Fair Odds Calculator

Fair odds (also called true odds or no-vig odds) are the decimal odds that would exist if bookmakers charged zero margin. They represent the mathematically correct price for each outcome based on true probability. Fair odds are always longer (higher decimal) than the odds actually offered, because bookmakers shorten every price to build in their margin.

To strip margin: first calculate the implied probability of each outcome (1/decimal), then sum all implied probabilities. This sum is greater than 1 — the excess is the overround. Divide each implied probability by the total sum to get true probability. Fair odds are 1/true probability. This calculator does all of this automatically.

You might override the margin if you know a bookmaker's standard market margin (e.g. 5%) and want to apply it consistently, or if you are working from a market where you only have odds for some outcomes. It is also useful for modelling what fair odds would look like at different margin levels for comparison.

This figure shows the expected long-run loss per £100 wagered in this market. If the margin is 5%, the bookmaker retains roughly £4.76 per £100 staked (margin / (1 + margin) × 100). Over a large number of bets at the same margin, this figure represents the theoretical cost of betting in this market.

A value bet exists when bookmaker odds exceed the fair odds you calculate from your own probability estimate. By stripping margin from one bookmaker and comparing with another, or comparing fair odds against your model's fair probability, you can identify whether a price represents genuine value or is simply a bookmaker mispricing their own market.