Staking

The Kelly criterion explained with worked examples

Updated 13 Nov 2026 · 3 min read

Most staking plans tell you how to change your bet after a win or a loss. The Kelly criterion asks a different question: given a price and your chance of winning, what share of the bankroll makes it grow fastest? The answer is one short formula, and it has a hard condition attached.

The condition: an edge of your own

Kelly only stakes when the bet is worth more than it costs. A price of 2.10 breaks even at a win rate of 47.62%. If you rate the bet at 52%, every unit staked earns 9.20% on average. That surplus is the edge.

Without it the formula returns zero. The break-even point itself sits above the true chance by the margin, which the guide to bookmaker margin takes apart.

The formula in one line

Divide the edge by the profit a winning unit makes, which is the price minus one. In the example that gives 8.36% of the bankroll. From 1,000 the full Kelly stake is 83.64.

A price quoted in another format goes through the odds converter first, and the Kelly calculator does the division for any price, chance and bankroll.

Why full Kelly is rarely staked

Full Kelly gives the fastest growth, 0.38% per bet in the example, and a rough ride on the way. Half Kelly stakes 41.82 and still keeps 75% of that growth, 0.29% per bet, with half the money at risk.

Staking more than full Kelly is worse on both counts: growth drops, and at double Kelly it is gone altogether.

The weak point is your own number

The formula trusts your chance completely. Suppose the bet really wins 50% of the time, not 52%. The right stake was 4.55% of the bankroll, and full Kelly on your estimate staked almost twice that.

This is the practical reason for half or quarter Kelly: they forgive an estimate that is a little too keen. A quarter stakes 20.91 in the example.

What Kelly says about casino games

A game at an RTP of 97% has an edge of 3% against you on every bet, whatever you stake. Put that into the formula and the stake is zero. No staking plan changes this, and does Martingale work on crash games shows what happens when one tries.

For play with a negative edge the useful questions are how long a bankroll lasts and how a session can end. The bankroll survival calculator and the strategy simulator answer those.

Kelly is a rule for sizing a bet you already have a reason to make. It does not find such bets and it cannot create an edge.

Questions about staking with Kelly

What is the Kelly stake for a coin that wins 55% at even money?

Exactly 10% of the bankroll. At a price of 2.00 the Kelly share equals the edge.

Is half Kelly much slower than full Kelly?

Less than it sounds. In the example it keeps 75% of the growth while risking half as much per bet.

What if Kelly gives a negative number?

Then the bet has no edge at your chance, and the stake is nothing. A negative result never means betting the other side at the same price.

Can I use Kelly without knowing my real win rate?

Not safely. The stake is only as good as the chance you feed in, which is why a cautious fraction is the usual choice.

Tools for sizing a stake

Kelly stake Bankroll survival

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