Kelly Criterion Calculator: Free Bet Sizing Tool

Quick take: The Kelly Criterion is a formula for working out how much of your bankroll to stake on a bet, based on your edge — not a fixed percentage, and not a guess. Enter your bankroll, the odds, and your estimated win probability below, and the calculator does the math instantly. No signup, no download.

Calculate your stake

Work out how much of your bankroll to stake based on your edge

Not sure how to read decimal odds? See our odds converter.

Full Kelly stake

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Half Kelly stake

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Half Kelly is the more common real-world choice — it sacrifices some long-run growth for meaningfully lower variance. This is a mathematical sizing tool, not betting advice, and assumes your win-probability estimate is accurate — the formula only protects you from bad staking, not from a bad estimate. See our Sports Investing Bible for the full explanation.

How to Use This Calculator

  • Bankroll — the total amount you have set aside for betting, not your entire savings.
  • Decimal odds — the odds offered for the bet you’re considering. If you only have American or fractional odds, convert them to decimal first.
  • Win probability — your own honest estimate of how likely the bet is to win, as a percentage. This is the number the whole calculation depends on — more on that below.

The calculator then shows two numbers: the full Kelly stake (the mathematically optimal amount to maximize long-run bankroll growth) and the half Kelly stake (half that amount, for meaningfully lower variance).

What the Kelly Criterion Actually Tells You

The Kelly Criterion answers one specific question: given your bankroll, the odds, and your estimate of winning, what stake maximizes long-run growth without risking ruin? It does not tell you whether a bet is good — that depends entirely on whether your win-probability estimate is accurate. The formula is only as good as the number you put into it.

This is the same formula covered in more depth, including the full derivation and worked examples, in our Sports Investing Bible.

Full Kelly vs. Half Kelly

Full Kelly is mathematically optimal for long-run growth, but it comes with large short-term swings — a string of losing bets at full Kelly stakes can shrink a bankroll dramatically even when the underlying edge is real. Most experienced bettors use half Kelly (or even quarter Kelly) instead: it gives up some long-run growth in exchange for meaningfully smoother results, which matters more in practice than the theoretical maximum.

Frequently Asked Questions

What happens if the calculator shows a 0% stake? That means there’s no mathematical edge at the odds and probability you entered — the Kelly formula is telling you not to bet, not producing an error.

Can the Kelly Criterion be negative? The underlying formula can go negative, which just means betting against the outcome (or not betting at all) is favored. This calculator floors the result at 0% rather than showing a confusing negative stake, since you can’t bet a negative amount on a single side.

Where do I get an accurate win probability? This is the hardest part of using Kelly in practice, and the calculator can’t do it for you. It comes from your own analysis, a model, or a trusted data source — not from the calculator itself.

Is full Kelly ever the right choice? For most individual bettors, no — the variance is too high in practice. Full Kelly is more of a theoretical upper bound than a practical recommendation, which is why half Kelly is shown alongside it here.

Does this work for other decisions besides sports betting? The Kelly Criterion originated in information theory and is used in investing and trading too — the math is identical, only the inputs (odds and probability) change meaning depending on context.

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