Free Tools
Enter your estimated win probability and the offered odds to calculate expected value, ROI, and the mathematically optimal bet size for your bankroll.
What do you think the true probability is?
What is the sportsbook offering?
Used to calculate Kelly bet sizes
Updates in real-time
The Concepts
The two most important tools in a sharp bettor's toolkit.
EV = (your_prob × profit_if_win) − (loss_prob × stake). A +EV bet means that if you placed it thousands of times, you'd profit on average. A single bet can lose while still being +EV — EV is about long-run expectation, not individual outcomes. Consistently finding +EV bets is the only sustainable path to profit.
The Kelly formula f = (b×p − q) / b tells you what fraction of your bankroll to wager, where b = decimal odds − 1, p = your win probability, q = 1 − p. Full Kelly maximizes long-run growth but creates wild variance. Most professionals use half or quarter Kelly to smooth the ride.
If you estimate 55% win probability and the book implies 52.4% (−110), you have a 2.6 percentage point edge. That's a meaningful +EV bet. The key is the accuracy of your probability estimate — overestimating your edge is the most common way bettors lose despite "doing the math."
Your probability estimate should come from a model, historical data, or genuine research — not gut feel. A useful calibration check: if you estimate 60% for many games, do you win approximately 60% of them? If you win only 50%, your estimates are systematically overconfident. Closing Line Value is a better proxy for edge than win rate alone.
Our NBA ML model generates calibrated win probabilities for every game — the exact input you need to calculate EV and Kelly stakes.