How to construct a market size, top-down and bottom-up with a triangulation, is sizing craft taught in market sizing methods. What belongs here is narrower, and it's where pitches actually fail: what a sizing slide may claim, and what a reader does to check it.
Why do most market size slides fail?
A failing market slide shows a single large number with a percentage applied to it. Yardpost's first market slide said the United States food and beverage distribution software market is $18.5 billion, and that capturing 1% of it produces $185,000,000 of annual revenue. Each statement can be defensible on its own and the slide is still worthless, for a reason that has nothing to do with the size of the number.
The 1% is unearned. Why 1%, and not 0.1% or 4%? Nothing on the slide says. A reader who has seen the same slide two hundred times knows the percentage was chosen to make the product of the two numbers look attractive. Worse, the reader can't check anything. A top-down number arrives as an assertion sourced to a research house, and the share arrives as an aspiration.
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Worked example: rebuilding Yardpost's market slide
To repair it, build the market from countable things and show the count. Yardpost's rebuilt slide starts from 21,400 food distribution facilities. It applies a dock-door filter of 38%, because the product needs twelve or more doors to pay for itself, and that leaves 8,132 facilities. Then it applies 55% for the share that runs scheduled rather than first-come receiving, leaving 4,473 realistic buyers. Contract value is tiered: 55% of facilities are small at $14,400, 33% are mid at $27,600, and 12% are large at $52,800, which blends to 0.55 × 14,400 + 0.33 × 27,600 + 0.12 × 52,800 = $23,364. That makes the serviceable market 4,472.6 × 23,364 = $104,497,826.
Now for the interesting part, which is what you do with the discrepancy. Yardpost's top-down claim was $185,000,000 and the build is $104,497,826. So the claim is 1.770 times the build, an overstatement of $80,502,174, and the build is 56.49% of what was claimed.
A weak pitch drops the top-down slide and shows only the build. A strong one shows both and states the ratio. A reader who watches a founder catch their own overstatement stops auditing and starts listening. On a market slide, volunteering the number that hurts you is the cheapest credibility you can buy.
Which habits help a market size slide survive scrutiny?
Two more habits help the built slide survive contact.
State every filter as an assumption with a source, and put the filters on the slide itself, where nobody has to dig through the notes for them. A reader who disagrees with the 55% can substitute their own and see what happens. Someone running their own sensitivity is engaging with the slide, and they've stopped dismissing it.
Show adjacency separately, never blended in. Yardpost's adjacent verticals, meaning pharmaceutical and building-products distribution, multiply the core market by an assumed 2.6 to reach $271,694,349. That belongs in a clearly labelled second bar with the multiplier stated. Fold it into a single number and the reader's confidence in the core build has to carry the adjacency assumption too.
What share of the market does the plan reach?
Sizing has a bottom too, and founders skip it. Yardpost's current $1,437,640 of annual recurring revenue is 1.376% of the built serviceable market, and the milestone the raise buys, $4,200,000, is 4.02% of it. Those figures make the ambition legible and the plan credible. A market slide that shows a $271 million opportunity and never says what fraction of it the plan reaches has changed the subject.