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 makes the arithmetic visible, but it does not decide whether a price fits your budget or information. Write down the stake before placing anything, check the market rules, and keep essential money separate. The result is a planning figure, not a prediction of what will happen.

For another view, visit the calculator hub, compare the Bankroll and staking and Free bet SNR, or read the guide to reading odds. These tools use the same price conventions, so you can check a number from more than one angle.

Reading a rounded result

Displayed money amounts are rounded to two decimal places, while a real settlement can apply its own rounding sequence. Keep the original inputs beside the result if you need to repeat the calculation later.

If an input is unclear, pause and find the stated market term rather than guessing. A useful check is to change one input by a small amount and see whether the result moves in the direction you expect.

That does not validate the market, but it can reveal a reversed sign, an incorrect price format or a duplicated stake. Use the final figure as an aid to understanding the arithmetic, not as a reason to ignore a limit you set in advance.