In a haze of late-night ambition, a young founder opened a spreadsheet and typed one number: $125 million. Five thousand colleges. Twenty-five thousand dollars each. The math was intoxicating. He had found his goldmine… or so he thought.

Have you ever built an entire business plan on a market size you never actually verified? Most founders do, and it's the single most expensive assumption they'll ever make.

In Episode 10 of Starting Up, host Jay Sensi breaks down TAM, SAM, and SOM, the three acronyms that separate the dreamers from the doers. Drawing from his own gut-punch experience launching My College Roomie (Campus Kaizen), Jay reveals how his "$125 million opportunity" collapsed into a fraction of its size the moment real research entered the room.

The truth was brutal. 5,000 potential customers became 2,000. The internatio ...