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The Kelly Criterion Explained

The Kelly criterion sizes your stake to your edge for optimal long-term growth. Here's how it works, and why most bettors should use a fraction of it.

Updated 18 August 2026.

The Kelly criterion is a mathematical staking formula that sizes each bet according to your estimated edge, maximising long-term bankroll growth. It's powerful but unforgiving of overconfidence. This guide explains it simply, extending staking plans compared.

What Kelly does

Kelly tells you what fraction of your bankroll to stake based on the odds and your estimated probability of winning. Bigger edges mean bigger stakes; no edge means no bet.

In theory it delivers the fastest sustainable bankroll growth.

The formula in plain terms

The stake fraction rises with your edge (how much the true probability beats the implied probability) and adjusts for the odds. When you have no edge, Kelly says stake nothing.

Our staking calculator can work out Kelly stakes for you.

Why use fractional Kelly

Full Kelly is volatile and assumes you know your edge exactly — which you don't. Most bettors use a fraction (half or quarter Kelly) to cut the swings and the risk of overestimating their edge.

Overestimating your edge with full Kelly can cause brutal drawdowns.

Kelly's big caveat

Kelly is only as good as your probability estimates, and overconfidence makes people overbet. If you can't reliably estimate your edge, a simple percentage plan is safer.

Treat it as a tool for disciplined, analytical bettors, not a shortcut to riches.

Frequently asked questions

What is the Kelly criterion?

A staking formula that sizes each bet according to your estimated edge — bigger edges mean bigger stakes — to maximise long-term bankroll growth. With no edge, it says stake nothing.

Should I use full Kelly?

Usually not. Full Kelly is volatile and assumes you know your edge exactly. Most bettors use fractional Kelly (half or quarter) to reduce swings and the risk of overestimating their edge.

What's the main risk with Kelly staking?

It's only as good as your probability estimates. Overconfidence leads to overbetting and brutal drawdowns, so if you can't reliably estimate your edge, a simple percentage plan is safer.

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