Overround and margin calculator

Outcome one decimal odds Outcome two decimal odds Outcome three decimal odds Outcome four decimal odds, optional

Example result

2.00, 3.50 and 4.00 imply a 3.57% overround.

What this calculator answers

Add a set of decimal prices to measure the implied total. The calculator reports the overround and rescales the implied probabilities to produce proportional fair odds.

Formula and worked example

Formula: Overround percent = (sum of 1 divided by each decimal price minus 1) multiplied by 100. Fair probability = raw implied probability divided by the implied total.

Prices of 2.00, 3.50 and 4.00 imply 50.00, 28.57 and 25.00 percent, or 103.57 percent in total. Removing that proportional margin gives fair decimal odds of about 2.07, 3.63 and 4.14.

Checks before using the result

• Adding decimal prices instead of inverse probabilities.
• Assuming margin is distributed equally in every market.
• Comparing fair odds from different outcome sets.

When the calculation does not apply

The fair odds are a proportional adjustment, not an estimate of true event probability. Markets can shade outcomes unevenly.

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 Kelly criterion and Poisson football score, 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.