Enter the price of every outcome in a market and this takes out the bookmaker margin, so you can see the fair chance and the fair price of each one.
Every bookmaker price has a margin built in, called the vig, juice or overround. Add up the implied chances of every outcome in a market and you get more than 100%: two sides at −110 (1.91) imply 52.4% each, 104.7% in total. The extra 4.7% is the margin. Take it out and each side is a fair 50%, a fair price of 2.00 (+100).
The no-vig price is the market's own view of the chance with the house edge removed. It is the number to compare other prices against. If one book's no-vig price says 50% and another book offers 2.20 on the same side, that price is longer than fair and has positive expected value against the first book's view.
This calculator uses the proportional method: each outcome's implied chance is divided by the total book, so the chances add up to exactly 100%. On a market at 1.50 and 2.60, the implied chances are 66.7% and 38.5%, a book of 105.1%. Divided through, the fair chances are 63.4% and 36.6%, fair prices of 1.58 and 2.73.
It is the most common method and it is a good default. In markets with a big favourite and long outsiders, bookmakers tend to load more of the margin onto the long shots, so the proportional method can slightly overstate an outsider's fair chance. For two-way markets like spreads and totals, the difference is tiny.
Use a sharp, low-margin market to find the fair price, then shop for a better price elsewhere. It also shows how expensive a market is: a two-way market at 102% is cheap to bet into, one at 108% is not.