Overround calculator

What overround is

Overround is the bookmaker's built-in margin in a betting market. You work it out by converting each outcome's odds into an implied probability and adding them together. In a perfectly fair market, every outcome would add up to 100%. In most bookmaker markets the total is above 100%, and the amount above 100% is the overround.

A worked example

Take a football match priced at 2.10, 3.40 and 3.60. The calculator opens on these odds.

Added together, those implied probabilities total 104.81%. That means the market has a 4.81% overround.

Why the overround matters

A lower overround usually means a more competitive market for the bettor. A market with a 102% book percentage is generally better value than a similar market priced at 110%, if the odds are otherwise comparable.

A book that adds up to less than 100% is called an underround. It is uncommon with a single bookmaker, but it can appear when you compare prices across several bookmakers or exchanges.

How this calculator works

• Odds can be typed as decimals (3.5), fractions (5/2) or evens.
• Add between two and eight outcomes. Every outcome of the market needs a price, or the calculator asks for the missing one.
• The margin-free probability of an outcome is its implied probability divided by the book percentage, so the set adds up to 100%. This removes the margin in proportion to each probability.
• Percentages are rounded to two decimal places.

Questions

What is overround?

Overround is the bookmaker's built-in margin in a market. Convert every outcome's odds to an implied probability and add them together. The amount above 100% is the overround.

How do I work out implied probability?

Divide 1 by the decimal odds and multiply by 100. Odds of 2.10 imply a probability of 47.62%.

Is a lower overround better?

A lower overround usually means a more competitive market for the bettor. A book of 102% is generally better value than a similar market priced at 110%, if the odds are otherwise comparable.

Can the overround be below 100%?

Yes. A book under 100% is called an underround. It is uncommon with a single bookmaker, but it can appear when you compare prices across bookmakers or exchanges.

Is overround the same as the bookmaker's margin?

They are closely related. Overround usually means the amount above 100% in the book percentage. Margin is often used more generally for the bookmaker's built-in edge.

Does an overround mean the bookmaker wins every event?

No. Bookmakers can still lose on individual results, especially when their liabilities are uneven.

How does this calculator take the margin out?

It divides each implied probability by the book total, so the probabilities add up to 100%. That takes the margin out in proportion to each probability. The margin-free odds are 1 divided by that probability.