Edge and bet size calculator
Being right isn’t enough. You need an edge after the fee, and a size small enough to survive the times you’re wrong.
Checked October 20, 2026The sample is Ana’s first trade under her plan in the book: a 54¢ maker bid on a team the sharp sportsbook prices at 58%, with about $320 in her accounts. Get the fee per share from the Kalshi or Polymarket calculator.
Why size matters as much as being right
Losses and gains aren’t symmetric. Lose 50% of your money and you need +100% to get back to even. A trader with a real edge who bets half the bankroll each time can still go broke after an ordinary run of losses; the same trader at 2% barely notices it.
The Kelly fraction gives the size that grows money fastest if your probability is exactly right. Your probability is never exactly right, so the book uses a quarter of it, capped at 2% of everything you have set aside for trading.
Where your probability should come from
Not from a hunch. Start from how often this kind of thing happens, adjust for the news, and write the number down before you look at the price. For sports, a sharp sportsbook’s price with its margin removed is the best starting point; the odds converter does that step.
Questions people ask
How do I calculate my edge on a prediction market?
Edge per share = your probability − price − fee per share. If you think an event is 70% likely, the price is 62¢ and the fee is 1.65¢, your edge is 70 − 62 − 1.65 = 6.35¢ a share. If it isn’t positive, there’s no trade.
What is the Kelly criterion for a prediction market?
The Kelly fraction is (p − cost) ÷ (1 − cost), where p is your probability and cost is the price plus fee. It assumes your number is exactly right, which it never is, so professionals bet a fraction of it.
How much should a beginner bet per trade?
The book’s rule: the smaller of a quarter Kelly and 1–2% of your whole bankroll, counted across every platform. At 2% of $500, that is $10 a trade. It keeps a bad run survivable.
The whole method is in the book
These tools come from 62% Chance, a plain-English guide for beginners by Armando Acevedo, who spent 25 years setting prices for sportsbooks. It follows one new trader from her first look at a price to a tested plan: fees, the order book, borrowing a sportsbook’s sharp prices, bet size, scams, taxes and when to stop.
