Kelly criterion calculator
How much to bet — sized to your edge.
Edge (EV)
+10%
Stake % of bankroll
4.17%
Recommended stake
41.67
Positive edge — half Kelly suggests staking 41.67 (4.17% of your bankroll).
Kelly sizes the stake to your edge. Most bettors use half or quarter Kelly to smooth variance — and your win-probability estimate has to be honest, or the number is meaningless.
What is the Kelly criterion?
The Kelly criterion is the bet size that maximizes long-run bankroll growth given your edge. It needs two honest inputs: the price, and your estimate of the true win probability. With an edge it stakes more; with none, it stakes nothing.
Full Kelly is aggressive and swingy, so most disciplined bettors use half or quarter Kelly to cut variance while keeping most of the growth. Bankroll Guardian applies the same Kelly and unit sizing to every bet you log — and tracks whether your probability estimates actually hold up over time.
Common questions
- What is the Kelly criterion formula for betting?
- Kelly stake = (b × p − q) ÷ b, where b is the decimal odds minus 1, p is your estimated win probability, and q = 1 − p. Example: at +100 (b = 1) with a 55% win probability, Kelly = (0.55 − 0.45) ÷ 1 = 10% of your bankroll.
- What is half Kelly or quarter Kelly?
- A deliberate fraction of the full Kelly stake — half or a quarter. Full Kelly is only optimal if your probability estimate is perfect; fractioning down sharply cuts the swings while keeping most of the growth, which is why most experienced bettors stake half or quarter Kelly.
- How much of my bankroll should I bet?
- Without a measured edge, a flat 1–2% of bankroll per bet is the standard answer. With a genuine, estimated edge, use a fraction (half or quarter) of the Kelly stake this calculator returns. Staking above 5% per bet risks ruin from ordinary variance alone.
- Can the Kelly criterion tell me not to bet?
- Yes — that's one of its virtues. If your estimated win probability doesn't beat the probability implied by the odds, your edge is zero or negative and the formula returns a stake of $0. No edge, no bet.
More free tools
Track every bet automatically
Stop doing the math by hand. Bankroll Guardian logs your bets and shows P&L, ROI, CLV and more — free to start, no card required.