The Kelly Criterion is a staking formula. Not a selection method, not a winners machine. It tells you how much of your betting bank to put on a bet, based on your edge and the price on offer.
For punters with a measurable edge (meaning you can estimate a horse’s true chance of winning more accurately than the market), Kelly is mathematically the optimal way to size each bet. Used properly, it maximises long-term bank growth without overexposing you to ruin.
Used incorrectly, or applied without an honest edge, it’ll empty your bank faster than flat staking. The catch isn’t in the formula. It’s in the inputs.
This guide covers what the formula is, how to apply it to Australian decimal odds, why most disciplined punters use a fractional version, and where Kelly stops being useful.
The formula
The Kelly Criterion was developed by mathematician John Kelly at Bell Labs in the 1950s. It’s been used by professional gamblers, hedge funds and quantitative traders ever since.
For a single bet at fixed odds, the formula is:
K = (bp − q) / b
Where:
- K is the fraction of your bank to stake
- b is the decimal odds minus 1 (your net profit per dollar staked, if the bet wins)
- p is your estimated probability of the bet winning
- q is the probability of the bet losing, which is 1 − p
If K comes out negative, the formula is telling you not to bet. The price is too short for the chance you’ve estimated. If K is positive, that fraction of your bank is the mathematically optimal stake.
A worked Australian example
You’ve done the form on a horse running at Flemington on Saturday. Your honest assessment is that the horse has a 33 per cent chance of winning the race.
The market is offering $4.00. The implied probability of $4.00 is 25 per cent (1 ÷ 4.00). Your assessment is 33 per cent. Your edge is the gap between the two.
Plug it in:
- b = 4.00 − 1 = 3
- p = 0.33
- q = 1 − 0.33 = 0.67
K = (3 × 0.33 − 0.67) ÷ 3 = (0.99 − 0.67) ÷ 3 = 0.32 ÷ 3 = 0.107
The Kelly Criterion is telling you to stake 10.7 per cent of your bank.
On a $5,000 bank, that’s $535 on one bet.
If that number makes you nervous, it should. Full Kelly is mathematically optimal in a vacuum, but it assumes your edge estimate is perfectly accurate. In the real world, your 33 per cent assessment might be closer to 30 per cent, or 27 per cent, or 25 per cent. When the input is wrong, the stake size based on it is wrong too, and full Kelly is unforgiving about that.
Why most punters use fractional Kelly
The fix is to bet a fraction of what full Kelly suggests. Most disciplined punters use half Kelly: take the K value the formula gives you and bet half of it.
In the example above, full Kelly suggested 10.7 per cent of your bank. Half Kelly suggests 5.35 per cent. On a $5,000 bank, that’s $267 rather than $535.
The trade-off is small in terms of long-term bank growth, but significant in terms of survivability. Half Kelly:
- Cuts your peak bet size dramatically
- Reduces the impact of an overestimated edge
- Smooths out the volatility of consecutive bad runs
- Still captures most of the long-term growth that full Kelly provides
Some punters go further, using quarter Kelly (a quarter of the formula’s stake). The smaller the fraction, the closer Kelly behaves to flat percentage staking. That’s by design. Fractional Kelly is the bridge between mathematical optimisation and practical discipline.
When Kelly doesn’t work
The Kelly Criterion isn’t magic. It depends entirely on three assumptions, and if any of them break, the formula breaks with them.
You actually have an edge. If your true win probability estimate isn’t more accurate than the market’s, Kelly will recommend stakes that lose you money over time. The formula assumes a real, measurable edge. No edge, no benefit.
Your edge estimate is honest. Punters chronically overestimate their accuracy. A horse you’d assess at 33 per cent might actually be a 25 per cent chance once you account for variables you didn’t consider. Be conservative with your probability inputs, especially when you’re starting out.
You’re betting on independent outcomes. Kelly works race by race. It doesn’t account for multi-bet structures, hedged exposures, or correlated positions. For a punter backing one horse per race, it’s appropriate. For a punter running multis or trading positions on Betfair, it isn’t.
There’s also the practical issue of bank timing. If you’re betting on five races a day, the formula doesn’t really account for what happens if races two and three resolve before you can adjust your bank size for race four. Most punters get around this by treating the bank as fixed across the day and only recalculating at end of day.
Bringing it together
The Kelly Criterion is the best mathematical answer to the question of how much to bet, for a punter with a measurable, honest edge. Use the full formula to calculate the stake, halve it for survivability, and recalculate your bank regularly.
If you don’t have a measurable edge yet, stick with flat 1 per cent staking from our sensible money management guide until you can prove out a process. Once you can, Kelly gives you a way to scale your stakes with confidence in your edge.
For more on staking approaches generally, our staking plans guide compares the main methods side by side. For the principles behind the value-based betting that makes any of this work, Six keys to punting success is the place to start.
Responsible gambling note. Gamble responsibly. If you or someone you know needs help, contact Gambling Help Online on 1800 858 858 or visit gamblinghelponline.org.au.