There is one customer whose invoice pays your rent. When their order comes in, the month is fine. When it is late, you cannot pay a supplier. You have never quite said this out loud, and they have almost certainly worked it out anyway.
You found out how much power that gives them the last time you tried to raise your price and did not. Or the time they asked for something unreasonable on a Friday and you did it. Or when their payment terms went from thirty days to sixty and you said nothing, because what were you going to do.
The day they leave is the day everyone worries about. But the cost starts long before that, and it is being taken out of your business every month in prices you cannot charge, terms you cannot enforce and work you cannot refuse.
By the end of this guide you will know your exact concentration figure, what it is costing you today, how many new customers it would take to replace your biggest one and how long that would realistically take, a plan to protect the account while you build alternatives, and what to do in the first week if they go.
(Written for any country. Money appears as plain numbers, read them in your own currency. The same arithmetic applies whether your big customer is a company, a wholesaler, a school, an agency, a hotel group or a single large family account.)
Measure it before you argue about it
Take last year's sales. For each customer, work out what share of the total they were. If you have hundreds of small customers, do it for your top ten and for anything that looks large.
Concentration = customer's annual sales ÷ your total annual salesThen read your biggest number against this:
| Largest customer | What it means |
|---|---|
| Under 10% | Healthy. Losing them is a bad quarter, not a crisis |
| 10-20% | Normal for a small business. Keep an eye on it |
| 20-33% | Real risk. You are already behaving differently towards them |
| Over 33% | They effectively control your pricing and your calendar |
| Over 50% | You are a subcontractor to their business, without any of the protections an employee would have |
Do the same for your top three combined. If your top three are more than about half your sales, the same rules apply, because those customers often share a market. When one goes quiet, the others frequently do too.
What it costs you before they ever leave
This is the part owners underestimate, because it never appears as a loss anywhere.
The price rise you did not make. You have not moved their rate in three years because you were afraid of the conversation. If they are 400,000 of annual sales and you have absorbed 10% of cost increases, that is 40,000 a year, every year, taken quietly.
The terms you cannot enforce. Late payment you would chase from anyone else. Deposits you do not ask for. The credit limit that exists for everybody except them.
The work you cannot refuse. Unprofitable jobs, rush jobs, weekend jobs, the things outside your scope. Each one is priced as a favour because saying no feels impossible.
The customers you did not serve. Their work fills your calendar, so other enquiries get a slower reply, a longer lead time, or a no. That is how a business ends up with one customer: not by choice, but by always being too busy with them to build anything else.
The risk you are carrying for them. Their cash flow problems become yours. Their delays become your idle week. Their change of strategy is your bad year.
Add those up honestly and the annual cost of concentration is usually a substantial fraction of what the customer contributes. That is the number to hold in mind while you read the rest, because it means reducing concentration is not just insurance. It pays.
The rest of this guide is paid
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