Every betting line has a hidden tax baked in. If you turn the odds into probabilities and add them up, they come to more than 100% — that excess is the bookmaker's margin, the vig (or overround). To find what the market really thinks, you have to strip it out. That's de-vigging.
Odds are probabilities in disguise
Decimal odds of 2.00 imply a 50% chance (1 ÷ 2.00). Odds of 1.50 imply 66.7%. But take a two-way market priced at 1.90 / 1.90: that's 52.6% + 52.6% = 105.2%. Probabilities can't sum above 100% — the extra 5.2% is the vig, the book's built-in edge.
How to de-vig
The simplest method is to normalise: divide each raw implied probability by the total. In the example, 52.6% ÷ 105.2% = 50% for each side — the fair, vig-free line. On a three-way football market (home/draw/away) you do the same across all three. What's left is the market's honest estimate of each outcome.
Why it's the baseline for value
You can't measure an edge against a number that includes a tax. De-vigging the sharpest available line gives you the cleanest read on the true probability — the anchor you compare your own model to. Bet against the raw, vig-inflated price and you'll think you have an edge you don't.
Momus de-vigs the sharp bookmaker line in code and blends it with its model before measuring any edge. See how that fits the full method in how Momus finds value on Polymarket, or the difference between venues in Polymarket vs bookmakers.