Once you know a bet is +EV, the next question is how much to wager. Bet too little and you're leaving profit on the table. Bet too much and you risk blowing up during a bad stretch. The Kelly Criterion is a mathematical formula that finds the sweet spot.
The formula
You find a bet at +150 odds (decimal 2.50) and estimate a 45% chance of winning.
b = 2.50 − 1 = 1.50
p = 0.45
q = 0.55
Kelly % = (1.50 × 0.45 − 0.55) ÷ 1.50 = (0.675 − 0.55) ÷ 1.50 = 0.125 ÷ 1.50 = 8.3%
Full Kelly says to bet 8.3% of your bankroll. On a $1,000 purse, that's $83.
Why full Kelly is too aggressive
In theory, Kelly maximizes bankroll growth. In practice, full Kelly is dangerously volatile. It assumes your probability estimate is perfectly accurate — and it never is. If you overestimate your edge even slightly, Kelly will have you betting way too much.
The standard in professional betting is to use fractional Kelly:
- Half Kelly: Divide the Kelly output by 2. Reduces variance significantly with minimal impact on long-term growth.
- Quarter Kelly: Divide by 4. Even safer. Many professional bettors use quarter Kelly.
In the example above, quarter Kelly would be 8.3% ÷ 4 = about 2% of your bankroll — roughly 2 units.
Why beginners should just flat bet
Kelly requires accurate probability estimates on every bet. If your estimate is off — and as a beginner, it will be — Kelly amplifies the error. Overestimate your edge by 5% and Kelly will have you oversizing bets on -EV propositions.
Flat betting (1 unit on every bet) removes this problem entirely. You don't need to estimate your edge precisely — you just need to identify +EV spots. The sizing stays constant, the variance stays manageable, and you learn without risking ruin.
Graduate to fractional Kelly after you have 500+ tracked bets with documented, verifiable edge in specific markets. Until then, the 1-unit system is safer, simpler, and nearly as effective for anyone who can't precisely quantify their edge.
The bottom line: The Kelly Criterion calculates optimal bet size based on your edge, but full Kelly is too aggressive and requires perfect probability estimates. Most pros use quarter or half Kelly. Most beginners should flat bet 1 unit per game until they have enough data to confidently estimate their edge.
Run the numbers before you bet.
The BeginnerBets +EV Calculator shows you instantly whether a bet is worth placing — based on math, not gut feeling.
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