Kelly Criterion Calculator
The Kelly Criterion finds the bet size that maximizes long-term bankroll growth given your true edge. Most experienced bettors use Half Kelly or Quarter Kelly to reduce variance. Enter your bankroll, your estimated win probability, and the offered odds.
Result
Kelly formula
f* = (b × p − q) / b, where:
f*= fraction of bankroll to stakeb= decimal odds minus 1 (net odds)p= your estimated probability of winningq= 1 − p (probability of losing)
Why Half Kelly
Full Kelly assumes your probability estimate is exactly correct. In practice, estimates carry uncertainty, and Full Kelly produces high variance and high drawdown. Half Kelly captures roughly 75% of the long-term growth rate of Full Kelly with about 25% of the variance. Quarter Kelly is conservative for bettors new to using probability estimates.
Negative Kelly
If the calculator shows a negative Kelly fraction, your estimated probability is below the book's implied probability. You have negative expected value at this price and should not place the bet.
Frequently Asked Questions
What is the Kelly Criterion in sports betting?
The Kelly Criterion is a formula that finds the bet size that maximizes long-term bankroll growth given your true edge. It assumes your probability estimate is correct. The formula is f* = (bp - q) / b, where b is decimal odds minus 1, p is your win probability, q is 1 minus p.
Why use Half Kelly or Quarter Kelly?
Full Kelly produces high variance and large drawdowns when probability estimates are slightly wrong. Half Kelly captures ~75% of the long-term growth with ~25% of the variance. Quarter Kelly is even more conservative and recommended for bettors new to probability estimation.
What does a negative Kelly fraction mean?
Negative Kelly means your estimated probability is lower than the book's implied probability. The bet has negative expected value at this price and you should not place it. Full Kelly would advise betting zero (or the opposite side if available).
How do I estimate my win probability for Kelly?
Compare your read against the no-vig fair odds from the no-vig calculator. If your number is consistently higher across many bets without confirmation, your estimates are probably inflated. Track results and recalibrate every few months.
What's a reasonable Kelly fraction for casual TN bettors?
Most recreational bettors should use Quarter Kelly or Eighth Kelly, capped at 1-2% of bankroll per bet. Even experienced sharps rarely exceed Half Kelly. The downside variance of Full Kelly is severe; one bad estimate can drop your bankroll 50% before recovery.