Fair odds and the bookmaker's margin
Fair odds are the break-even price for a given probability. The gap between fair odds and the price you're offered is where the bookmaker's margin lives — and it's on every bet you make.
What 'fair odds' means
Fair odds are simply 1 divided by the probability. If a player truly has a 50% chance, the fair price is @2.00. At 60%, it is @1.67. At 25%, @4.00.
At fair odds, a bet breaks even over the long run: you neither gain nor lose on average. It is the reference point every value judgment is measured against.
The margin: vig, overround, juice
Bookmakers do not offer fair odds. They shade every price a little shorter so that the implied probabilities across a market sum to more than 100%. That surplus is the margin — also called the vig, the juice or the overround.
Example: a match where both players are genuinely 50/50. Fair odds would be @2.00 / @2.00. A book might post @1.90 / @1.90 instead — implied 52.6% each, summing to 105.2%. The 5.2% over 100% is the margin the book keeps.
Why the margin matters to you
The margin is a fee you pay on every bet, win or lose. It is the reason a long run of coin-flip bets at @1.90 slowly bleeds money even though the underlying calls are 50/50. To profit over time, your edge has to be big enough to overcome the margin first — a high bar.
Sharper markets (big ATP and WTA matches) carry thinner margins; smaller events carry fatter ones. Either way, the margin never works in your favor.
Fair odds vs the odds you're offered
Put the two side by side. If your best estimate of the probability gives fair odds of @1.67 and the market offers @1.95, the price is longer than fair — that is the shape of a value bet. If the market offers @1.50, you would be paying the margin and then some.
Honesty check: because markets are efficient and the margin is real, prices that are genuinely longer than fair are rare, and an apparent gap is often just estimation noise. Baseline publishes its own fair odds next to the market price and its settled record on the Transparency page so you can judge for yourself.
Frequently asked questions
What are fair odds?
Fair odds are 1 divided by the true probability — the break-even price. At a 40% chance, fair odds are @2.50. A bet placed at fair odds neither wins nor loses money on average.
What is the bookmaker's margin?
It is the amount by which a market's implied probabilities exceed 100%. Bookmakers shorten every price slightly so they profit regardless of the outcome; it is also called the vig, juice or overround.
How do I calculate the margin on a two-way market?
Add the implied probabilities of both sides (1/odds each). @1.90 and @1.90 give 52.6% + 52.6% = 105.2%, so the margin is about 5.2%.
Does a lower margin mean I should bet?
No. A thinner margin just means a sharper, more efficient price. It lowers the cost of betting but does not create an edge on its own.