Kelly Criterion Calculator
Size a bet as a fraction of your bankroll from your edge. Enter decimal odds, your true win probability and bankroll to get the full-Kelly and half-Kelly stake. Runs entirely in your browser.
Kelly only works with an honest probability estimate. If you overestimate your edge, Kelly overstakes — half-Kelly is the common, more conservative choice.
What is the Kelly criterion?
The Kelly criterion is a bankroll-management formula that turns your edge into a stake size. When your probability estimates are accurate, it produces the fastest long-run bankroll growth: it stakes more as the edge grows and recommends nothing when there’s no edge. It’s a discipline tool as much as a maths one — it stops you overbetting a small edge or chasing a bet with none.
The formula
q = 1 − p
f = ( b × p − q ) ÷ b → clamp to 0 if negative
stake = f × bankroll (half-Kelly = f ÷ 2 × bankroll)
where O = decimal odds, p = your true win probability as a fraction, and f = the fraction of bankroll to stake.
Worked example
Odds of 2.50 give b = 1.50. If your true win chance is 45% (p = 0.45, q = 0.55), then f = (1.50 × 0.45 − 0.55) ÷ 1.50 = (0.675 − 0.55) ÷ 1.50 = 0.125 ÷ 1.50 ≈ 8.33%. On a $1,000 bankroll that’s about $83.33 at full Kelly, or roughly $41.67 at half-Kelly.
When to use it
Reach for Kelly when you have a genuine, honest probability estimate — from a model or a sharp reference — and want a principled stake instead of a flat unit. Because it’s sensitive to error, most bettors use half-Kelly or less. Pair it with the EV calculator to confirm the bet is +EV before sizing it.
An honest probability makes Kelly work
HaterPicks projects player-prop lines and grades every pick win or lose. Free trial · no card to start.
Start freeFAQ
What is the Kelly criterion?
The Kelly criterion is a staking formula that sizes each bet as a fraction of your bankroll based on your edge. It maximises the long-run growth rate of the bankroll when your probability estimates are accurate, betting more when the edge is larger and nothing when there is no edge.
How is the Kelly stake calculated with decimal odds?
With decimal odds O, let b = O − 1 (profit per unit) and p your true win probability with q = 1 − p. The Kelly fraction is f = (b × p − q) ÷ b. If f is negative there is no edge, so the recommended stake is zero. Multiply f by your bankroll to get the stake.
What is half-Kelly and why use it?
Half-Kelly simply stakes half of the full-Kelly fraction. Because full Kelly is very sensitive to errors in your probability estimate — and can swing the bankroll sharply — most practitioners use half-Kelly (or less) to cut variance while keeping most of the growth benefit.
What happens if I overestimate my edge?
Kelly assumes your win probability is correct. If you overestimate it, full Kelly overstakes and can do real damage to your bankroll. That is why an honest probability — and a fractional Kelly — matters. This tool is for research and education, not betting advice.