Kelly Criterion & Unit Size Calculator
The Kelly Criterion is a mathematical formula that calculates the optimal bet size based on your edge and the odds offered. Published by John Kelly at Bell Labs in 1956, it maximizes long-term bankroll growth while minimizing risk of ruin. Most bettors use fractional Kelly (half or quarter) for added safety. This calculator also includes unit size recommendations — helping you set a consistent, bankroll-appropriate bet size whether you have an identified edge or simply want disciplined flat-unit staking.
Formula
Kelly % = (bp - q) / bWhere b = decimal odds - 1 (net odds), p = win probability, q = loss probability (1 - p). Example: At +150 (b=1.5) with 45% win chance: Kelly = (1.5 × 0.45 - 0.55) / 1.5 = 8.3% of bankroll. Half Kelly = 4.2%.
How to Use This Calculator
- Enter the odds offered by the sportsbook
- Enter your estimated win probability
- Enter your current bankroll
- View Full Kelly, Half Kelly, and Quarter Kelly recommended stakes
When to Use This
- •Determining optimal bet size when you have an identified edge
- •Managing bankroll risk across a portfolio of bets
- •Comparing aggressive (Full Kelly) vs. conservative (Quarter Kelly) approaches
Kelly Criterion & Unit Size Calculator
Calculate optimal bet size based on your edge (Kelly Criterion) or use flat unit sizing for bankroll management.
Half Kelly (Recommended)
Full Kelly
Quarter Kelly
Edge Analysis
Flat Unit Sizing
No edge estimate? Bet a fixed percentage of your bankroll on every wager.
Your Unit Size
$20.00
2% of $1,000 bankroll
Units in Bankroll
50
10-Unit Loss
$200.00
Quick Reference
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For informational and educational purposes only. Not financial or gambling advice. All calculations assume odds remain constant and bets execute as entered. Actual results may vary. Gambling involves risk of loss.
Frequently Asked Questions
What is the Kelly Criterion?
The Kelly Criterion is a formula published by John Kelly at Bell Labs in 1956. It calculates the mathematically optimal percentage of your bankroll to wager based on your edge. It maximizes long-term growth rate while protecting against ruin.
Why do most bettors use Half Kelly instead of Full Kelly?
Full Kelly assumes perfect edge estimation, which is rare in practice. If your estimated probability is even slightly off, Full Kelly can lead to overbetting. Half Kelly (betting 50% of the Kelly amount) provides ~75% of the growth rate with significantly less variance.
What happens if Kelly Criterion gives a negative number?
A negative Kelly value means the bet has negative expected value — you don't have an edge. The recommendation is to not place the bet. The Kelly Criterion only suggests positive bet sizes when you have a genuine mathematical advantage.
Related Glossary Terms
Kelly Criterion
A formula for calculating the optimal bet size to maximize long-term bankroll growth.
Bankroll Management
The practice of managing your gambling funds to minimize the risk of going broke.
Edge
Any advantage a bettor has over the bookmaker or casino on a specific wager.
Unit
A standardized bet size used to track performance, typically 1-2% of your bankroll.
Risk of Ruin
The probability of losing your entire bankroll, even with a positive edge.
Expected Value (EV)
The average amount you can expect to win or lose per bet over time.