What Are Fair Odds? The Bookmaker's Margin (Vig) Explained

Every betting price you see is lying to you a little. Not about who's favoured — about how much. Understanding the bookmaker's margin, and how to strip it out, is the single most useful piece of math in betting. It takes five minutes to learn.

The margin hides in the probabilities

Convert decimal odds to an implied probability: 1 ÷ odds.

Take a match priced at 1.91 / 1.91 on the two sides of a total:

• Over: 1 ÷ 1.91 = 52.4%
• Under: 1 ÷ 1.91 = 52.4%

Together: 104.8%. Real probabilities add up to 100% — the extra 4.8 points is the bookmaker's margin (the "vig" or "overround"). It's how the book profits regardless of the result.

Stripping the vig: fair odds

To find what the market actually believes, rescale the probabilities so they sum to 100%:

• Over: 52.4 ÷ 104.8 = 50%
• Under: 52.4 ÷ 104.8 = 50%

So a 1.91/1.91 line is really a coin flip, and the fair odds are 2.00 on each side. The gap between 1.91 and 2.00 is what you pay for the bet to exist.

That's the whole trick. Any two-way market, same recipe. (Three-way markets like 1X2 work the same, just with three probabilities in the sum.)

Why this matters: the honest definition of an edge

An edge is not "I think this team wins." An edge is: my probability for this outcome is higher than the de-vig probability at the price I can actually get.

If the fair line says 50% and you believe 55%, you have a claimed edge of 5 points. If you believe 52%, you have almost nothing — the margin eats it.

This is also how you spot inflated tipster math. A common trick is comparing a model probability against the raw implied probability (vig included), which flatters every claimed edge by several points. Always ask: edge against what — the posted price, or the de-vig fair line?

The Quant's rules at our bar

When the Quant quotes an edge on a Three Pundits pick, three things are non-negotiable:

1. The edge is computed against the de-vig fair line, never against the raw price.
2. The posted price and the book get named — an edge you can't actually bet doesn't exist.
3. Claimed edges are capped and negative-EV picks are blocked at the publish gate, because models run hot and the market usually knows more than the model.

Try it yourself

Next time you see a price, do the two-step: implied probabilities, rescale to 100%. You'll start seeing margins everywhere — and you'll never read "huge value!" the same way again.

Every pick we publish shows its price, book, and the computed edge, on the record before kickoff: threepundits.com/picks.

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This is general information about how betting markets work, not betting advice. 18+ · bet responsibly.