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Kelly Criterion Calculator: Bankroll & Unit Size
Turn your edge into a bet size. Enter your bankroll, the odds and your win probability to see the full, half and quarter Kelly stake in dollars and units.
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| Sizing | % of bankroll | Stake | Units | Growth per bet |
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Caution: Kelly is only as good as your win probability. If you overestimate it by even 2–3 points, full Kelly overbets and your bankroll can swing hard. Most pros use quarter to half Kelly and cap any single bet at a few percent.
How to use this calculator
Type your bankroll, the American odds you can bet, and your honest win probability (a no-vig market price is a good start). Set your unit size if you use something other than 1% of bankroll. The green row is quarter Kelly, the size most bettors should start with. Copy the result to share your sizing.
How to read the growth column
“Growth per bet” is the expected change in your bankroll’s long-run (log) growth rate for each bet of that size. Full Kelly maximizes it; half Kelly keeps most of the growth with far less risk; at 2× Kelly growth falls to about zero even though each bet is +EV. That is why overbetting a real edge can still lose money over time.
Your win probability should come from a market, not a hunch: the no-vig price from the hold calculator is a solid baseline. Check the bet’s value first in the EV calculator. For PrizePicks slips, the Power vs Flex picker shows EV per entry.
FAQ
What is the Kelly criterion?
Kelly is the bet size that maximizes long-run bankroll growth: f = (b × p − (1 − p)) ÷ b, where p is your win probability and b is decimal odds minus 1. At +150 with a 42% chance, Kelly says bet 3.33% of your bankroll.
Should I bet full Kelly, half Kelly or quarter Kelly?
Half Kelly gives about three-quarters of full Kelly’s growth with half the swings, and quarter Kelly is safer still. Because win probabilities are estimates, most bettors use quarter to half Kelly. Betting twice Kelly gives roughly zero growth with huge risk.
How big should a betting unit be?
A common rule is 1 unit = 1% of your bankroll, with most bets 1–3 units. Kelly turns your edge into units: if quarter Kelly is 0.8% of bankroll, that is 0.8 units at 1% per unit. Recalculate units when your bankroll changes.