What is the Kelly criterion in simple terms?

The Kelly criterion tells you how much of your bankroll to bet based on your edge. Bigger edge = bigger bet. It's designed to grow your bankroll as fast as possible without risking too much. The formula is: Kelly% = Edge / (Odds - 1).

Is Kelly criterion better than flat staking?

Kelly produces faster bankroll growth when your probability estimates are accurate. But flat staking is more forgiving of estimation errors and simpler to follow. For most recreational bettors, flat staking at 1-3% is safer. For experienced bettors with reliable edge estimates, half Kelly is optimal.

What happens if Kelly gives a negative number?

A negative Kelly result means the bet has negative expected value — you shouldn't bet on it at all. This happens when the offered odds are lower than the true fair odds, meaning the bookmaker's price doesn't represent value.

Can I use Kelly for accumulators?

Kelly can be applied to accumulators by treating the combined odds and overall probability. However, estimating the true probability of a multi-leg accumulator is much harder than a single bet, so fractional Kelly (quarter Kelly) is strongly recommended for accumulators.

Strategy

Kelly criterion for sports betting

Learn how to use the Kelly criterion to calculate optimal bet sizes. Covers full Kelly, fractional Kelly, and when to use each approach.

6 min readUpdated 9 Apr 2026Written by the Trackbet team

Key takeaways

The Kelly criterion calculates the mathematically optimal stake to maximize bankroll growth
Kelly% = Edge / (Odds - 1), where Edge = (Probability × Odds) - 1
Full Kelly is too aggressive for most bettors — use half or quarter Kelly instead
Kelly requires accurate probability estimates to work correctly
Over-staking (above Kelly) actually reduces long-term growth, not increases it

What is the Kelly criterion?

The Kelly criterion is a mathematical formula developed by John Kelly at Bell Labs in 1956. It calculates the optimal fraction of your bankroll to stake on a bet with positive expected value, maximizing long-term growth rate.

Unlike flat staking where you bet the same amount regardless of edge, Kelly tells you to bet more when your edge is larger and less when it's smaller. This is mathematically proven to be the fastest way to grow a bankroll — but it comes with higher variance.

The key insight: there's an optimal stake size. Betting too little leaves money on the table. Betting too much actually reduces your growth rate and increases risk of ruin.

The Kelly formula for betting

For decimal odds:

Kelly% = (p × (d - 1) - (1 - p)) / (d - 1)

Where:

p = true probability of winning
d = decimal odds offered

Simplified:

Kelly% = Edge / (Odds - 1)
Edge = (p × d) - 1

Example 1: 55% win probability, odds 2.00

Edge = (0.55 × 2.00) - 1 = 0.10
Kelly = 0.10 / (2.00 - 1) = 10% of bankroll

Example 2: 30% win probability, odds 4.00

Edge = (0.30 × 4.00) - 1 = 0.20
Kelly = 0.20 / (4.00 - 1) = 6.67% of bankroll

Calculate instantly with the Kelly criterion calculator.

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Why you should use fractional Kelly

Full Kelly maximizes theoretical growth, but in practice it's too aggressive because:

Probability estimates are imperfect. Kelly assumes you know the true probability exactly. Overestimating your edge leads to catastrophic over-staking.
Variance is extreme. Full Kelly produces drawdowns of 50-80% — psychologically brutal even if you recover.
Simultaneous bets. Kelly is designed for sequential bets. When placing multiple bets at once, full Kelly stakes can exceed 100% of your bankroll.

Fractional Kelly comparison:

Full Kelly (100%): Maximum growth, maximum variance. Risk of ruin is theoretically 0 but drawdowns are severe.
3/4 Kelly (75%): 75% of growth rate, significantly less variance.
Half Kelly (50%): 75% of the growth rate of full Kelly with dramatically less variance. Most popular choice among professionals.
Quarter Kelly (25%): 44% of full Kelly growth but very smooth equity curve. Good when edge estimates are uncertain.

Half Kelly is the sweet spot for most bettors — you sacrifice only 25% of the growth rate while cutting variance roughly in half.

Kelly vs flat staking

Flat staking bets the same percentage regardless of edge. Kelly adjusts stake size based on edge magnitude.

Use flat staking when: You don't have precise probability estimates, you prefer simplicity, or you're just starting out.
Use Kelly when: You have reliable probability estimates (e.g., from devigging sharp odds), you understand the math, and you can handle the variance.

In practice, many professional bettors use a hybrid: fractional Kelly when they have strong edge estimates, with a minimum and maximum stake cap (e.g., never below 0.5% or above 4% of bankroll regardless of what Kelly suggests).

Common Kelly mistakes

Overestimating your edge. If you think your edge is 10% but it's really 3%, Kelly will have you staking 3x too much. Always be conservative with probability estimates.
Using full Kelly. Unless you have perfect probability estimates and nerves of steel, use half or quarter Kelly.
Ignoring correlation. Kelly assumes independent bets. If you're betting multiple correlated outcomes (e.g., same match), treat them as a single bet or reduce stakes.
Staking above Kelly. This is the biggest mistake. Staking above the Kelly fraction doesn't just increase risk — it actually decreases your expected growth rate. More risk, less reward.