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Expected Value (EV) is the average amount you can expect to win or lose per bet if you placed the same bet thousands of times. It is the single most important concept in professional sports betting.
A positive EV (+EV) bet means you have a mathematical edge. Over hundreds of bets, you will come out ahead. A negative EV (-EV) bet means the sportsbook has the edge, and you will lose money over time.
Example:
You believe a team has a 55% chance to win. The sportsbook offers +120 odds ($100 bet wins $120).
EV = (0.55 x $120) - (0.45 x $100)
EV = $66 - $45 = +$21 per $100 bet
This is a +EV bet. Over 100 bets of $100 each, you would expect to profit roughly $2,100.
The key insight: you do not need to win every bet. You need to find bets where the odds are better than the true probability. Win rate does not matter as much as whether you consistently bet with positive expected value.
The Kelly Criterion is a mathematical formula that tells you exactly how much of your bankroll to wager on a bet, based on your edge and the odds. It was developed by John L. Kelly Jr. at Bell Labs in 1956 and has been used by professional gamblers and investors ever since.
Example:
You have a 55% chance to win at +150 odds (decimal 2.50, so b = 1.50).
Kelly = (1.50 x 0.55 - 0.45) / 1.50
Kelly = (0.825 - 0.45) / 1.50 = 0.25 = 25%
Full Kelly says bet 25% of your bankroll. On a $1,000 bankroll, that is $250.
Full Kelly is aggressive. It maximizes long-term growth but creates wild bankroll swings. In practice, your probability estimates are never perfect, so overbetting is a real risk.
Most professional bettors use Half Kelly (50% of the full Kelly recommendation) or Quarter Kelly (25%). This sacrifices some growth for much smoother results and protection against estimation errors.
A common rule: if your edge estimate could be off by more than a few percentage points, use Quarter Kelly. If you are very confident in your model, Half Kelly is reasonable. Almost nobody should use Full Kelly.
Even with a genuine +EV strategy, short-term results are driven by variance. You could lose 10 bets in a row and still have a profitable edge.
The "Bets for 95% Confidence" metric above tells you roughly how many identical bets you would need to place before you can be 95% sure your results will be positive. The smaller your edge, the more bets you need. This is why volume matters in +EV betting.
The formula uses the normal approximation to the binomial distribution. For a bet with win probability p, loss probability q, and net profit b per unit wagered:
In plain terms: a 2% edge might need 2,000+ bets to prove itself. A 10% edge could show profit in under 100 bets. This is why finding high-EV opportunities and betting volume both matter.
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