Kelly criterion calculator

Bankroll Decimal odds Estimated true probability % Kelly fraction Full Kelly Half Kelly Quarter Kelly

Example result

At 2.50 and 45%, half Kelly on 100.00 is about 4.17.

What this calculator answers

Calculate the Kelly fraction from a price and estimated true probability, then apply a selected fraction of that figure to a stated bankroll.

Formula and worked example

Formula: Full Kelly fraction = (b times p minus q) divided by b, where b is decimal odds minus 1, p is probability and q is 1 minus p. Stake = bankroll times fraction.

At 2.50 with a 45 percent estimate, b is 1.50 and q is 0.55. Full Kelly is (1.50 times 0.45 minus 0.55) divided by 1.50, or 8.33 percent. Half Kelly on 100.00 is 4.17.

Checks before using the result

• Using the market implied probability as the true probability estimate.
• Ignoring uncertainty in an estimate near the break even point.
• Applying the full fraction to money needed for essentials.

When the calculation does not apply

It assumes a reliable probability estimate and repeatable independent opportunities. It does not fit a single uncertain outcome or money needed for living costs.

Keep the number in context

A calculator shows the arithmetic. It doesn't tell you whether the price fits your read of the outcome or your budget. Decide the stake before you enter it. Check the market rules. Keep essential money out of it entirely.

Cross-check on a second tool. The calculator hub, Bankroll and staking or Free bet SNR cover the same numbers from a different angle. Same price conventions, so a decimal price in one is a decimal price in the other.

Read the guide to reading odds if you're new to the formats. Two minutes.

Reading a rounded result

Money is rounded to two decimals here. A real settlement applies its own rounding sequence, which can differ. Keep the original inputs written down if you're going to check the number later.

Unclear about an input? Find the stated market term. Don't guess. A quick sanity check: change one input by a small amount and see whether the result moves the way you expect.

That doesn't validate the market. It can flag a reversed sign, a mis-entered price format or a duplicated stake. Use the number to understand the arithmetic. It's not a reason to ignore a limit you set in advance.