Quick answer
Enter the odds for every outcome in one market. The calculator adds up the implied probabilities, shows the Overround (the bookmaker's built-in margin) and removes it to give fair, no-vig probabilities and odds. At 1.90 / 1.90 the overround is 5.26% and the fair odds are 2.00 / 2.00 [1].
Calculator
How it works, with an example
Each price implies a probability of 1 ÷ odds. In a bookmaker market the implied probabilities sum to more than 100%; the excess is the overround [2]. This calculator uses the proportional (multiplicative) method: divide each implied probability by the total so they sum to 100%. Example for a three-way football market at 2.40 / 3.30 / 3.10: implied 41.67% + 30.30% + 32.26% = 104.23%, overround 4.23%, fair probabilities 39.98% / 29.07% / 30.95%. Other de-vigging methods (additive, power, Shin) can shift fair prices slightly for big underdogs.
Formula
booksum = Σ 1 / oddsᵢ
overround = booksum − 1
margin = 1 − 1 / booksum
fair (no-vig) probability pᵢ = (1 / oddsᵢ) / booksum
fair odds = 1 / pᵢFeed it live odds
Fair prices are most useful when you compare them across many bookmakers at once. Odds-API.io Partner link returns every bookmaker's prices for an event in one call, so you can compute the overround of each book and build a consensus fair line in code [3].
Partner link — we may earn a commission. See our affiliate disclosure.
Frequently asked questions
What is a normal overround?
It varies by bookmaker and market. Low-margin (sharp) books on major markets can be a few percent; recreational books and niche markets are often higher. Use the calculator on real prices to compare.
What is the difference between overround and margin?
Overround is how far the implied probabilities exceed 100% (booksum − 1). Margin is the share of stakes the book keeps on a balanced book (1 − 1/booksum). Both are shown.
Can I use this for prediction markets?
Yes. Enter each outcome's price as a probability-style decimal (0.40 Yes → 2.50) or use the odds converter first. Exchange prices often sum close to 100%; the gap is the spread and fees.
About this guide
Pred-Data publishes independent, source-linked explainers about prediction markets and the AI tools used around them. Every factual claim on this page links to a primary or reputable secondary source, checked on the date shown above. We do not run a trading venue, take bets, or give personal advice.
Author: Ken Lin · Corrections: if you spot an error, we review it and date-stamp the fix.
Affiliate disclosure
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Sources
- Bookmaker Margin Calculator (overround, fair odds) — Odds-API.io tools, accessed 2 Nov 2026
- What Is Implied Probability in Betting? Formula & Example — Odds-API.io glossary, accessed 2 Nov 2026
- Quickstart (get a key, first calls, example responses) — Odds-API.io docs, accessed 2 Nov 2026