Flat betting grinds a bankroll down slowly and predictably. The Kelly Criterion calculator below sizes each wager against your actual edge, and the fractional slider keeps volatility capped so your Risk of Ruin stays at zero.
Kelly Criterion Calculator
How to calculate your optimal bet size in 3 steps
How Does This Compare to 0% House Edge Protocols?
Audited 100% RTP
Instead of standard 1.00% to 4.00% casino house margins, compare with mathematically verified 100% RTP Originals (0.00% House Edge) on Duel Casino:
Professional bettors and commodity traders use the Kelly Criterion to pin down the exact share of a bankroll that should ride on one wager. Three numbers feed it, and all three matter:
- Your Bankroll: Money reserved for betting alone. Not rent money, not grocery money.
- Net Decimal Odds: Payout odds with the stake removed. A book showing decimal odds of 2.10 gives net odds of 1.10.
- Your Win Probability (True Probability): Your honest estimate of winning. This input makes or breaks everything. Guessing here produces garbage output; you need an objective basis for believing an edge exists.
The math behind the Kelly formula
Bell Labs physicist John L. Kelly Jr. published the formula in 1956 to tackle signal noise in telephone lines. Gamblers spotted its real use soon after: it maximizes long-term logarithmic bankroll growth. The formula itself:
f* = (b * p - q) / b
Where:
f*is the fraction of your current bankroll to bet.bis the net odds received on the wager (decimal odds – 1).pis your true probability of winning (expressed as a decimal from 0 to 1).qis your true probability of losing, which is simply1 - p.
A concrete example:
Suppose a coin-toss bet pays decimal odds of 2.10 (net odds 1.10). A fair coin means your 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%)
So the correct stake is 4.54% of bankroll. Across hundreds of trials, sizing bets this way compounds faster than flat staking or gut-feel amounts.
Why “Full Kelly” is practically insane
Full Kelly sizing can erase half your bankroll over one bad weekend. That outcome isn’t bad luck — it’s baked into the math.
Even with perfectly estimated edges, Full Kelly carries a 33% chance of halving your bankroll during a rough stretch. Most professional sports bettors therefore run quarter Kelly instead.
Scaling the suggested stake down to half or quarter Kelly sacrifices a sliver of growth speed in exchange for far lower volatility and shallower drawdowns.
Frequently asked questions
What does a negative Kelly percentage mean? (This occurs when the bet has a negative Expected Value)
A negative output signals no mathematical edge. Either the odds are priced too rich or the true win probability is too low, making the bet a negative expected value (-EV) proposition. Correct play: don’t place it.
How do I calculate my true win probability?
Through statistical modeling, historical data analysis, or by spotting gaps between public consensus prices and sharp market lines. Intuition and vibes produce wrong probabilities, which produce wrong stakes, which drain bankrolls.
Can I use the Kelly Criterion for multiple simultaneous wagers?
No. Standard Kelly assumes sequential betting — one wager settles before the next goes on. Simultaneous bets stack correlated risk and raise the chance of joint ruin sharply. In that case you need a “Multi-Kelly” algorithm, which scales every stake down until their combined exposure fits your bankroll parameters.