What are fair odds in sports betting?
Fair odds are the price that would pay out with no margin for anyone — the odds that exactly reflect an outcome’s true probability. If an event happens 55% of the time, its fair price is the one that breaks even at a 55% win rate: −122 in American terms. Bet better than fair and you have positive expected value; bet worse and you are paying someone else’s margin.
The conversion is simple: fair decimal odds are 1 ÷ probability. A 55% event is 1 ÷ 0.55 = 1.818 in decimal, which converts to −122 American. A 25% event is 4.00 decimal, or +300. Probability in, price out — no vig attached.
Where fair probabilities come from
Two sources, in order of trust. The first is the market itself: devig a liquid two-way market and the cleaned probabilities are the market’s fair estimate. The second is a model — your own estimate built from data, like the pitcher-strikeout engine behind the OddsGuy board. Market fair is the benchmark; model fair is how you find disagreements with it.
Either way, the workflow is identical. Establish a fair probability, convert it to fair odds, then compare against the best price any book is actually dealing. The market price must be better than your fair price — longer odds, higher payout — or there is no bet.
The gap is the edge
Fair odds turn betting into a comparison you can see. Fair −122 against a market −110 means the book is paying you as if the event were 52.4% likely when it is really 55% — a 2.6-point gap worth about +5.0% per dollar staked. Fair −122 against a market −130 means the book is charging you for 56.5% — walk away. Every surface on this site that shows a fair price next to a live price is drawing exactly this picture.
Worked example — from probability to price to decision
- Model probability
- 55.0%
- Fair decimal odds
- 1 ÷ 0.55 = 1.818
- Fair American odds
- −122
- Market price / implied
- −110 → 52.4%
- Edge
- 55.0% − 52.4% = 2.6pp
- EV per unit staked
- +5.0%
The market is pricing a 55% event at 52.4%. Buying it at −110 when fair is −122 is worth +5.0% on every dollar — that gap is the whole trade.
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Related terms
Frequently asked questions
How do fair odds differ from the odds sportsbooks post?▾
Posted odds include the bookmaker’s margin; fair odds do not. A market that devigs to a 50/50 probability has fair odds of +100 on both sides, but no book will ever deal you that — you will see −110/−110 instead. The difference between the two is the vig you pay.
How do I compute fair odds from a probability?▾
Fair decimal odds are 1 divided by the probability: 0.55 becomes 1.818. To American: for probabilities above 50%, the fair price is −(p ÷ (1 − p)) × 100, so 0.55 becomes −122. Below 50%, it is ((1 − p) ÷ p) × 100, so 0.25 becomes +300.
Should I ever bet at worse than fair odds?▾
Not if profit is the goal. A bet below fair price is −EV by definition — you are being paid less than the risk deserves. Parlays and entertainment bets aside, every serious staking decision is a search for prices at or better than fair.
Where can I see fair odds live?▾
The OddsGuy scanner computes a fair probability for every MLB prop on the board — devigged from the market and checked against the model — and displays it next to the best available price. When the best price beats fair, the row is +EV and ranked accordingly.
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