Betting calculator
Overround Calculator
Use this calculator to work out the implied probability, total book percentage and bookmaker margin in a betting market.
Enter the odds
Add each outcome in the market, then enter the decimal odds.
| Outcome | Decimal odds | Implied probability | |
|---|---|---|---|
| 47.62% | |||
| 29.41% | |||
| 27.78% |
This market totals 104.81%, which means the odds include an overround of 4.81%.
What is overround?
Overround is the bookmaker’s built-in margin in a betting market. It is calculated by converting each outcome’s odds into implied probability, then adding those probabilities together.
In a perfectly fair market, all outcomes would add up to 100%. In most bookmaker markets, the total is above 100%. The amount above 100% is the overround.
Overround formula
Implied probability = 1 ÷ decimal odds × 100
Book percentage = all implied probabilities added together
Overround = book percentage − 100
Example
Imagine a football match with these odds:
| Outcome | Odds | Implied probability |
|---|---|---|
| Home | 2.10 | 47.62% |
| Draw | 3.40 | 29.41% |
| Away | 3.60 | 27.78% |
Added together, those implied probabilities total 104.81%. That means the market has a 4.81% overround.
Why does overround matter?
Overround helps explain how bookmakers price markets in their favour. It does not mean a bookmaker is guaranteed to profit on every individual event, but it does show that the odds are usually built with a margin above the true 100% market.
What is underround?
If a market adds up to less than 100%, it is called an underround. This can sometimes indicate an arbitrage opportunity, although real-world factors such as stake limits, changing odds, account restrictions and exchange commission can affect whether it is actually usable.
Overround calculator FAQs
What is a good overround?
A lower overround usually means a more competitive market for the bettor. For example, a market with a 102% book percentage is generally better value than a similar market priced at 110%, assuming the odds are otherwise comparable.
Can overround be below 100%?
Yes. If the total implied probability is below 100%, the market is known as an underround. This is uncommon with a single bookmaker, but it can appear when comparing odds across multiple bookmakers or exchanges.
Is overround the same as bookmaker margin?
They are closely related. Overround usually refers to the amount above 100% in the book percentage. Bookmaker margin is often used more generally to describe the bookmaker’s built-in edge.
Does overround guarantee the bookmaker makes money?
Not on every single event. Bookmakers can still lose on individual results, especially if liabilities are uneven. Over time, margin, pricing and liability management are part of how bookmakers aim to operate profitably.