Flat betting is a slow, predictable death for your bankroll. This Kelly Criterion calculator helps you mathematically optimize your bet sizing based on your true edge, keeping volatility under tight control to keep your Risk of Ruin at zero.
The Kelly Criterion is a formula used by professional bettors and commodity traders to determine the mathematically perfect percentage of their bankroll to risk on a single wager. To get started, you need three key numbers:
Developed by Bell Labs physicist John L. Kelly Jr. in 1956, the formula was originally designed to resolve signal noise issues in telephone lines. Gamblers quickly realized it could maximize the long-term logarithmic growth of a bankroll. Here is the formula:
f* = (b * p - q) / b
Where:
f* is the fraction of your current bankroll to bet.b is the net odds received on the wager (decimal odds – 1).p is your true probability of winning (expressed as a decimal from 0 to 1).q is your true probability of losing, which is simply 1 - p.Imagine you find a coin-toss bet that pays out at decimal odds of 2.10 (net odds of 1.10). Since it’s a standard coin toss, your true win probability is 50% (0.50).
b = 1.10 p = 0.50 q = 0.50 f* = (1.10 * 0.50 - 0.50) / 1.10 f* = (0.55 - 0.50) / 1.10 f* = 0.05 / 1.10 ≈ 0.0454 (4.54%)
The math says you should risk exactly 4.54% of your total bankroll on this bet. Over hundreds of trials, risking exactly this amount will compound your bankroll faster than flat betting or arbitrary sizing.
Losing half your bankroll on a single bad weekend—numbing. But that is exactly what will happen if you use “Full Kelly” sizing.
Most professional sports bettors scale down to a 0.25 fractional Kelly. Smart move. Full Kelly has a 33% chance of cutting your bankroll in half during a bad run, even if your edge estimations are 100% correct.
By reducing the suggested bet size to a fraction (e.g., half Kelly or quarter Kelly), you trade a small amount of growth speed for a massive reduction in volatility and drawdown risk.
If the calculator outputs a negative number, it means you have no mathematical edge. The sportsbook has priced the odds so high (or the true win probability is so low) that placing this bet has a negative expected value (-EV). The correct strategic action is to skip the bet.
True probability is calculated through statistical modeling, historical data analysis, or finding discrepancies between public consensus and sharp betting markets. Simply guessing or following “gut feelings” will result in incorrect Kelly suggestions and bankroll loss.
No. The standard Kelly formula assumes sequential betting (one bet settles before you place the next). If you place multiple bets at the same time, the risk of joint ruin increases dramatically. For simultaneous bets, you must use a “Multi-Kelly” algorithm, which scales down all wagers so their combined risk does not exceed your bankroll parameters.