AFL Kelly Criterion Calculator
Enter the price on an AFL match, your win probability, your bankroll and a Kelly fraction, and get the stake the Kelly criterion suggests in both dollars and per cent of bankroll.
Kelly Calculator
Inputs
The price on offer at the bookmaker.
Your own estimate, from a model or a devigged price.
The amount set aside for betting, across all accounts.
Results
Full Kelly stake is 8.33% of bankroll
The edge is +12.50% of stake, divided by the profit multiple of 1.50.
- Full Kelly stake
- $83.33
- Half Kelly stake
- $41.67
- Edge
- +12.50%
- Expected log growth
- 0.515%
8.33% of bankroll
4.17% of bankroll
As a percentage of stake
Per bet at full Kelly
| Kelly fraction | Share of bankroll | Stake |
|---|---|---|
| Full Kelly | 8.33% | $83.33 |
| Half Kelly | 4.17% | $41.67 |
| Quarter Kelly | 2.08% | $20.83 |
Kelly assumes your probability estimate is exact. It is not, and overstating an edge makes full Kelly overbet, which costs more growth than underbetting does. That is why most people use half or quarter Kelly. A negative fraction is a no bet, not an instruction to lay, unless you actually have a lay market available.
How the AFL Kelly Criterion Calculator works
The Kelly criterion sets the stake that maximises the long-run growth rate of a bankroll. It scales with your edge and shrinks as the price shortens, so a small edge at short odds produces a small stake and a large edge at long odds produces a larger one. If the formula returns zero or a negative number, the price carries no edge on your numbers and the criterion stakes nothing.
Full Kelly is aggressive and assumes your probability estimate is exactly right. Because it never is, most people bet a fraction of it, commonly a half or a quarter. Fractional Kelly gives up a modest amount of theoretical growth for a large reduction in drawdown, and it protects you from the main practical failure mode, which is overstating your own edge.
f = ((decimal odds - 1) x probability - (1 - probability)) / (decimal odds - 1) Stake = f x Kelly fraction x bankroll
Example
A price of 2.50 with a 45.00% win probability gives a full Kelly stake of 8.33% of bankroll. On a $1,000 bankroll that is $83.33 at full Kelly, or $41.67 at half Kelly. The edge is 12.50% of stake, divided by the profit multiple of 1.50.
How to use it
- Enter the decimal odds on offer, for example 2.50.
- Enter your estimated win probability as a percentage, for example 45.
- Enter your betting bankroll, for example 1000.
- Pick a Kelly fraction such as half Kelly and read the suggested stake.
AFL Kelly Criterion Calculator FAQ
Why do most people use half Kelly rather than full Kelly?
Full Kelly is optimal only if your probability estimate is exact. Overstate your edge and full Kelly overbets, which erodes growth far faster than underbetting does. Half Kelly retains around three quarters of the theoretical growth rate while roughly halving the volatility and the depth of drawdowns, and it stays sane if your estimate is a little off. Quarter Kelly is common for markets where the probability input is soft, such as AFL player props with thin samples behind them.
What happens if the calculator returns a negative stake?
A negative Kelly fraction means the price is shorter than your estimated probability, so the bet has negative expected value and the criterion says to stake nothing. In a market where you can take the other side, the negative figure hints that the opposite selection may be the value one, but you should price that side directly rather than inverting the number. Treat a negative result as a no bet, not as an instruction to lay unless you actually have a lay market available.
Should the bankroll be my whole betting balance?
It should be the amount you have genuinely set aside for betting and are willing to see fall, not your total savings and not just the balance sitting in one bookmaker account. Kelly is a percentage rule, so understating the bankroll makes every stake too small and overstating it makes every stake too large. If you spread funds across several accounts, use the combined figure and keep it consistent, otherwise the stake sizing drifts every time you move money around.
Does Kelly work for multis and same game multis?
The formula still runs on any single price and probability pair, so you can feed it a multi. The problem is the probability input. Multi legs are rarely independent, and same game multi legs are strongly correlated, so a naive probability built by multiplying leg estimates will be wrong, usually optimistically. Combined with the larger margin held on multi markets, Kelly applied to a multi tends to produce a stake that looks reasonable on numbers that are not.
How often should I recalculate the stake?
Recalculate the bankroll figure on a regular schedule rather than after every bet. Updating after every result makes stakes jump around and can lead to chasing after a loss. Many people reset the bankroll weekly or at the end of an AFL round, which keeps stakes proportional without reacting to single outcomes. What matters more is consistency: use the same bankroll definition and the same Kelly fraction across all your bets so that stakes stay comparable.
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