When an investor asks how big your market is, the weakest answer you can give is a top-down one: "The Indian edtech market is worth 10 billion dollars, and we only need one percent of it." Every founder in the room has quoted the same analyst report, so the number tells the investor nothing about your specific business, and "just one percent" signals that you have no real plan to capture anything. A bottom-up estimate does the opposite. It builds your market from countable units you can defend, and it forces you to understand exactly who pays you, what they pay, and how often.
The formula
Bottom-up sizing is one line of arithmetic:
Market size = number of target customers times price per purchase times purchases per year.
Each term is something you can source or reason toward, rather than a figure you copied from a slide. The number of customers comes from a real population you can point to. Price is your actual or intended price, not a market average. Frequency is how many times a year a customer buys, which matters enormously: a wedding photographer and a coffee shop can serve the same person, but one sells once a decade and the other sells daily.
A worked example
Say you are building GST-filing and invoicing software for small service businesses. Start from a countable population. India has more than 1.5 crore active GST taxpayers, and over 7.8 crore enterprises are registered on the Udyam portal for MSMEs. You decide your realistic target is small service firms that already file GST, and you estimate that segment at roughly 20 lakh businesses. You price the product at 6,000 rupees per year, billed as an annual subscription, so frequency is one paid renewal per year.
That gives 20,00,000 customers times 6,000 rupees times 1 = 1,200 crore rupees per year. That is your bottom-up market. Notice how much more you learn than from a top-down figure: if the number looks too small, you can see whether the fix is more customers, a higher price, or an add-on that increases frequency. The math is a business model, not just a headline.
Turn it into TAM, SAM, SOM
Investors expect three nested layers. Build all three from the ground up.
- TAM (total addressable market): everyone who could ever buy, for example all 1.5 crore GST-registered businesses.
- SAM (serviceable addressable market): the slice you can actually serve today given your product, language, and geography, for example the 20 lakh small service firms above.
- SOM (serviceable obtainable market): what you can realistically win in the next two to three years given your team and sales capacity, often a low single-digit share of SAM. Justify it with your channel and conversion assumptions, not a round percentage.
Where to find real Indian customer counts
The credibility of a bottom-up estimate rests on the source behind your customer count. Use official, current data:
- MSMEs: Udyam Registration Portal figures (over 7.8 crore registered enterprises).
- Formal businesses and tax base: GST System Statistics for active taxpayer counts by state and type.
- Startups: DPIIT recognition data on the Startup India portal (more than 1.97 lakh recognised startups as of late 2025).
- Companies: MCA filings for registered company counts.
- Consumers and connectivity: TRAI telecom subscription reports and Census data for population segments.
Common mistakes to avoid
- Confusing a population with a market. "1.4 billion Indians" is not your TAM unless every one of them can and will pay you.
- Forgetting frequency. A one-time 10,000 rupee purchase and a 10,000 rupee annual subscription look identical in a spreadsheet but are very different businesses.
- Pricing at the average. Use your price, then show how it changes as you move upmarket or add tiers.
- Skipping the source. Every number should have a link an investor can check. An unsourced figure is a guess.
Do the bottom-up math before you ever open an analyst report. If a top-down number is far larger than your build-up, that gap is not free upside: it is a set of questions you must answer about who those extra customers are and why you cannot reach them yet.