Free tool
Kelly Criterion Calculator
How much to stake when you actually have an edge — and how little that is. Give it the price you're offered and what you think the real chance is; it returns full, half and quarter Kelly together, because the gap between them is the point.
The price you think is fair — the de-vigged number. Its implied probability is what Kelly uses.
All three are shown below whichever you pick — this just marks the one you use.
Enter the price you're being offered and this fills in. There's no stake to recommend until you've supplied a probability — we won't invent one.
How it works
Kelly sizes a bet from the edge, not from confidence. With b the decimal odds minus one, p your win probability and q = 1 − p, the fraction of bankroll is f* = (b·p − q) / b. When that comes out at zero or below there is no edge, and the calculator says no bet rather than a small one.
The probability is the whole ballgame. Kelly is exactly as good as the number you feed it, and an estimate that's a point or two generous turns a real edge into an overbet — which is why most people stake a fraction of it. Betting more than full Kelly lowers long-run growth even when the edge is genuine.