Kelly Criterion Calculator
Work out the optimal stake size based on the odds offered and your own probability estimate. The Kelly Criterion maximises long-term bankroll growth while controlling risk.
Bet Details
Implied probability: —
Optimal Stake
Enter odds and probability to calculate
How It Works
The Kelly Criterion formula is:
f = (b × p − q) / b
- f = fraction of bankroll to stake
- b = net odds (decimal odds minus 1)
- p = your estimated probability of winning
- q = probability of losing (1 − p)
The result tells you what percentage of your bankroll to risk. If the number is negative, the bet has no edge and should be avoided.
FAQs
What is the Kelly Criterion?▼
The Kelly Criterion is a mathematical formula that calculates the optimal stake size for a bet based on the odds offered and your estimated probability of winning. It maximises long-term bankroll growth while minimising the risk of ruin.
Why do many bettors use Half Kelly instead of Full Kelly?▼
Full Kelly can be highly volatile — a single bad run can draw down your bankroll significantly. Half Kelly (staking 50% of the formula's suggestion) delivers roughly 75% of the growth rate with far less variance, making it the preferred choice for most professional bettors.
What happens if the Kelly percentage is negative?▼
A negative result means the odds are not in your favour — your estimated probability is lower than the implied probability of the odds. The correct action is to not bet (or to lay the selection if on an exchange).
Should I enter decimal or fractional odds?▼
This calculator uses decimal odds (e.g. 3.00, 2.50). If you have fractional odds, convert them first — for example, 2/1 = 3.00 decimal, 7/2 = 4.50 decimal.
Kelly Criterion Calculator for Horse Racing Betting
The Kelly Criterion is one of the most powerful staking strategies in horse racing betting, and our free Kelly Criterion Calculator makes it easy to apply. Developed by Bell Labs scientist John L. Kelly Jr. in 1956, the formula calculates the exact fraction of your betting bankroll you should stake on any given horse to maximise long-term growth. Unlike flat staking or percentage-of-bank methods, the Kelly approach adjusts your stake dynamically based on the perceived edge — the gap between your own probability estimate and the odds the bookmaker is offering.
To use the calculator, simply enter the decimal odds for your selection and your estimated probability of that horse winning. The tool instantly computes the optimal stake as a percentage of your bankroll, along with the expected value (EV) of the bet and the market's implied probability. If you enter your total bankroll, the calculator converts the Kelly percentage into a concrete stake in pounds. You can also choose between Full Kelly, Half Kelly, and Quarter Kelly — most professional punters use Half Kelly to reduce variance while still capturing roughly three-quarters of the maximum growth rate.
The Kelly formula is particularly valuable in horse racing because the sport offers so many variables — going, draw, jockey, trainer form, pace — that a well-researched punter can often derive a probability estimate that differs from the market. When your estimate is more accurate than the implied probability baked into the odds, the Kelly Criterion tells you precisely how much of that edge to monetise. A positive edge means a bet is justified; a negative result means you should pass entirely, protecting your bankroll from negative-EV wagers that erode long-term returns.
When to Use the Kelly Criterion in Racing
The Kelly Criterion shines when you have a reliable method for estimating a horse's true chance of winning. This might come from speed figures, sectionals, pedigree analysis for specific race conditions, or a combination of factors built into a ratings system. If your probability estimates are systematically more accurate than the market's, Kelly staking will grow your bankroll faster than any fixed-stake approach over a large sample of bets.
However, the formula is only as good as the probabilities you feed it. If your estimates are noisy or biased, Full Kelly can amplify losses quickly — which is why Half Kelly is the default for most serious bettors. It halves the recommended stake, dramatically reducing drawdowns while still concentrating more capital on your strongest edges. Quarter Kelly is even more conservative and suits those still validating their model or betting in volatile markets like small-field handicaps and maiden races.
The Kelly Criterion is less suited to each-way and multiple bets without modification, because those wagers have more than two outcomes. For straight win singles — the bread and butter of value betting — it remains the gold standard for optimal stake sizing. Pair it with our Odds Converter to check decimal equivalents, or use the Pace Map Builder to form the race-reading opinions that feed your probability estimates.
Kelly Criterion vs Other Staking Plans
| Staking Method | How It Works | Best For |
|---|---|---|
| Kelly Criterion | Stake scales with perceived edge — bigger edge, bigger stake | Punters with reliable probability models |
| Flat Staking | Same stake on every bet regardless of confidence | Beginners; simple bankroll protection |
| Percentage Staking | Fixed % of current bankroll, recalculated each bet | Gradual compounding without edge-based sizing |
| Confidence Staking | 1–5 unit scale based on subjective confidence | Tipsters and qualitative cappers |
The key advantage of Kelly over these alternatives is that it ties stake size directly to the mathematical value in the bet. Flat and percentage staking treat a 20/1 shot with a massive edge the same as an even-money favourite with a tiny edge — Kelly does not.
Frequently Asked Questions
Is the Kelly Criterion good for horse racing?
Yes — horse racing is one of the best sports for Kelly staking because skilled form analysts can produce probability estimates that diverge from the market. As long as your estimates are well-calibrated, Kelly will grow your bankroll optimally. Most punters use Half Kelly to smooth the ride.
What odds format does the Kelly Calculator use?
The calculator uses decimal odds (e.g. 3.00 for a 2/1 shot). If you work in fractional odds, convert them first — divide the first number by the second and add 1. For example, 7/2 becomes 3.50 decimal. Our Odds Converter tool handles this instantly.
Should I ever stake more than Kelly suggests?
No. Staking above Full Kelly does not increase your expected growth rate — it reduces it and increases the risk of bankrupting your bankroll. Full Kelly is already the mathematical optimum; going beyond it is pure gambling.
Can the Kelly Criterion guarantee profits?
No staking plan can turn negative-EV bets into long-term winners. Kelly maximises growth given a genuine edge — if your probability estimates are no better than the market's, no staking method will save you. The formula's real power is that it tells you when not to bet (negative Kelly) and how much to stake when you do have an edge.