The Kelly Criterion: How Much Should You Bet?

By Cobia Picks · Updated July 3, 2026 · ~7 min read

Finding a good bet is only half the job. The other half — the half that quietly decides whether you compound or go broke — is how much to stake. Bet too little and you leave growth on the table; bet too much and one cold streak wipes you out even when you're right on average. The Kelly criterion is the math that answers this, and it's the sizing framework most serious bettors and traders anchor to. Here's how it works and how to use it without blowing up.

What the Kelly criterion is

Kelly is a formula that outputs the fraction of your bankroll to wager on a bet, given two inputs: your estimated probability of winning and the odds you're being paid. It's the stake size that maximizes the long-run growth rate of your bankroll. Bet more than Kelly and your growth rate actually falls while your risk of ruin climbs; bet less and you grow more slowly but sleep better. It is not a betting system that picks winners — it assumes you already have an edge and tells you what to do with it.

The formula

For a simple win/lose bet, the Kelly fraction is:

A worked example

Suppose you think a coin-flip-looking market is really 55% to hit, and it's priced at even money (a bet where you win $1 for every $1 risked, so b = 1). Plug it in:

• p = 0.55, q = 0.45, b = 1
• f = (1 × 0.55 − 0.45) / 1 = 0.10

Full Kelly says stake 10% of your bankroll on this bet. On a $2,000 bankroll that's a $200 wager. Notice how a modest-sounding 5-point edge (55% vs the 50% the even-money price implies) translates into a chunky 10% stake. That's the first warning sign: Kelly gets aggressive fast.

Now change the price. Same 55% read, but you get it at plus-money — say $1.50 profit per $1 (b = 1.5). Then f = (1.5 × 0.55 − 0.45) / 1.5 = 0.375 / 1.5 = 0.25. A better price on the same edge quadruples the recommended stake to 25% of bankroll. That is a lot of money on one bet — which is exactly why almost nobody bets full Kelly.

Why full Kelly is too aggressive

Full Kelly is mathematically optimal only if your probability estimate is exactly right. In real betting it never is. You're estimating 55% but the truth might be 52%, or 48%. Full Kelly is also brutally volatile: even with a real edge, a full-Kelly bankroll routinely suffers 50%+ drawdowns that most people can't stomach and that can end a bankroll if they hit early. The math rewards the long run, but you only get the long run if you survive the short one.

Kelly only works if your edge is real

Here's the part fake cappers never mention: Kelly is only as good as your probability estimate. The formula takes your win probability as gospel. If you feed it a fantasy — "I'm 60% here" when you're actually a 50% coin flip — Kelly will confidently tell you to bet a large slice of your bankroll straight into a negative-expectation wager, and the sizing turns a leak into a hemorrhage.

That's why Kelly can't be separated from expected value. You need a genuine, evidence-based reason to think your number beats the market's number before Kelly means anything. And in practice the cleanest proof that your edge is real is closing line value — consistently getting a better price than the market closes at. Positive CLV over a large sample is the signal that your win-probability inputs are honest; without it, any Kelly stake is just confident guessing.

Using it in practice

• Estimate your true win probability honestly — and then shade it toward the market, because the market is usually smarter than you think.
• Compute the full-Kelly fraction from that probability and the price on offer.
• Bet a fraction of it — half or quarter — never the full number.
• Recompute off your current bankroll as it grows or shrinks, so stakes scale with what you actually have.
• Track CLV to confirm your probability inputs are real before you trust the sizing at all.

You don't have to do the arithmetic by hand. Cobia's Tools tab includes a Kelly calculator — enter your estimated probability and the price, and it returns the full-, half-, and quarter-Kelly stakes so you can size a position in seconds without over-betting.