2026 describes profit margins as slightly less compressed, helped by efficiency savings and by passing some costs into prices. It also says weak demand limits how far firms can do that, and that some firms protected by hedges or fixed contracts may try to raise prices in 2027. Those 2027 conversations are where a large customer's leverage shows up.

The fix: rank customers by profit, then test two scenarios

Take the figures below, which are illustrative only. A business has £2,000,000 turnover. Its largest customer buys £500,000 (25%) at a 22% gross margin, which is £110,000. Serving that customer costs £35,000 a year in rush orders, delivery and admin, so it leaves £75,000. The other customers leave £360,000 after their own serving costs. Together that is £435,000 against £315,000 of overheads, so profit is £120,000.

Now run the two scenarios:

A 5% discount for the biggest customer costs £25,000. Profit falls to £95,000, a drop of about 21%.
That customer leaves. If overheads stay the same, profit falls from £120,000 to £45,000, a drop of 62.5%.

This customer is 25% of turnover but about 17% of contribution. Its loss still removes more than half the profit, because the overheads do not leave with it. That is the number to know before a negotiation, not after.

Tips and tricks for this week

Rank by profit, not turnover. List customers with turnover, gross margin, serving cost and what is left. It takes about 30 minutes for your top ten.
Run the "what if" for your top three. Work out profit after a 5% discount and after losing the customer, with overheads held constant.
Set a limit. Decide the largest share of profit any one customer may hold, and write down the date you want to be under it.
Judge new business by profit share. A new account that adds turnover but needs rush work can leave you more exposed, not less.
Mark any fixed-price or long-running contracts that come up for renewal in 2027, because that is when price pressure tends to arrive.

Why this method is sound

Ranking customers by what is left after serving them is a standard way to separate turnover from profit. FreeAgent's guide suggests measuring concentration as the top customer's revenue divided by total revenue. Adding the serving cost to that calculation turns a revenue warning into a profit decision, and the scenario test shows what it is worth to you. It gives direction, not measurement: your own costs will move the numbers.

If you want a structured view of where your margin goes, the Business Health Check is built for that. One change, one date.