De-Vigging: Normalise a Market's Odds
Convert decimal odds into implied probabilities, measure a market's overround, and normalise the probabilities while keeping the method's assumptions visible.
What is implied probability?
For decimal odds O, the implied probability is 1 / O. In a two-outcome market with prices 1.95 and 1.90, the raw probabilities are approximately 51.28% and 52.63%. Their total is 103.91%, so the market has an overround of about 3.91 percentage points.
The overround is a property of the quoted price set. It is not an extra outcome or a prediction of the result.
Proportional normalisation
One common de-vigging method divides each raw implied probability by their sum. In the illustrative example above:
- first outcome:
0.5128 / 1.0391 = 49.35% - second outcome:
0.5263 / 1.0391 = 50.65%
The normalised values total 100%. Converting a normalised probability p back into decimal odds uses 1 / p.
What does de-vigging assume?
Proportional normalisation assumes the margin is distributed proportionally across the outcomes. Other methods make different assumptions, and none turns a market consensus into a fact. The result is a transparent reference estimate, especially useful when comparing like-for-like prices—not a promise that any quote is value or that an outcome will occur.
For three-outcome markets, include every mutually exclusive settlement outcome in the sum. For markets with different lines or rules, do not combine prices as if they were the same market. Arbitrage stake-split maths uses the same reciprocal-price logic for a different calculation.
Pair the estimate with data quality
Check the event, market, line, time, source, and available amount before using any normalised price in a review. Limits, liquidity, and signal quality explains why a number on a board can be incomplete context.
Inspect price context in PhotonOdds
Use the EV+ view to inspect the displayed price context alongside the underlying market details. Treat any estimate as a prompt to review assumptions, not as a recommendation.
Try it with your own prices in the de-vig calculator — it runs the same proportional normalisation and shows the overround explicitly.
18+ only. Betting carries risk. PhotonOdds provides analytical and educational tools, not a promise of profit or a recommendation to place a bet. If gambling is causing harm, see Responsible Gambling.