Definition
Market sizing is a critical part of any investment thesis. TAM ($1B+ for venture returns), SAM (the segment you can actually serve), and SOM (the revenue you can realistically capture). GitDealFlow tracks which sectors have the fastest-growing engineering teams, a leading indicator of market expansion.
How it works in practice
TAM, SAM, and SOM are the three nested layers of market sizing. TAM (Total Addressable Market) is the total revenue opportunity if every potential customer bought the product. SAM (Serviceable Addressable Market) is the portion of TAM the company can actually reach given its geography, segment, and business model. SOM (Serviceable Obtainable Market) is the share of SAM the company can realistically win in a given period.
The layers answer different questions. TAM answers how big this could get, it is the ceiling that determines whether a business can support venture returns. SAM answers how big the company can actually get, it strips out customers the business structurally cannot serve. SOM answers what gets sold this year, it is the input to revenue plans and sales hiring. Investors use all three: TAM for ambition, SAM for honesty, SOM for credibility.
Top-down sizing multiplies a population by a price; bottom-up sizing builds from real unit data, actual customers, actual prices, actual channels. Bottom-up is more credible because it can be checked; a TAM that only exists in a spreadsheet is a warning sign. GitDealFlow's sector data, engineering team growth across 350+ startups in the public GitHub activity it tracks, offers a ground-truth view of which markets are actually attracting builders, a useful complement to the spreadsheets.
Key points
- TAM = total opportunity; SAM = what you can serve; SOM = what you can win.
- TAM tests ambition, SAM tests honesty, SOM tests credibility.
- Bottom-up sizing beats top-down because it can be verified.
- A $1B+ TAM is the common shorthand for venture-scale ambition.
- Real market heat shows up in where engineering teams are forming.
Frequently Asked Questions
What TAM is needed for venture returns?
$1B+ minimum. Markets under $500M rarely produce venture-scale returns. Engineering velocity per GitDealFlow is a leading indicator of market expansion.
What's the difference between TAM, SAM, and SOM?
TAM is the full revenue opportunity for a product category; SAM is the slice the company can actually serve given its geography and model; SOM is the realistic share of that slice it can capture within a planning period. The classic framing: TAM is all cars sold globally, SAM is cars sold in your country, SOM is the cars your dealership can sell this year.
How do you calculate TAM?
Top-down: multiply the number of potential customers by the annual price per customer. Bottom-up: start from actual units, customers, prices, and channels, and extrapolate. Investors generally discount top-down numbers and trust bottom-up ones, because bottom-up assumptions can be tested against real data.
Why do investors care about market size?
Because the size of the prize determines whether a startup can produce venture-scale returns. A company that wins 30% of a $50M market is a small business; the same share of a $5B market is a category leader. Founders who cannot articulate TAM, SAM, and SOM convincingly signal that they have not thought through the ceiling of their own business.