The Kelly Criterion is a mathematical formula that tells you exactly how much of your bankroll to bet on a single bet. In 2026, most professional bettors and poker players use it. It prevents overbetting and maximises long-term growth.
The Kelly Criterion is a formula that calculates the optimal bet size given your edge and the odds you are getting. The output is a percentage of your bankroll.
Formula: Kelly % = (bp - q) ÷ b, where: - b = odds received minus 1 - p = probability you win - q = probability you lose (1 - p)
Example: You are betting on a coin flip where you win $1.10 for every $1 wagered. The true probability is 50/50, but the odds imply only 47.6% probability (because the bookmaker takes a cut). Your edge is negative. The Kelly formula would tell you to bet 0%, meaning do not bet.
Now imagine a modified game where you win $2.20 for every $1 wagered (the bookmaker is offering generous odds). Your edge is positive. The Kelly formula might tell you to bet 3% of your bankroll on this single bet. If your bankroll is $1,000, you bet $30.
The Kelly Criterion prevents overbetting. Many losing bettors fail because they bet too much on each individual bet. If you have a 55% win rate and you bet 20% of your bankroll on each bet, a bad run of 10 losses in a row (which is statistically normal) wipes you out. The Kelly Criterion keeps you from ever going broke if you have a true edge.
Professional bettors often use "Fractional Kelly," betting only a percentage of the Kelly amount. Instead of full Kelly, they use 50% or 25% Kelly. This is because Kelly assumes perfect probability assessment, which is impossible. If you overestimate your edge, full Kelly destroys your bankroll. Fractional Kelly provides safety.
Most 2026 professional bettors use 25% Kelly or less. This trades some long-term growth for dramatically reduced risk of ruin.
| Your Win Probability | Decimal Odds | Kelly % | Practical Action |
|---|---|---|---|
| 55% | 1.91 | 0.9% | Bet less than 1% of bankroll |
| 60% | 1.91 | 5.2% | Bet 5% (or use fractional kelly: 1-2%) |
| 65% | 1.91 | 10.5% | Bet 10% (or use fractional kelly: 2-5%) |
| 70% | 1.91 | 17.3% | Bet 17% (or use fractional kelly: 4-8%) |
No. Kelly is optimal, but you can be profitable with simpler strategies (like flat-staking or unit-based betting). Kelly just accelerates your bankroll growth if you have a true edge. It also prevents catastrophic losses.
This is the biggest risk with Kelly. If you think you have a 60% edge but actually have a 50% edge, full Kelly will eventually destroy your bankroll. This is why fractional Kelly (25-50%) is safer. It gives you a buffer for probability estimation errors.
No. In pure luck games (slots, roulette), you have no edge. The Kelly formula would tell you to bet 0%. Kelly only works when you have a statistical edge (like in poker, blackjack with perfect strategy, or sports betting with better analysis than the market).
Verified against 0 primary sources. Last reviewed April 14, 2026.