Your own estimate, not derived from the odds — that's the whole point of Kelly sizing.
Full Kelly is high-variance. Most real bettors use half or quarter Kelly to reduce swings.
Recommended stake
8.33%
of bankroll.
Dollar stake
$83.33
Implied probability at these odds: 40.00%. Your estimate needs to beat that for Kelly to recommend a positive stake.
What this is (and isn't)
This is a documented reference implementation of Kelly stake sizing — including the negative-edge clamp that keeps a meaningless negative formula output from ever being shown as a recommended stake. It doesn't show live sportsbook lines, link to a sportsbook, or take a stake for real money — every number here, including your win probability estimate, is one you type in, computed on your device. If you need live odds data behind this math, that's what the API is for.
Frequently asked
- What is the Kelly criterion formula?
- f* = (b×p − q) / b, where b is the net odds (decimal odds minus 1), p is your estimated true win probability, and q is 1 − p. The result is the fraction of your bankroll to stake for maximum long-run growth, given your own probability estimate.
- Why does this calculator ask for MY win probability instead of using the odds?
- Kelly sizing is only meaningful when your estimate of the true win probability differs from what the market odds imply — that gap is your "edge." The odds alone can't tell you your edge; only your own model or judgment can. This calculator never derives probability from the odds you enter.
- What happens if I have no edge, or a negative edge?
- If your estimated probability doesn't clear the odds' implied probability, the raw Kelly formula returns a negative number. That is not a valid stake — this calculator clamps it to exactly 0% and shows "No edge — Kelly recommends not betting this" rather than ever displaying a negative bankroll percentage.
- What is half-Kelly or quarter-Kelly, and why would I use it?
- Full Kelly is the mathematically optimal growth rate, but it comes with high variance — big drawdowns are common even when your edge estimate is correct. Betting half or a quarter of the full Kelly stake trades some long-run growth for meaningfully smaller swings, which is what most practical bettors use instead of full Kelly.
- Does this calculator use live sportsbook odds?
- No. The offered odds and your win probability estimate are both numbers you type in yourself. It runs entirely in your browser, with no live odds feed and nothing sent to a server.
Need the live odds behind the math?
This calculator is free forever, no key required. When you're ready for real sportsbook lines, scores, and stats behind it, the API has a free tier too.
250 req/day free (500 with GitHub). No credit card.