fair odds = 1 ÷ estimated probability. At 50%, you get fair odds of 2.00. Don't mix them up with margin-free odds, which come from a bookmaker's prices. Fair odds come from an estimate made elsewhere, and they can be wrong: they're worth exactly what the estimate is worth.
What are fair odds?
Fair odds are the odds that match an estimated probability exactly, with no margin added. They turn an estimate into a price, in the unit the market speaks.
That's what makes them useful for comparing. A probability of 50% can't be set directly against odds of 2.20, because the units differ. Convert it to fair odds of 2.00 and the estimate sits next to the displayed price, so the gap is easy to read.
What is the formula?
fair odds = 1 ÷ estimated probability, with the probability written as a fraction.
At 50% you work out 1 ÷ 0.50 = 2.00. At 40% you get 2.50. At 25%, 4.00. It's the exact inverse of the implied probability calculation. Only the source of the number you convert changes.
Convert an estimated probability into fair odds
Enter your estimated probability as a percentage. The result is shown in all three formats.
Your own estimate, which must come from a source other than the odds themselves.
Result
- Decimal odds
- 2.00
- Fractional odds
- 1/1
- American odds
- +100
Warning:
Fair odds follow from the probability you enter. Their quality depends entirely on the quality of that estimate.
Displayed, margin-free and fair odds: three things not to mix up
These three sets of odds look alike on paper. They don't answer the same questions.
| Odd | Where it comes from | What it answers | What it does not say |
|---|---|---|---|
| Displayed odd | The bookmaker | At what price this bet is offered | The share of margin it contains |
| Margin-free odd | Market prices, margin removed | How the market splits the chances | Whether the market is right |
| Model fair odd | An independent estimate | What that estimate would be worth as a price | Whether the estimate is correct |
In this table, the last column matters more than the others. Knowing what a number doesn't tell you protects you better than knowing what it does.
Margin-free odds come from an operator's prices, so they can't be any fairer than the market they're taken from. Model fair odds come from somewhere else. That makes them useful and risky at once: they can beat the market or land well away from it. Margin removal is covered in margin-free odds.
A real case: Borussia Dortmund v Eintracht Frankfurt (Bundesliga), on a completed match. The chart and the first table show the OddScore margin-free odds, our market-derived reference price. The second table applies the proportional method to the raw odds of a reference operator, unnamed. The two estimates don't match exactly.
From 1 May to the start, the margin-free odds on “Borussia Dortmund win” moved from 1.60 to 1.78 (+11.40%), a margin-free probability of 62.62% then 56.21%. Lowest value seen: 1.57; highest: 1.79. This chart describes a past market movement: it says nothing about the match result and is not betting advice.
| Outcome | First tracked odds | Last odds before the start | Change | Margin-free probability (first, then last) |
|---|---|---|---|---|
| Borussia Dortmund win | 1.60 | 1.78 | +11.40% | 62.62% then 56.21% |
| Draw | 4.90 | 4.55 | −7.12% | 20.41% then 21.97% |
| Eintracht Frankfurt win | 5.89 | 4.58 | −22.17% | 16.99% then 21.82% |
Raw odds from a reference operator, not named, before the start: the implied probabilities add up to 103.42%, a margin of about 3.42%. Dividing each probability by that total gives the margin-free probability (proportional method, slightly different from the OddScore consensus in the chart).
| Outcome | Raw odds | Implied probability | Margin-free probability |
|---|---|---|---|
| Borussia Dortmund win | 1.70 | 58.82% | 56.88% |
| Draw | 4.44 | 22.52% | 21.78% |
| Eintracht Frankfurt win | 4.53 | 22.08% | 21.34% |
| Total | 103.42% | 100.00% |
Source: OddScore. Margin-free odds computed from the odds of the international reference bookmakers, snapshots from 1 May to 8 May 2026, match already played. Data frozen on 11 October 2026.
Fair odds and fair price: a difference in vocabulary
"Fair odds" is the generic calculation. At OddScore, "fair price" is one specific, documented estimate.
OddScore's fair price is a reference price estimated from the odds of several bookmakers. The margin is removed from each one before aggregation, and the price is recalculated at every new reading. It comes from the market and isn't an independent sporting forecast: it tells you where the consensus of available prices sits at a given moment.
We never present it as "the true odds". That would assume exact odds exist and that someone knows them. A market consensus stays an estimate, even when it aggregates dozens of operators.
Can fair odds be wrong?
Yes. The division is exact, but the probability you feed it isn't necessarily.
Here are three common ways to end up with wrong fair odds:
- an overfitted estimate, calibrated on too few matches or on an unrepresentative history;
- a badly calibrated estimate, which says 60% where the outcome happens 45% of the time;
- a stale estimate, made before information the market has already absorbed (team sheet, a withdrawal, playing conditions).
In all three cases the fair odds look as credible as good ones. Nothing in the number flags the problem, so fair odds should never be read on their own. Of the three, I'd say miscalibration is the one that fools people most easily, because the division itself stays spotless. If you then compare them with a market price to spot an opportunity, be very careful. It's covered in value bet.
The formula corrects nothing. Fair odds inherit the quality, or the flaws, of the estimate behind them.
What OddScore does
OddScore estimates a reference price from several bookmakers and tracks it over time. The platform compares that estimate with the available prices to make gaps readable. It draws no betting recommendation from them.
That price moves. Calculated three days before a match or one hour before, it doesn't tell the same story. Odds movements cover that evolution, and closing line value looks at its final version.
Fair odds show what an estimate would be worth as a price. They let you estimate a gap with the market. They don't tell you whether the estimate is any good.
Put it into practice
Apply this guide to real odds.
See a market reading
A real match with its odds, movements and 0-100 signal.
Open the demoMargin calculator
Measure a bookmaker's margin on any market.
Calculate the marginDig into the market
Odds movements are only part of the story. Here are the next topics to read.
Odds and probability: the complete guide
The whole journey, from the displayed odds to the closing line.
Read the guideMargin-free odds: the proportional method
The other reference price, taken from market prices.
Remove the marginValue bet: what the gap measures
What you can do with fair odds, and what you can't conclude from them.
Understand valueImplied probability: working it out from odds
The reverse calculation, applied to the price an operator displays.
Convert oddsFrequently asked questions
What are fair odds?
They're the odds that match an estimated probability exactly, with no margin added. You get them by dividing 1 by that probability.
How do you calculate fair odds?
Divide 1 by the estimated probability, written as a fraction. A probability of 50% gives 2.00, 40% gives 2.50 and 25% gives 4.00.
What is the difference between fair odds and margin-free odds?
Margin-free odds are worked out from an operator's prices once its overround has been removed. Fair odds are worked out from a probability estimated separately, by a model, a consensus or an analysis.
What is the difference between fair odds and fair price?
Fair odds is the generic calculation, and it works for any estimated probability. OddScore's fair price is one specific estimate, built by the platform's method from several bookmakers.
Can fair odds be wrong?
Yes. The formula is exact, but it corrects nothing. If the estimated probability is poor, the fair odds that come out of it are just as poor.
Do fair odds tell you whether to bet?
No. They give you something to compare with the displayed price. That comparison belongs to theoretical value, which depends on how good the estimate is.
Sources & methodology
This page draws on the standard formulas for converting between probabilities and odds, on research into how probability estimates are calibrated, on OddScore's price-estimation method and on official British resources (Gambling Commission, BeGambleAware).
- Convert the estimated probability into decimal odds (1 ÷ probability).
- Express those odds in fractional and American format, without touching the probability they represent.
- Keep fair odds (from an estimate) clearly apart from margin-free odds (from market prices).