An owner told me about their best account. Roughly $85,000 a year, reliable, genuinely profitable, about 12% of revenue. They were right that it was a good account.
The business was running an operating margin in the high teens. Which means that account was not 12% of the business. In practical terms it was most of the profit.
That is the arithmetic almost nobody does. Concentration gets measured against revenue, because revenue is the number that is easy to get. It should be measured against margin, because margin is what actually disappears.
The calculation
Two numbers.
Account revenue ÷ total revenue = concentration
Account contribution ÷ operating profit = what you actually loseThe second one is the honest number, and it is almost always larger than people expect.
Work it through. A business doing $700,000 at a 15% operating margin earns roughly $105,000. An account at 12% of revenue is $84,000 of top line. If it contributes at, say, 40% after its direct costs, that is about $33,600 of contribution — around a third of the operating profit, from one relationship.
Lose it and revenue falls 12%. Profit falls by a third. And the overhead it was helping carry does not fall at all, at least not for a while, because you cannot shed rent or a manager or your insurance in the month an account leaves.
That is the gap between how concentration feels and what it does.
Do it for your top five
Not just the biggest one. List your top five accounts, calculate contribution for each and express each as a percentage of operating profit rather than revenue. Owners routinely find that three accounts carry the entire profit and the rest of the book roughly breaks even.
That is not a crisis. It is extremely common in owner-operated businesses, and knowing it changes what you do about growth.
Concentration is not automatically a problem
I want to be careful here, because this topic attracts a lot of advice that amounts to “never let any customer get large,” which is bad advice for a small business.
Large accounts are usually your best accounts. They are cheaper to serve per dollar, they have lower acquisition cost, they often have better payment behavior, and building one is frequently the smartest commercial thing an owner has ever done. Telling someone to deliberately stay small in their best relationship is telling them to be worse at business.
The problem is not size. The problem is an account that is large, undocumented and unmanaged at the same time. Those three together are what turns a good account into an existential one.
So the goal is not to shrink it. The goal is to make its size survivable.
What makes concentration dangerous
Four things, and they compound.
No contract. The most common and the most fixable. A large account on a handshake means there is no defined scope, no notice period, no liability language and no agreed process for a dispute. The same owner above was doing roughly six figures a year of work for a multi-site institutional client with nothing signed. Their own concern about the exposure: if someone slipped on a floor their team had just cleaned, the claim goes to the property owner and the property owner comes to them, and there is nothing on paper defining what was agreed.
The owner holds the relationship. If the client’s only real contact is you, the account leaves when you are unavailable, distracted or replaced by their new procurement process. It also means you cannot take a holiday without the relationship noticing.
No notice period. A large account that can end on thirty days is a completely different risk from one that has to give ninety. Notice is the single cheapest protection available and it is usually free to ask for at renewal.
No replacement pipeline. If you have never sold anything comparable, you do not know whether you could replace it, how long it would take, or what it would cost in acquisition spend. That uncertainty is what makes owners tolerate bad behavior from large accounts, which is its own problem.
What to do, in order
1. Size the risk honestly
Contribution as a percentage of operating profit, for your top five. Then one more figure: how many months of overhead your cash reserve covers. Those two together tell you whether an account leaving is a bad quarter or an emergency.
My general rule: any account above 10% of revenue gets a written agreement, a named relationship owner who is not you, and a documented view of what happens if it ends. Above 20%, I would treat replacement as an active project regardless of how good the relationship is — not because you expect to lose it, but because the option to walk away is what keeps the terms fair.
Those thresholds are starting points from businesses I have worked with, not benchmarks. Move them to suit your margin and your reserve.
2. Paper it, framed as process rather than distrust
Introduce an agreement as something that is happening rather than something being negotiated. Insurance and liability review is the honest and usable frame, because it is generally true and it moves the conversation from “I want something from you” to “this is a requirement we both have to satisfy.”
Bring the existing terms, written down and unchanged. Do not change the price in the same conversation. If you paper the deal and reprice it at the same moment, you have reopened everything, and a routine adjustment becomes a full renegotiation. Price first, in a separate conversation, then the agreement. That sequence is covered in more detail in how to raise prices on an account you are afraid to lose.
Tradeoff: you are spending a conversation, and some goodwill, on something that produces no immediate revenue.
On the fear that asking will lose the account: owners consistently over-read the emotional temperature of large institutional clients. You are dealing with people executing a role. The person reviewing agreements cares whether the document exposes their employer, not whether you asked for one. In my experience these requests get processed rather than debated.
That said, if a client genuinely refuses to document work they have been doing for years, you have learned something worth knowing. Weight it.
3. Get the relationship off yourself
Name someone else as the day-to-day contact and make sure the client knows who they are. This is the step owners skip, because the relationship feels personal and handing it over feels like a downgrade.
It is the step that converts a personal dependency into a business asset. An account that only works because of you is not transferable, which means it does not survive your absence and it does not count for much if you ever sell.
Tradeoff: service will be different, and probably worse for a period. That is the cost of the account becoming an asset rather than a liability attached to your calendar.
4. Build the replacement capability before you need it
Not a replacement account. The capability to win one.
The useful question is not “who could replace this account.” It is “do we know how to sell this, and have we done it recently?” If your last comparable account arrived by referral four years ago, you do not have a repeatable sales process, you have a lucky history.
Sell one more of the same type while you still have the original. That is the only version of this that actually reduces risk, and it takes a year, which is why it has to start before the question is urgent.
5. Decide in advance what you would do
Write down what happens if the account gives notice tomorrow. Which costs come out, in what order and how fast. Which people are affected. How long the reserve lasts.
Owners who have written this down negotiate completely differently, because they are no longer negotiating from a place where the answer is unknown.
The trap this creates
There is a knock-on effect worth naming, because it is where concentration does the most quiet damage.
An owner who cannot afford to lose an account starts making concessions to keep it. Scope creep gets absorbed. Prices do not move for three years. Poor treatment of staff gets tolerated. Payment terms slip and nobody chases.
None of those decisions look like concentration risk. Each one looks like commercial pragmatism in the moment. Together they are the account slowly repricing itself downward while consuming more of the business.
That is why the reserve matters as much as the contract. An owner with two months of operating expenses in the bank has different conversations than one without, with the same customer, about the same issue, on the same day. The account did not change. The owner’s position did.
What about the other direction?
Worth a note, because owners sometimes over-correct.
Deliberately turning down a large opportunity to avoid concentration is usually wrong while you are still building the book. Growth from a large account is real growth, and the risk is manageable with the four steps above. Refusing it to keep a tidy distribution chart means choosing a smaller, more fragmented, more expensive-to-serve book of business for the sake of a ratio.
The exception is when a single account would require you to restructure the business around it — dedicated staff, dedicated equipment, systems you would not otherwise buy. That is not a customer any more, it is a business model change, and it should be evaluated as one.
Do this
- Contribution as a percentage of operating profit, top five accounts
- Months of overhead your reserve covers
- Anything over 10% of revenue: contract, named owner, written exit view
- Price first, paper second, never together
- Sell one more of the same type while you still have the original
Customer Audit Scoring Sheet → The sheet includes a concentration flag. Set your total company revenue on the first tab so it divides by the right number.
Frequently asked
What is a safe level of customer concentration? There is no universal figure, and it depends more on your margin and reserve than on the percentage. As a starting point I would want a contract and a named relationship owner above 10% of revenue, and an active replacement effort above 20%. A business with six months of reserve can carry more concentration than one living week to week.
Should I fire a large account to reduce concentration? Almost never. Reducing concentration by removing revenue makes the business smaller and does not make it safer. Grow the rest of the book instead, and make the large account survivable through contract, notice period and a transferable relationship.
How do I ask for a contract after years without one? Frame it as an operational or insurance requirement rather than a change in trust, bring the existing terms unchanged and do not touch price in the same conversation.
What notice period should I ask for? Long enough to replace the revenue, which for most owner-operated service businesses is sixty to ninety days. Ask for what you actually need rather than what sounds reasonable.
My biggest account is also my most profitable. Is that better or worse? Both. It is a better account and a larger exposure, and the two are not in tension. Treat it as the most valuable thing you have and protect it accordingly, which means documenting it, not shrinking it.
How do I know if I could replace it? Ask when you last sold something comparable and how it arrived. If the honest answer is “years ago, through a referral,” you have a history rather than a process, and building the process is the actual risk reduction.
Concentration is one of the first things I look at, because it changes how much risk every other decision carries. If you want help sizing it against your own numbers, here is how I work.