What is expected value (EV) in betting?
Expected value is the average amount a bet wins or loses if you could place it over and over. A positive EV (+EV) bet makes money long term; a negative EV bet loses it. EV is the single most important concept in betting because it separates a good bet from a bet that simply won. You can win a -EV bet and still have made a bad decision, and lose a +EV bet and still have made a great one.
How to calculate expected value
- Convert the odds to decimal, then to implied probability (100 / decimal odds). That is your break-even win rate.
- Estimate your true win probability for the bet.
- EV per $1 staked = (true win % x decimal odds) - 1. Multiply by your stake for the dollar EV.
- If your true win % is higher than the implied %, the EV is positive and the bet is worth making.
What is the Kelly criterion?
The Kelly criterion is a formula for how much of your bankroll to stake on a +EV bet to grow it as fast as possible without risking ruin. Bet too little and you leave money on the table; bet too much and variance can wipe you out. Kelly finds the mathematically optimal middle. It only recommends a bet when you actually have an edge; with no edge, Kelly says stake nothing.
The Kelly formula and half Kelly
Kelly fraction = (decimal odds x true win % - 1) / (decimal odds - 1). Multiply by your bankroll for the stake. Because real win probabilities are estimates, most sharp bettors use half Kelly (or quarter Kelly): half the recommended stake. That keeps almost all of the growth while sharply reducing the swings, which is why this tool shows both.
The Wise Guy Team way
We bet to the edge, not to a hunch. Honest probabilities, disciplined unit sizing, and fractional Kelly are how a bankroll compounds instead of busting. The whole game is finding bets where your real win rate beats the implied price, then sizing them with a clear head.
Frequently asked questions
What is expected value in sports betting?
The average profit or loss of a bet over the long run. Positive EV bets make money over time; negative EV bets lose it.
How do you calculate expected value on a bet?
EV per $1 = (true win % x decimal odds) minus 1. Multiply by your stake. If your true win % beats the implied %, EV is positive.
What is the Kelly criterion?
A formula for the bankroll fraction to stake on a positive EV bet to maximize long-term growth without risking ruin. No edge means no bet.
What is the Kelly criterion formula?
Fraction = (decimal odds x true win % minus 1) divided by (decimal odds minus 1), times your bankroll for the stake.
Should I use full or half Kelly?
Most bettors use half Kelly because true probabilities are estimates. Half Kelly keeps most of the growth with far less variance.
Does the Kelly criterion work?
Yes, when your win probabilities are accurate. Its weakness is bad inputs: overestimate your edge and Kelly overstakes, which is why fractional Kelly is standard.
21+. For entertainment and educational purposes, not financial advice. If gambling stops being fun, take a break. 1-800-GAMBLER.