Guide
Estimating Market Size: TAM, SAM and SOM, Top-Down and Bottom-Up
“How big is the market?” is one of the first questions a lender, an examiner, a partner or your own common sense will ask. This guide explains TAM, SAM and SOM, compares top-down and bottom-up estimates, and walks through a fully hypothetical example.
Please note: every figure in this guide is hypothetical, invented purely for teaching. None of them is an estimate of any real market.
TAM, SAM and SOM in plain English
- TAM (total addressable market): total demand for your type of product or service if every possible customer bought it. It is the outer boundary.
- SAM (serviceable addressable market): the part of TAM you can actually serve, given your location, product, language, price point and channels.
- SOM (serviceable obtainable market): the share of SAM you can realistically win within a set time, given competition and your capacity.
Always state the time period (usually one year) and the unit (customers, units sold or rupees) you are using.
Top-down: start big and narrow down
A top-down estimate begins with a large figure, such as a population, a number of households or a published industry total, and applies a series of percentages to narrow it down. It is quick, but every percentage is an assumption, and small errors multiply.
Bottom-up: count from the ground
A bottom-up estimate starts from things you can observe or count: specific places, customers you can reach, how often they buy and what they pay. It takes more legwork but is usually more convincing for a small business, because each number can be checked.
Worked example: a tiffin service (hypothetical)
Imagine you plan to start a home-style lunch tiffin service in a mid-sized city. A quick reminder on Indian number terms: 1 lakh = 1,00,000 and 1 crore = 100 lakh = 1,00,00,000.
Top-down estimate
| Step | Hypothetical assumption | Result |
|---|---|---|
| City population | 30 lakh people | 30,00,000 people |
| Working adults and students living away from family | 10% of population | 3,00,000 people |
| Would consider a paid tiffin service | 30% of those | 90,000 people |
| Average spend per subscriber | ₹2,500 per month | – |
| TAM (per year) | 90,000 × ₹2,500 × 12 | ₹270 crore |
| Share of those people inside your delivery area | 15% | 13,500 people |
| SAM (per year) | 13,500 × ₹2,500 × 12 | ₹40.5 crore |
| Realistic share of SAM by year two | 2% | 270 subscribers |
| SOM (per year) | 270 × ₹2,500 × 12 | ₹81 lakh |
Bottom-up estimate
| Step | Hypothetical assumption | Result |
|---|---|---|
| PG hostels and co-living buildings in your delivery area | 40 buildings × 30 residents | 1,200 people |
| Small offices without a canteen | 25 offices × 20 staff | 500 people |
| Total reachable people | 1,200 + 500 | 1,700 people |
| Share who subscribe after a free trial | 10% | 170 subscribers |
| Kitchen capacity check | Up to 200 lunches a day | 170 fits within capacity |
| Obtainable revenue (per year) | 170 × ₹2,500 × 12 | ₹51 lakh |
Comparing the two
The hypothetical top-down SOM (₹81 lakh) is higher than the hypothetical bottom-up figure (₹51 lakh). That gap is useful: it points to the assumptions that most need testing. Is a 2% share realistic? Is a 10% trial conversion realistic? You could test the second cheaply by running a free-trial week in two or three buildings and counting sign-ups. When the two methods roughly agree, you can be more confident; when they don’t, you know where to dig.
Building your own estimate
- Define the product, the customer and the time period precisely.
- Do a quick top-down estimate to see the rough scale.
- Do a bottom-up estimate from things you can count or observe.
- List every assumption in a table with its source: “counted”, “published source” or “my guess”.
- Compare the two results and investigate the biggest differences.
- Test your weakest assumptions with small real-world experiments or a short survey.
- Present a range (low, middle, high) rather than one precise-looking number.
Common mistakes
- Treating TAM as a sales forecast. Almost no business captures its whole TAM.
- Stacking optimistic percentages: several generous assumptions in a row can inflate the answer many times over.
- Mixing units, with people in one row and households in the next.
- Using a published figure without checking what it covers, which year it refers to and who produced it.
- Ignoring capacity: a SOM you cannot physically deliver is not obtainable.