Here's a number that decides more funding conversations than founders expect: what percentage of your revenue comes from your biggest customer? If the answer is "most of it," your impressive-looking startup is one email away from a crisis — and investors, lenders and acquirers all know to check.

What it is

Customer concentration risk is the danger created when too much of your income depends on too few customers. A startup doing £200,000 with one client providing £150,000 of it isn't a £200,000 business in risk terms — it's a business whose survival is controlled by someone else's decision to renew. Lose that client and 75% of your revenue vanishes overnight.

Why the smart money flags it

  • Investors see fragility, not scale. A diversified £200k business is worth more than a one-client £200k business, because the risk profile is completely different.
  • Lenders worry about repayment — a single lost contract could sink your ability to service debt.
  • Acquirers discount heavily for it, because they're buying a relationship they can't control.

It also quietly weakens you: a client providing most of your revenue has enormous leverage on price, terms and how they treat you. Concentration doesn't just risk your business — it costs you negotiating power every day.

The rough rule of thumb

There's no magic threshold, but as a guide: any single customer above ~20–25% of revenue is worth watching, and above ~40% is a genuine risk you should actively be reducing. Your accounts should make this visible — if you don't know your top customer's share of revenue off the top of your head, that's the first thing to fix.

Know the number before an investor asks it Clean management accounts show revenue by customer at a glance. Walking into a funding conversation already able to say "our largest client is 18% and falling" signals a founder in control. Being surprised by the question signals the opposite. This is exactly the kind of visibility our management accounts provide.

How to reduce it

  • Deliberately win smaller clients even when the big one is comfortable — diversification is a growth strategy, not just a safety one.
  • Turn one-off clients into recurring revenue so your base is broader and stickier.
  • Lock in longer terms with your key client to reduce the immediacy of the risk while you diversify.
  • Track it monthly — concentration creeps up quietly when a big client grows faster than the rest.

The founder takeaway

Growth that all comes from one customer feels great and is quietly dangerous. The startups that raise well and sell well are the ones that saw the concentration and diversified before they had to. We give founders the management information to see it coming — part of moving beyond basic compliance as you grow. Get started.