Kelly Criterion Bet Sizing Calculator

Work out the optimal bankroll % to stake on an edge.

Calculate the mathematically optimal percentage of your bankroll to stake on a bet using the Kelly Criterion, from the odds offered and your own estimated true win probability. Shows full Kelly alongside the more commonly used half-Kelly and quarter-Kelly fractions for lower variance. Runs fully in your browser — nothing is uploaded.

Kelly Criterion Bet Sizing Calculator features

• Full, half & quarter Kelly stake percentages
• Dollar stake amounts from your bankroll
• Edge percentage from your win-probability estimate
• Warns when there's no mathematical edge
• 100% free, in-browser, no sign-up

How to use Kelly Criterion Bet Sizing Calculator

1. Enter your bankroll

   The total amount you're managing across all your bets — not just this one wager.

2. Enter the bet's odds

   Type the odds being offered on the bet in American, Decimal or Fractional format.

3. Enter YOUR win probability estimate

   This is the key input — your own honest, researched estimate of the true chance this bet wins, not the odds' implied probability.

4. Read the suggested stake

   See the full Kelly stake (%, and in dollars), plus the more commonly used ½ Kelly and ¼ Kelly fractions for a smoother bankroll curve.

Tricks & interesting facts

Using Kelly without blowing up your bankroll

The formula is exact — but that's exactly why it needs a light touch in practice.

• TIP Most professional bettors use HALF Kelly, not full Kelly — it gives up very little long-run growth for a much smoother, less stressful bankroll curve.
• TIP If you're not fully confident in your probability estimate, go even smaller — quarter Kelly is a common, conservative default.
• FACT Full Kelly can suggest staking 20%+ of your bankroll on one bet when the edge looks big — which is exactly when a probability estimate is most likely to be wrong.

Kelly Criterion assumes you already know your true win probability — it tells you HOW MUCH to bet, not WHETHER your pick is actually good.

Frequently asked questions

What is the Kelly Criterion in betting?

A formula for the mathematically optimal fraction of your bankroll to stake on a bet with a perceived edge, balancing maximum long-run growth against the risk of ruin from betting too aggressively.

What is the Kelly Criterion formula?

f* = p − q ÷ b, where b is the net decimal odds (decimal odds minus 1), p is your estimated true win probability, and q is 1 minus p. f* is the fraction of your bankroll to wager.

Why do bettors use half or quarter Kelly instead of full Kelly?

Full Kelly assumes your probability estimate is exactly correct — in reality it never is, and full Kelly's stake size makes losing streaks very painful. Half or quarter Kelly gives up some long-run growth for a much smoother, lower-variance bankroll curve.

What does it mean if Kelly says to bet 0%?

It means your estimated win probability doesn't clear the odds being offered — there's no mathematical edge, so the formula says not to bet at all rather than risk money on a losing proposition.

Do I need to know the 'true' probability to use Kelly?

Yes, and this is the hardest part in practice — Kelly's output is only as accurate as your own probability estimate. Sharp bettors build models or use no-vig market prices as a baseline before adjusting for their own research.

Is this Kelly Criterion calculator free?

Yes — free, unlimited and 100% in-browser. No sign-up, no third-party API, nothing uploaded.

Related tools — Sports betting calculators

• Betting Odds Converter — Convert American, Decimal & Fractional odds instantly.
• Parlay Payout Calculator — Combine multiple legs into total parlay odds & payout.
• Implied Probability Calculator — Turn betting odds into win probability — with vig removed.
• Arbitrage Betting Calculator — Find guaranteed-profit stakes across two or three sportsbooks.
• Hedge Bet Calculator — Lock in profit (or cut your loss) on a bet before it settles.