Market sizing slides present enormous totals and investors discount them almost entirely, for reasons worth understanding.

The three layers

Total market, the part you could serve and the part you could realistically capture.

Which are usually presented and only the last matters.

Top-down sizing

Taking an industry figure and applying a percentage.

Which is fast and carries no information.

Bottom-up sizing

Number of potential customers multiplied by realistic price.

Which is defensible and demonstrates that you understand the buyer.

What it signals

Whether the founder has thought about who actually buys.

Common errors

Counting everyone who could conceivably buy.

Which produces figures nobody believes.

A smaller credible number is more persuasive than a large unsupported one.

Market growth

Whether the category is expanding or static.

Which affects whether growth requires taking share.

Taking share from established competitors is substantially harder.

New categories

Markets that do not yet exist cannot be sized from data.

Which is honest and is better stated than fabricated.

Analogous categories provide the only reasonable reference.

What to present

The bottom-up build, the assumptions in it, and the first segment you will actually pursue.

Where the numbers come from

Industry reports, government statistics and your own customer data.

Which vary enormously in reliability.

Citing the source and the date is a small thing that improves credibility considerably.

Research report figures

Commercially published market sizes with opaque methodology.

Which are quoted constantly and rarely examined.

Two reports on the same market frequently differ by multiples.

Segment sequencing

Which part of the market you enter first and why.

Which says more about strategy than the total ever does.

Pricing assumptions

The price in a bottom-up build carries most of the sensitivity.

Which makes it worth grounding in actual willingness to pay.

What investors are really assessing

Whether the outcome could be large enough to matter, and whether you think clearly about your market.

Sizing for internal decisions

The same work informs which segments to pursue.

Which is a better reason to do it than the pitch deck.

Sizing done only for investors tends to be done badly.

Customer counts

How many organisations or people actually match your buyer profile.

Which is frequently countable and rarely counted.

For business-to-business markets this is often a few thousand rather than millions.

Frequency and duration

How often they buy and for how long they stay.

Which converts a customer count into revenue.

Geographic scope

Where you can actually sell given language, regulation and support.

Which narrows the addressable figure considerably in practice.

Presenting honestly

State the method, cite the sources and show the assumptions.

Which is more persuasive than a large number without provenance.

A general note

Market data is estimated rather than measured, and reasonable people arrive at different figures.

Adjacent markets

Expansion opportunities beyond the initial segment.

Which is how a small initial market becomes a large outcome.

The sequence has to be credible rather than aspirational.

Timing

Whether a market exists now or will exist later.

Which is the difference between early and wrong.

Being too early is a common cause of failure with a correct thesis.

Bottom-up worked properly

Buyer count, penetration assumption, price and frequency.

Which is four numbers you can defend individually.

Sanity checks

Compare against the revenue of existing players.

Which catches sizing errors of an order of magnitude.

What to avoid

A large total, a percentage claimed without reasoning, and no segment sequence.

Which is the pattern investors have seen thousands of times.

Using it after funding

The segment analysis informs where sales effort goes.

Which is the version that actually affects the business.

Sizing filed after the raise was probably not worth doing.

The one-line summary

Build bottom-up, show your assumptions, name your first segment, and skip the enormous total.

A closing caution

None of this is prescriptive. Businesses differ by sector, by scale and by stage, and practices that work well in one context fail in another for reasons that are not always visible from outside.

What is consistent is that the businesses handling these questions well tend to have written something down, measured it in a defined way, and reviewed it on a schedule rather than when a problem forces the issue.

Where a decision carries legal, tax or employment consequences, professional advice specific to your jurisdiction is worth the cost, and this article is general description rather than advice.

One more thing worth saying

Most of what is written about running a business is written by people selling something, which shapes what gets emphasised and what gets left out.

The unglamorous parts, keeping records, reading the contract, updating the forecast, rarely feature because nobody can sell them. They are also the parts that most reliably separate businesses that survive from businesses that do not.

Whatever you take from this, take the habit of writing the number down and looking at it again next month.