Your biggest client pays a week late, and suddenly you are the one asking a supplier for a few more days. On Friday they send a rush job, and you say yes, because saying no to them feels like a risk you cannot afford this month.
When one client is half your income, your business runs on their payment habits rather than your own. The answer is not to drop them. It is to know the exact share they hold, keep enough cash to cover the month they stop, and grow the next two clients before you are forced to.
This guide covers how to work out your own number, what counts as too much, what the dependence quietly costs you long before anyone leaves, and what to do about it this month with what you already have.
Is this you?
- One or two clients make up most of what you bill, and you know roughly which without checking.
- When they pay late, you pay someone else late. Their delay becomes yours.
- You have not raised your price with them in a long time, and you are not sure how you would.
- You take their last-minute work over better-paid work from someone smaller.
- You have not pitched anyone new in months, because there has been no need.
- You avoid thinking about the day the contract ends, or the day their contact leaves.
If two or more of those sound familiar, one client is carrying more of your business than is comfortable. That is worth naming early, because none of it looks like a problem while the client is happy.
How much from one client is too much?
There is no legal limit for a small business, but there is a useful marker. Public companies in the US have to declare it when a single customer crosses 10% of revenue: under accounting standard ASC 280-10-50-42, quoted here in a letter to the SEC, "if revenues from transactions with a single external customer amounts to 10% or more of a public entity's revenues, the public entity shall disclose that fact". Investors are told at 10% because that is the level at which one customer starts to move the whole business.
A one-person or ten-person business cannot work to 10%, and nobody expects it to. As a rule of thumb rather than an accounting standard:
- Under 25%: low exposure. Losing them hurts and does not threaten the business.
- 25% to 49%: worth watching. You would need a plan and some cash to get through it.
- 50% or more: one client is carrying the business. Their decisions are now your decisions.
Two things move those bands. A long contract with a real notice period lets you live with a higher share than job-to-job work does, because a notice period turns a sudden loss into a planned one. And a client who pays reliably is a different risk from one who pays whenever it suits them, even at the same share of income.
The share is more common than most owners think. For the typical small business, the single biggest client accounts for 43% of a year's billing (58 businesses with two or more clients). In 45% of small businesses, one client accounts for half or more of a year's billing (58 businesses). Source: Billbooks invoice data, 8,483 invoices from 112 small businesses, September 2024 to September 2026 (aggregate only, figures shown only where 25+ businesses contribute). The working is in our invoice payment study.
So if one client is half your billing, you are not careless and you are not unusual. You are in the same position as nearly half the businesses in that data.
Why it happens to careful people
Nobody sets out to build a business on one account. It happens in a sequence that looks sensible at every step.
A good client sends more work. The work is steady and the invoices get paid, so you take the next piece rather than going looking for something uncertain. Winning new work costs a pitch, a quote and a wait; more work from an existing client costs an email. Over a year or two the easy yes becomes the account that pays the rent.
Then selling stops, because there is no time and no obvious need. That is the part that takes longest to undo, and it is why the problem is usually discovered too late. Our own client data shows how thin most books are underneath the big name: about 40% of clients are billed exactly once in two years; the rest are repeat business (1,934 clients, 112 businesses). A long list of clients is often one or two relationships plus a tail of one-off jobs.
The last part is the uncomfortable one. A client who knows they are your biggest can ask for more, pay later and push on price, and it is hard to push back when the answer to "what if they walk" is "then I am in trouble". None of that makes them a bad client. Most big clients are good clients. The risk is not the client, it is having no plan for the day they pay late, cut back or leave.
What it costs before anyone leaves
Most of the cost arrives quietly, while the relationship is still going well.
Their payment habits become your cash flow. When half your income arrives on one date, a week's slippage on that one payment moves your whole month, and there is nothing else large enough to cover it. That is how a profitable business ends up short, which is the pattern behind most cash flow problems in small business.
You stop pricing properly. Rush work at the old rate, scope that grows without the fee growing, a price review you keep postponing. Each one is a small decision and the discount compounds year after year.
You have less buffer than the size of the risk deserves. The JPMorgan Chase Institute studied 470 million transactions from 597,000 small businesses and found that half of all small businesses hold a cash buffer large enough to support 27 days of their typical outflows (data from February to October 2015, published 2016). Twenty-seven days is enough to absorb a quiet fortnight. It is not enough to replace half your income.
And it costs you something if you ever sell. A buyer looks at where the revenue comes from before they look at how much there is, because they are buying next year, not last year.
A worked example
The numbers below are illustrative, not from a real business. They are there to show the arithmetic, which is the part that surprises people.
You billed $180,000 over the last twelve months. Your biggest client was $90,000 of it, so exactly half. Your outgoings, including what you pay yourself, are $11,000 a month. You have $9,000 in the bank.
- Total billing per month: $15,000. Their share of it: $7,500.
- If they stopped tomorrow and nothing else changed, you would be billing $7,500 a month against $11,000 of outgoings.
- That is a shortfall of $3,500 a month, so your $9,000 covers about two and a half months.
- To get their share under a third, your other clients would have to go from $7,500 a month to $15,000. You would have to double everyone else, not add a client.
Two and a half months is the real number to sit with. It is not the drama of losing a client, it is the length of the runway, and it is usually shorter than people expect. Our burn rate calculator does the same sum on your full cost base if you want the wider view.
Find out what share of your income one client holds
Put in last year's billing, what your biggest client paid you and your monthly outgoings. The One Client Risk Check shows their share, the monthly gap they would leave and how long your cash would cover it. Free, no signup.
What to do this month, with what you already have
None of this needs new software or a difficult conversation on day one. Take them in order.
- Work out the number properly. List last year's billing by client, largest first, and calculate the top one as a share of the total. One export or one evening with your invoices. Use billing dates rather than payment dates so a slow payer does not distort the picture. Until you have the figure, everything else is a feeling.
- Treat their late payment like anyone else's. The fear of annoying a big client is exactly why their invoices drift. Send the reminder on the day it is due, in the same words you would use with a smaller client. Our payment reminder templates keep the tone neutral, and getting clients to pay on time covers the habits that work before the invoice goes out.
- Ask for a notice period at the next natural moment. A renewal, a price review or a new project is the moment to put 30, 60 or 90 days' notice in writing. Frame it as planning rather than distrust: you are asking for warning so you can keep their work staffed to the end. A notice period is the single change that converts this risk from sudden to manageable.
- Build the reserve out of their own payments. Set aside a fixed slice of every payment from the biggest client, in a separate account, until you could cover the shortfall for three months. It is the cheapest insurance available to you and it needs no one's agreement.
- Grow clients two and three before you go looking for strangers. They already trust you, and asking what else they need is faster than a cold pitch. Client acquisition and building a client base both cover the slower work of replacing the share, which is worth starting while you do not need it.
If they have already cut back or left
The order changes when the income has already gone. Work in this sequence.
Size the gap first, in months. Redo the sum above with them removed and your real outgoings, so you know how long you have. A number you can act on beats a week of worrying about a number you have not worked out.
Collect what you are owed before anything else. Their final invoices are the fastest money available, and they get harder to collect the longer the relationship is over, because the person who approved your work moves on. Start with the oldest, work down, and see how to handle outstanding invoices if any have drifted past 60 days.
Cut the costs that belonged to them. A subcontractor, a licence, a piece of software or materials you only used for their work. Those go before anything that helps you win the replacement.
Go back to former clients before you go to strangers. The people who have paid you once are the shortest route back to billing, and they are the 40% in the data above who were billed exactly once and never followed up.
Then say it out loud to someone. An accountant, a peer, anyone who will look at the numbers with you. Losing the account that paid the rent is a hit to confidence as much as to cash, and owners routinely make the cut too late because they are making it alone. If tax or debt is part of the picture, an accountant or a debt adviser is the right call, not a blog.
How to stop it coming back
Concentration is not a thing you fix once. It rebuilds itself every time a good client sends more work, which is to say every time things go well.
Three habits keep it in check. Check the share every quarter rather than every few years, so a drift shows up while it is still small. Keep one hour a week for finding work, even in a busy month, because that hour is the first thing to disappear and the hardest to restart. And set a ceiling you will not cross without a deliberate decision, for example: no client above 40% without a notice period and three months of cash in reserve.
Whatever you keep the records in, the quarterly check should take minutes, not an evening. In Billbooks, the invoices report filters by client and period and exports, and the outstandings report shows which clients carry the most unpaid balance, so the share and the risk are both a couple of clicks rather than a spreadsheet rebuild. Every paid plan includes all ten reports, from $7.95 a month, and the trial is free for 30 days with no credit card. A spreadsheet does the same job perfectly well if you already keep one up to date.
More problems like this one, with the first step for each, are collected on our cash flow problems hub.
One next step
Work out the share today, before you change anything. Ten minutes with last year's invoices tells you whether you are at 20% or 60%, and those are different problems with different answers. Everything else on this page depends on knowing which one you have.
Three free checks to size the risk, no signup
Frequently asked questions
What percentage of revenue from one client is too much?
As a rule of thumb, under 25% is low exposure, 25% to 49% is worth watching, and 50% or more means one client is carrying the business. There is no legal limit for a small business, though US public companies must disclose any customer worth 10% or more of revenue. A long contract with a real notice period makes a higher share safer than job-to-job work does.
Is it bad to have one very big client?
No. Most big clients are good clients, and a steady account is usually how a small business becomes stable enough to grow. The risk is not the client, it is having no plan for the day they pay late, cut back or leave: no notice period, no cash reserve and no second client close behind them.
How do I work out my client concentration?
List your billing for the last twelve months by client, largest first, then divide the biggest client's total by the total for all clients. Use billing dates rather than payment dates so a slow payer does not distort the result. A full year works better than a quarter, because it smooths out one-off large jobs.
What should I do first if my biggest client leaves?
Work out the monthly shortfall and how many months your cash covers it, so you know how long you have. Then collect every invoice they still owe, cut the costs that existed only for their work, and go back to former clients before pitching strangers, because people who have paid you once are the fastest route back to billing.
How do I chase a big client for a late payment without damaging the relationship?
Send the reminder on the day it falls due, in the same neutral wording you would use for any other client, and address it to accounts payable rather than to your contact. Chasing on a fixed schedule reads as a process rather than a complaint, and it is far less awkward than the conversation you have after 60 days of silence.
