Kelly Criterion Calculator
Finding a good bet is half the job; sizing it is what decides whether you survive. Enter your bankroll, the price, and your win probability to get the optimal stake — and the safer fractional-Kelly versions we actually use.
The formula
b = decimal odds − 1 · p = win prob · q = 1 − p
stake = f × bankroll
f is the share of your bankroll full Kelly would stake. It rises with your edge and falls as the odds shorten. If your probability only matches the price's implied probability, f is zero — no edge, no bet.
FAQ
How is a Kelly bet calculated?
f = (b×p − q) / b — decimal odds minus 1, times your win probability, minus the lose probability, over b. Multiply by bankroll for the stake.
Why bet a fraction of Kelly?
Full Kelly overbets hard whenever you overestimate your edge, and the swings are brutal even when you're right. A quarter or half keeps most of the growth with far less ruin risk.
What if the result is zero?
Your probability doesn't beat the price's implied probability, so there's no edge. Kelly says bet nothing — and most of any board is exactly that.