No-vig calculator: strip the bookmaker margin and find the fair price

Convert every price in a market to implied probability and add them up. In a fair market the total would be exactly 100%. In a real one it is always more — typically 102% to 107% on a football match, and considerably more on markets with many runners. That excess is the overround, also called the vig or the juice.

It exists because a bookmaker who priced every outcome fairly would make nothing. Building in a few percent means that a balanced book profits regardless of the result. It is the price of the service, and it is entirely legitimate — but it means the prices you see are systematically shorter than the market's genuine opinion.

Two prices of 1.91 each imply 52.4%, and together that is 104.8%. The market thinks each side is a 50% chance; the 4.8% is margin. If you assume 1.91 means "the bookmaker thinks this is a 52.4% shot", you will consistently misjudge how confident the market actually is.

The simplest method, and the one this calculator uses, divides each implied probability by the total. Two prices at 52.4% each divided by 104.8% give 50% each, so the fair price on both sides is 2.00. This is called proportional or multiplicative devigging.

It has a known weakness: margin is not usually spread evenly across a market. Bookmakers load more of it onto longshots, which is the well-documented favourite-longshot bias. In a market with a 1.20 favourite and a 15.00 outsider, proportional devigging will slightly overstate the outsider's fair chance.

More sophisticated methods exist — additive, power and Shin devigging all attempt to model where the margin is concentrated. For two-way markets with prices near evens, the differences between them are tiny, and the proportional method is a perfectly good working tool. On lopsided multi-runner markets, treat the output as an approximation rather than a precise figure.

The most valuable use of a no-vig price is as a reference point. If a market's fair price on a selection is 2.20 and another bookmaker is offering 2.40, you have found a genuine discrepancy without needing any opinion of your own about the event.

This is the mechanism behind most systematic value betting. Rather than forming an independent view, you use the sharpest available market as your estimate of true probability and look for prices elsewhere that beat it. It requires no expertise in the sport at all — only the discipline to compare consistently and the acceptance that accounts finding these prices tend to get limited.