What are the most common mistakes founders make when sizing their market, and how do I avoid faking a big TAM?
The short answer
The classic mistake is starting from a giant global number and multiplying down with invented percentages (the 1% of a billion-dollar market trap), which no serious investor believes and which fools you more than them. Build it bottom-up: realistic number of buyers times a price they'll actually pay, then sanity-check against comparable companies' revenue. A smaller, defensible number you can explain beats a huge one you can't, and it forces you to know who your customer really is.
Go deeper, your way
3 hand-picked resources, 3 link-checked. Pick how you want to dig in.
▶️ Video
✓ Link checkedFreeBeginner
Why we picked it
Kevin Hale (YC partner, Wufoo co-founder) walks through how to package your idea so an investor believes it can grow fast, and a big part of that is telling the founder story so 'why you' lands instead of sounding like a boast. It is practical and script-level: how to open, what to lead with, and how to make your unfair edge legible in the first minute. Watch it once before you write a single pitch line.
A pitch is really a hypothesis about why this company can grow quickly, so your 'why you' has to feed directly into that growth story, not sit as a separate bio slide.
Investors weigh how well you can sell and tell the story, so evidence you understand your customer beats abstract claims about your background.
Lead with clarity: make the problem, your insight, and why you are the one to solve it understandable in a sentence or two.
Why we picked it
When there is no category report to point at, you have to build the number yourself, and this is the essay that teaches you how. It walks through bottoms-up sizing (start from your actual customer, their willingness to pay, and how you will reach them) and shows why the top-down 'we just need 1 percent of a huge market' story falls apart. Treat it as the method for a defensible estimate, not a promise about how big you will get.
From
Andreessen Horowitzby Anu Hariharan, Frank Chen, Jeff Jordan~20 min read
Build TAM from the bottom up: real customer profile times realistic price times how many you can actually reach and sell to.
Top-down percentages inflate the number and hide the hard part, which is distribution and go to market.
Some of the best companies (eBay, Airbnb) started against a market that looked small, then expanded the use case, so a modest starting number is not a dealbreaker.
Why we picked it
This piece names the exact trap you are trying to avoid: googling an industry stat and assuming you just need 1 percent of it to build a billion-dollar company. It calls that the wrong approach and walks you to the fix, sizing TAM as annual revenue per customer times the number of customers who actually match your profile and would pay. It is short, practical, and aimed at the pitch slide where founders most often fake a big TAM.
