Kelly Criterion Calculator

A value bettor spots odds of 2.10 on a market they price at 52 percent. The edge is real, but the next question is harder: how much of the bankroll goes on it? Bet too big and one bad run erases months of work. Bet too small and the edge barely compounds. The Kelly criterion answers that with a formula, and the calculator above runs it for you.

Convert any price to decimal first with our odds converter tool so the number you paste in is clean. The probability input is the one that matters most, and our guide to positive EV betting covers how to derive a fair probability from Pinnacle or Betfair prices.

There is no universally correct fraction. For a bettor with a well-tested model and a large sample behind their edge, half Kelly is defensible. For someone newer to +EV betting, or working from noisy estimates, quarter Kelly protects against the very real chance the true edge is smaller than it looks. When your inputs are uncertain, staking less is the honest choice. The theory sits in our explainer on the Kelly criterion, worth reading once before you trust any staking output.

Two habits close the loop. Cap your staking fraction in advance, a quarter to a half, and never override it mid-session. And track closing line value on every bet, since positive CLV across a few hundred bets confirms your probabilities are sound long before the profit and loss does. Our bankroll management guide ties the staking maths to the discipline that makes it work.

What if the calculator returns a negative number?

A negative Kelly fraction means the odds offer no value at your estimated probability. The implied probability of the price is higher than your own estimate, so the expected value is negative. The correct action is to place no bet at all. Treat a negative output as a filter that just saved you a losing stake, not as a figure to bet small.

Should I use full Kelly or a fraction?

Almost everyone should use a fraction. Full Kelly is optimal only with perfect probability estimates, which do not exist in sports betting. Half Kelly keeps most of the growth with far less variance, and quarter Kelly is calmer again. If your edge is unproven over a large sample, quarter Kelly is the safer starting point.

Where do I get the win probability to enter?

From a repeatable method, not a feeling. Many value bettors devig the odds from a sharp book such as Pinnacle or Betfair Exchange to estimate a fair probability, then compare it to the soft book price. A model or a documented system also works. The one thing to avoid is entering a number because the bet looks appealing.

Does a Kelly stake stop my account getting restricted?

No. Kelly sizes a bet for bankroll growth, not for account safety. Soft bookmakers restrict winning accounts regardless of staking method, often within 50 to 500 bets. Staking to the market limit or placing conspicuously large bets can speed that up. Sizing sensibly and spreading action across books helps, but no calculator removes restriction risk.

Can I use the Kelly criterion for anything besides betting?

Yes. Kelly originated in information theory and is used in investing to size positions under uncertainty. The maths is identical: an edge, a payoff and a probability. The caution is the same too, since full Kelly is volatile wherever the probability estimate is imperfect, which in real markets it always is.

Is a bigger edge always worth a bigger stake?

In theory yes, and the calculator reflects that by raising the fraction as the edge grows. In practice, a large fraction on a single bet concentrates risk, and your edge estimate on that bet may be wrong. This is why a staking cap and fractional Kelly matter more as the suggested stake climbs.