overround calculator

What is overround?

Overround is the built-in margin a bookmaker includes in a market. If you convert every listed price into an implied probability and add them up, a perfectly fair market totals exactly 100%. In a bookmaker market, the total is usually above 100%. That extra percentage is the overround (also called the bookmaker margin or vig).

Example: if a two-outcome market sums to 106%, the overround is 6%. That 6% is the bookmaker's edge before considering risk management, customer behavior, and line movement.

How this calculator works

1) Convert odds into implied probability

• Decimal odds: P = 1 / odds
• American odds (+): P = 100 / (odds + 100)
• American odds (-): P = |odds| / (|odds| + 100)
• Fractional odds (A/B): P = B / (A + B)

2) Sum probabilities

The calculator adds all implied probabilities to produce the book percentage. A fair market is 100%. Anything above 100% is overround.

3) Compute margin and fair no-vig probabilities

Margin is: Overround = Book% - 100%. To remove vig, each implied probability is normalized by dividing by total implied probability. These normalized values are the no-vig probabilities, which can be converted back to fair odds.

Why overround matters

• Price quality: Lower overround generally means better value for bettors.
• Comparison shopping: You can compare books on margin, not just single prices.
• Model calibration: Traders and analysts can compare model probabilities to vig-free probabilities.
• Arbitrage detection: If combined best prices across books total below 100%, there may be an arbitrage opportunity.

Quick example

Suppose a soccer 1X2 market has decimal odds:

• Home: 2.20
• Draw: 3.40
• Away: 3.20

Implied probabilities are approximately 45.45%, 29.41%, and 31.25%. Total = 106.11%, so overround is 6.11%.

The no-vig probabilities become each implied probability divided by 106.11%, producing a normalized distribution that sums to 100%.

Common mistakes to avoid

• Mixing odds formats in one calculation.
• Treating overround as guaranteed profit in all circumstances.
• Ignoring market movement and limits.
• Using tiny sample sizes when benchmarking bookmaker margin.

Practical tips

For bettors

• Track overround by sport and market type (moneyline, totals, props).
• Use no-vig lines to estimate your true edge against your model.
• Shop multiple books before placing a bet.

For analysts and traders

• Monitor margin shifts before major events and near kickoff.
• Separate sharp and recreational market segments.
• Use fair no-vig probabilities as a cleaner signal for forecasting.

Responsible gambling note: Use tools like this for analysis and budgeting discipline. Never risk more than you can afford to lose.