Kelly Calculator
Work out how much of your bankroll to stake based on your odds and your estimated win probability.
Your own estimate of the chance of winning — not the odds' implied probability. The Kelly criterion only works if your estimate is realistic.
Enter your bankroll, odds and win probability to see your recommended stake.
This calculation is a mathematical guide and relies entirely on your own estimate of the win probability. It is not a guarantee of profit and does not replace responsible gambling.
The Kelly criterion is a formula for how much of your bankroll to stake when you believe you have an edge over the odds. Used correctly it maximises your long-run growth — used incorrectly (staking too much) it can drain your bankroll fast. This calculator works out full, half and quarter Kelly for you, so you can pick a level that matches your risk tolerance.
How the Kelly Criterion Works
The Kelly criterion takes three things into account: your odds, your estimated probability of winning, and how much you have to stake with. The bigger your edge (the gap between your probability and the odds' implied probability), the larger the share of your bankroll the formula recommends.
f* = (b × p − (1 − p)) / b, where b = decimal odds − 1 and p = your win probability
f* is the share of your bankroll (full Kelly) that is mathematically optimal. If f* is zero or negative, the bet has no value and the correct stake is zero. Many players stake half or quarter Kelly instead of full, since it gives almost as much long-run growth with far smaller bankroll swings — good protection if your probability estimate turns out to be too optimistic.
Worked Example
Say you have a bankroll of 10,000 and find a bet at odds of 2.50, where you judge the win chance to be 45%. Then b = 2.50 − 1 = 1.5, and f* = (1.5 × 0.45 − 0.55) / 1.5 = 0.125 / 1.5 ≈ 8.33%. Full Kelly is therefore about 833, half Kelly about 417 and quarter Kelly about 208. The odds' implied probability is 1/2.50 = 40%, so your assessed edge is 5 percentage points — enough for the formula to recommend a stake.
Frequently Asked Questions
Why do most people stake only half or quarter Kelly?
Full Kelly maximises the theoretical growth rate, but is extremely sensitive to errors in your probability estimate — overestimate your edge even slightly and full Kelly risks large bankroll drawdowns. Half Kelly delivers roughly 75% of the growth but only about 25% of the swings, making it a far safer everyday strategy.
What happens if I stake more than Kelly recommends?
Staking more than full Kelly ("overbetting") actually lowers your expected long-run growth compared to full Kelly, despite the larger stake — and sharply increases the risk of major bankroll drawdowns. The Kelly criterion is the upper limit of what's mathematically sound, not a floor.
Does the Kelly criterion work if my probability estimate is wrong?
The Kelly criterion is only as good as your win-probability estimate. If it's too optimistic, the formula will recommend too large a stake, and vice versa. That's why half or quarter Kelly is a wise choice for most people — it builds in a buffer against your own judgment not being perfect.
