One percent of a big market is not a market.
Why top-down sizing fails, how to decompose an opportunity across the verticals a product can credibly reach, and why the honest number is usually smaller and worth more.
There is a slide that appears in a large fraction of decks and costs more meetings than almost anything else on the page. It cites an industry report, states that the market is worth some very large number, and observes that capturing one percent of it would produce a substantial business.
Every experienced investor has seen this slide more times than they can count. What it communicates is not the size of the opportunity. It is that nobody has done the work.
The top-down number is wrong in both directions at once
The obvious failure is overstatement. A headline industry figure includes buyers the company cannot reach, geographies it does not operate in, budget lines it does not touch, and incumbents who will not be displaced. Very little of it is addressable in any meaningful sense, and the one percent is not a modest claim, it is an unexamined one.
The less obvious failure is understatement, and it is the more expensive of the two. A single industry category rarely describes what a product can actually sell into. The same technology usually reaches adjacent use cases, different buyer types, and other geographies, and each of those is a real part of the opportunity. Compressing all of it into one category headline throws away the most interesting thing about the company.
The top-down number simultaneously claims too much and asks for too little.
Decompose, then build each segment from the bottom
The credible method treats sizing as several small pieces of research rather than one large citation. Pin the core vertical the product serves today. Then reason explicitly about the adjacent and expansion verticals the same product, technology or platform can credibly touch: new use cases, new buyer types, new geographies, and the upsell or platform plays that follow.
Each segment is then sized from the bottom up, and the arithmetic is the same every time: reachable customers, multiplied by realistic price or contract value, multiplied by an adoption assumption that can be defended. Each segment carries its own source and its own growth rate, because a mature segment and an emerging one do not grow at the same speed and averaging them hides the part that matters.
Aggregate those segments and the three familiar figures fall out with actual meaning behind them. The total is the sum of credible reachable segments, core plus the adjacencies that genuinely expand. The serviceable portion is what current segment focus, geography and route to market can reach now. The obtainable portion is what three to five years of execution plausibly wins.
Conservative per segment, comprehensive across segments
That phrase is the whole discipline. Each individual assumption should be one an investor would accept without argument, and often lower than the founder’s instinct. The comprehensiveness comes from the number of segments considered, not from optimism inside any one of them.
The result is almost always a smaller headline figure than the top-down version, and it is worth considerably more. It survives interrogation, because every component of it can be walked through. It shows the reader how the company thinks about expansion, which is what a growth investor is actually buying. And it demonstrates that the founder can distinguish between the market they are in and the market they cite, which is a proxy for a great deal else.
It is also the section founders are least equipped to build alone, because it is research work rather than founder knowledge, and doing it properly takes days. That is a reason to have it done, not a reason to cite a report.