Customer concentration is the share of your revenue that comes from a single customer or a small group of them. It is the risk that behaves best right up until it doesn't. A large account smooths your cash flow, justifies capacity, and makes forecasting feel easy — until the day that account renegotiates, gets acquired, or brings the work in-house.
Most owners know roughly who their biggest customer is. Far fewer know the actual percentage, and almost nobody knows what it was eighteen months ago compared to today. That gap is where the risk lives.
How to measure it
Pull trailing twelve months of revenue by customer. Not calendar year — trailing twelve, because seasonality distorts partial-year numbers badly in most product businesses.
Then calculate three things:
Top-one share. Largest customer's revenue divided by total revenue.
Top-five share. Your five largest customers combined, divided by total. This catches the case where no single account looks alarming but a handful of accounts collectively own you.
Direction of travel. Run the same two numbers for the prior twelve months. A top-one share of 28% that was 19% last year is a different situation than 28% that was 34%. One is a growing dependency, the other is a diversifying business.
What the thresholds actually mean
There is no universal danger line, but there are practical bands.
Under 10% — no single account can materially hurt you. This is unusual for small businesses and often means you're leaving relationship depth on the table.
10–20% — normal and generally healthy. Losing your top account would be a bad quarter, not an existential event.
20–35% — this is where it becomes a real strategic consideration rather than a number on a page. You can survive the loss, but it would likely mean layoffs, a credit line draw, or both. Lenders and acquirers start asking about it here.
Above 35% — you are, in practice, a supplier division of that customer's business. Their strategy is now your strategy. Their payment terms are your working capital policy. This is survivable and plenty of businesses run this way for years, but it should be a decision you've made deliberately, not one you drifted into.
Worth noting: concentration matters more when switching costs are low. If your largest customer could replace you in a month with a phone call, 25% is riskier than 40% would be for a supplier who is embedded in a certified production process.
The signals that precede a loss
Concentrated accounts rarely disappear without warning. The warnings just don't look like warnings at the time.
- Order pattern changes. Same annual volume, but ordering shifts from monthly to quarterly. Often means they're building a buffer while they qualify a second source.
- A new contact appears. Procurement gets involved in a relationship that used to run through operations. Someone is being asked to justify the spend.
- Payment terms drift. Not a formal renegotiation — just invoices going from 32 days to 45 without a conversation. This is the same signal worth tracking in your AR aging report, and it's usually where the first sign shows up if you're watching for it.
- They ask for your cost breakdown. Sometimes this is genuine collaboration. Sometimes it's the groundwork for a bid comparison.
- RFP language enters the conversation. By this point the decision is often already partly made.
None of these individually mean anything. Two or three together in the same quarter warrant a direct conversation.
What to actually do about it
Don't fire the customer. The advice to "diversify away" from a large account usually comes from people who have never had to replace 30% of revenue. You diversify by adding, not subtracting.
Fix the terms first. If concentration is your risk, the cheapest mitigation is contractual. A notice period, a minimum volume commitment, or a term contract instead of PO-by-PO doesn't reduce the percentage but does convert a cliff into a slope. This is the highest-leverage move available and it costs nothing but a conversation.
Deepen before you widen. More products into the same account is not diversification, but it does raise switching costs — which reduces the probability of loss even if it doesn't reduce the impact. Both matter.
Set a growth target in dollars, not percentage. "Get the top account under 25%" tempts you to shrink it. "Add $400K of revenue outside the top three accounts" is the same goal with a healthier incentive.
Know your break-even without them. The number worth having on hand isn't the concentration percentage. It's what your cost structure would need to look like if the account went away — which costs are variable, which contracts you could exit, how many months your cash covers the gap. Owners who have run that math sleep better and negotiate harder, because the account can tell when you can't afford to lose them.
The version that matters
Concentration is a number, but the decision it informs is about leverage. A customer who represents 30% of your revenue knows it. The question that actually matters isn't "what percentage is too high" — it's whether you'd be able to say no to their next request, and what it would take to be able to.
Get the real number before the call comes
Most owners can guess at concentration. Few can produce the actual break-even-without-them math on demand, and that's the number that matters when the conversation with your biggest customer gets hard.
Helm's CFO advisor connects directly to QuickBooks and can walk through the break-even-without-them math with you using your real P&L numbers. Pull a by-customer sales report from QuickBooks for the exact top-one and top-five revenue share, then bring it into the chat — the COO advisor can cross-reference it against your uploaded contracts to check whether you already have a notice period or minimum-volume clause protecting you. Ask either one to run the numbers before the next renegotiation, not after.