Size a stake with the Kelly criterion
The Kelly criterion sets the stake that maximises the long-run growth rate of a bankroll when you hold a genuine edge. It needs your own estimate of the win probability, so this Kelly criterion calculator is only as reliable as that estimate. If you do not have one yet, build it first with the implied probability calculator and adjust from there.
Formula and worked example
Net odds b = Decimal odds - 1
Full Kelly fraction = (Estimated probability x Decimal odds - 1) / b
Half Kelly = Full Kelly / 2, Quarter Kelly = Full Kelly / 4
Stake = Kelly fraction x Bankroll
At decimal odds of 2.50, the market-implied probability is 40%. With a 50% estimate, the edge is 10 percentage points and the net odds b are 1.50, so full Kelly is (0.50 x 2.50 - 1) / 1.50 = 16.67% of the bankroll. On a 1,000 bankroll that is a full Kelly stake of 166.67, a half Kelly stake of 83.33 and a quarter Kelly stake of 41.67.
Why full Kelly is rarely the right stake
Full Kelly maximises long-run growth only when your probability is exactly right. Betting probabilities are estimates, and full Kelly is unforgiving when they are too high: it stakes aggressively, so a run of losses or an overstated edge can draw the bankroll down sharply. That deep drawdown is the practical face of risk of ruin, the chance of grinding a bankroll down to nothing before the edge can play out.
Fractional Kelly is the standard response. Half Kelly keeps most of the theoretical growth rate while roughly halving the size of the swings; quarter Kelly is more conservative again. Betting a fraction protects against the estimate being wrong, not just against bad luck, which is why most disciplined bettors stake at half Kelly or lower. If your edge is uncertain, size down rather than up. A zero or negative edge returns a zero stake here, because the Kelly criterion then recommends no bet at all.
Reading the edge
The edge compares your estimated win probability with the market-implied probability from the price. A positive edge is the gap the Kelly fraction sizes against; a negative edge means the offered price is shorter than your estimate justifies. Because bookmaker prices carry margin, normalise a full market with the vig calculator before trusting a single implied probability, and confirm the expected value with the value betting calculator.
Limits and responsible use
Kelly sizing assumes a fixed, correct edge and a bankroll you can commit fully. It does not model correlated bets, price movement, limits or operator settlement rules, and it is not a guarantee of profit. See Gambling Commission LCCP 4.2.6.
Set a spending limit before gambling and do not chase losses. Read GambleAware's advice and our responsible gambling guidance.
Related tools and guides
After sizing a stake and checking the risk, you can compare betting partners. Oddly Likely may earn a commission from partner links; check current odds, eligibility and terms. Read the affiliate disclosure.
Sources
- Gambling Commission, LCCP condition 4.2.6. Current code accessed 4 August 2026.
- GambleAware, Advice to consider if you're gambling. Accessed 4 August 2026.