The Existential Dozen — top-10 concentration risk.

Diversification isn't something small agencies have by default — it's something they have to build on purpose, and most don't. Scale produces it mechanically: a billion-dollar agency needs a hundred clients in its top bucket to cross 10% concentration, while a $500K agency reaches 5% concentration with a single $25K commercial account. So the smaller the agency, the more deliberately concentration has to be managed — and the more carefully a buyer has to read it.

§ 01 · The threshold and the benchmarksTop-10 above half the book.

The Existential Dozen threshold is a top-10 concentration above 50% of commission revenue. Below it, the rest of the book can absorb a single account loss; above it, two or three departures push the agency past the point where capacity and cash flow recover. Where healthy books land depends heavily on scale.

Read concentration as a three-signal stability picture rather than one number: the single-client share (flagged by the 15% rule), the top-10 share (the Existential Dozen line at 50%), and the active client count — the denominator that makes diversification possible. A healthy sub-$1M agency keeps its largest client under 15% (ideally at or below 10%), its top 10 in the 25–40% range, and 300 or more active clients. A thin denominator is the quiet killer: 150 clients can't dilute a whale through new business the way 400 can.

§ 02 · When structure can't save itFour walk-aways.

Most concentration is priceable through a haircut and an earn-out. Four scenarios aren't.

The other three: a top-10 concentration of 55%+ in a book under 200 clients, where the denominator is simply too thin for new business to dilute; concentrated accounts in industries undergoing structural change, where carrier non-renewal, regulatory disruption, and competitive compression form a double-trigger problem; and concentration tied to a single non-transferable carrier appointment, where concentration and transferability compound into one unhedgeable risk. In each, the discount that looks generous is really a warning — the risk isn't priceable because it isn't survivable.

§ 03 · The seller's runwayDiluting the top 10.

With a two-year horizon — meaningful runway, where 12 months is a sprint that rarely shifts more than a few points — a seller has four levers. Prune the bottom-half tail: 60 to 100 tiny accounts produce a disproportionate service load, and shedding them frees capacity for mid-market production. Target the middle: $8K–$15K commission commercial accounts are big enough to dilute concentration but not so big they join the top 10. Document durability on the concentrated accounts — a ten-plus-year relationship with a cross-sold set of lines and a second touchpoint reads completely differently from a three-year single-line account managed by the departing owner. And, counterintuitively, strategically replace one concentrated account: losing 20% of one client and replacing it with five $10K accounts can raise the multiple enough to produce a larger sale price than keeping the whale.

§ 04 · Why buyers price to the lineThe math is discrete.

A 52% top-10 isn't dramatically worse than a 48% top-10 in any continuous sense — but it sits on the wrong side of the line where diversified risk management is still possible, and buyers price to the line. That's not arbitrary: below 50% the book has enough independent revenue that a normal year of new business and a normal level of attrition net out to stability; above 50% the agency's survival depends on a handful of relationships behaving, which is a fundamentally different risk to underwrite. So treat the threshold as real even though the underlying number is continuous, and pair the top-10 read with the single-client number and the client count before you decide whether the discount is a buying opportunity or a reason to walk.

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