Quantitative Betting Calculator
Kelly Criterion Calculator
The Kelly Criterion calculator tells you what fraction of your bankroll to bet based on your edge - the gap between your estimated win probability and the odds on offer. Enter the odds and your true win probability, then choose full, half, or quarter Kelly for staking that maximizes long-term bankroll growth without overbetting.
How the kelly criterion calculator works
Enter the odds and your win probability
The odds set your payout; your estimated true probability of winning sets your edge. Kelly only recommends a bet when your probability beats the odds' implied probability.
It sizes the bet to your edge
Bigger edges get bigger stakes, smaller edges get smaller ones. If there's no edge, Kelly returns zero - telling you not to bet.
Pick a Kelly fraction
Full Kelly maximizes growth but swings hard. Most pros use half or quarter Kelly to cut volatility and survive the inevitable estimate errors.
Kelly formula
f* = (b·p − q) ÷ b, where b = decimal odds − 1, p = your win probability, and q = 1 − p. f* is the fraction of bankroll to stake; multiply by 0.5 or 0.25 for half or quarter Kelly.
Example — At +150 (b = 1.5) with a true 50% win chance: f* = (1.5 × 0.5 − 0.5) ÷ 1.5 = 16.7% of bankroll at full Kelly, or about 8.3% at half Kelly.
Frequently Asked Questions
What is the Kelly Criterion?
The Kelly Criterion is a formula that sets the bet size which maximizes the long-term growth rate of a bankroll, given your edge and the odds. It scales stakes up with your advantage and down without one.
Why do bettors use half Kelly?
Full Kelly assumes your probability estimate is exact - rarely true in betting. Half or quarter Kelly sharply reduces bankroll swings and the damage from overestimating your edge, at a small cost to growth.
Is Kelly better than flat betting?
Kelly grows a bankroll faster than flat staking when your edge estimates are accurate, because it presses winners and protects against ruin. Its weakness is sensitivity to bad probability estimates, which fractional Kelly mitigates.
What happens if my win probability is wrong?
Kelly is very sensitive to the probability you enter - overestimating your edge leads to overbetting and larger drawdowns. That's the main reason experienced bettors use a fraction of full Kelly.
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