De-vigging: finding the fair price

A market that adds up to 104.8% cannot be read as probability. De-vigging scales it back to 100% so it can be — and the result is the number every +EV decision is measured against.

The quick version

If a market is -110/-110, the fair price is +100 on both sides. That is the most useful shortcut in betting: anything better than +100 on a -110/-110 market is a bet worth making. No modelling, no opinion, just a comparison.

The real version

Take a sharp book at -135/+115. Convert both to percentages, scale them so they sum to 100, and convert back. You get roughly +123 on both sides — the market's honest opinion with the house's cut removed.

Why the simple split is not always right

Dividing evenly assumes the margin sits evenly across both sides. It usually does not — books charge more on longshots, where casual money goes and where a mistake costs them least. On a heavy favourite, the even split systematically overstates the underdog. There are better methods that correct for this, and which one is best is a genuinely open question, so treat a fair price as an estimate with an error bar rather than a fact.

One book is not a market

A single book's de-vigged number is that book's opinion, including its mistakes. This app requires at least two books before it will call anything a fair price, and blends several, so one stale or wrong quote cannot set the reference alone. You can raise that floor in Settings if you want an even more conservative board.