How to Turn Odds Into Probability
One formula converts any price into a percentage. Learn how to strip out the bookmaker's margin and use the cleaned number to judge whether a bet is worth taking.
Odds are a price. Behind every decimal number sits a probability the bookmaker has assigned to an event, plus a mark-up that pays for the business. Learning to move between the two — odds to probability and back — is the single most useful skill in betting, and it takes one formula.
This guide covers the conversion, why the numbers add up to more than 100%, how to strip the margin out, and how to use the cleaned figure to judge whether a price is worth taking.
In short.
- Probability = 1 ÷ decimal odds. Odds of 2.50 imply 40%.
- Add up the implied probabilities of every outcome and you get more than 100% — the excess is the bookmaker's margin.
- To clean a price, divide its implied probability by the total. That gives the market's real view.
- A bet is worth taking only when your own probability is higher than the cleaned one, with room to spare.
- The same maths works for ₦100 stakes and ₦100,000 stakes — the percentages do not change.
Part of the series
This article builds on how to read betting odds. Once you can turn a price into a probability, the next step is comparing it with your own estimate — see sports betting for beginners for the full picture.
The formula
Divide one by the decimal odds. Multiply by 100 if you prefer percentages.
| Odds | Implied probability | Meaning in plain words |
|---|---|---|
| 1.20 | 83.3% | Expected to happen five times out of six |
| 1.50 | 66.7% | Two times out of three |
| 1.90 | 52.6% | Slightly better than a coin flip |
| 2.50 | 40.0% | Two times out of five |
| 3.40 | 29.4% | Roughly three times out of ten |
| 6.00 | 16.7% | One time out of six |
The reverse works the same way: if you think an event happens 35% of the time, the fair price is 1 ÷ 0.35 = 2.86. Anything above that is worth a look; anything below it costs you money on repetition.
Why the percentages add up to more than 100
Take a two-way market priced 1.90 and 1.90. Implied probabilities are 52.6% and 52.6%, which sums to 105.3%. The world does not contain 105% of anything — those extra 5.3 percentage points are the bookmaker's mark-up, built into both prices.
Example. A three-way football line: 2.30 for the home side, 3.30 for the draw, 3.10 for the away side. Implied probabilities are 43.5%, 30.3% and 32.3%. Total: 106.1%. The margin is 6.1% of the book, or about 5.7% of everything staked.
Cleaning the margin out
The simplest method divides each implied probability by the total. It is not perfect — bookmakers spread the mark-up unevenly across outcomes — but it is close enough to be useful.
| Outcome | Odds | Raw probability | Cleaned | Fair odds |
|---|---|---|---|---|
| Home | 2.30 | 43.5% | 41.0% | 2.44 |
| Draw | 3.30 | 30.3% | 28.6% | 3.50 |
| Away | 3.10 | 32.3% | 30.4% | 3.29 |
| Total | 100% | — | ||
The cleaned column is what the market actually believes. Compare your own estimate against that, never against the raw number — otherwise you are treating the mark-up as if it were information.
Turning this into a decision
Once you have a probability, the calculation is one line: expected return = your probability × odds − 1.
- You rate the home side at 45%, the price is 2.30: 0.45 × 2.30 − 1 = +3.5%. Worth considering.
- You rate them at 41%, the same price: 0.41 × 2.30 − 1 = −5.7%. Skip it.
- You rate them at 50%, the price is 1.90: 0.50 × 1.90 − 1 = −5.0%. Skip it, even though you fancy them.
Important. Notice the third line. Being right about who wins is not enough — the price decides whether being right pays. That is the whole reason this conversion matters.
How much edge is worth having
Small gaps are noise. Your estimate is rougher than the market's, so demand a buffer before staking.
| Gap between your probability and the cleaned one | What it usually means |
|---|---|
| 1–2 percentage points | Rounding and noise — no bet |
| 3–5 points | Marginal; only if your method has a track record |
| 6–10 points | Worth a bet at your normal stake |
| More than 15 points | Suspicious — check whether you have missed team news |
The last row matters. A huge gap usually means the market knows something you do not: a suspension, an injury, a rested first team. Before congratulating yourself, check the team sheet.
Money example
Example. You stake ₦500 per bet, 100 times, on prices averaging 1.90 where you win 55% of the time. Return: 55 × ₦950 = ₦52,250 against ₦50,000 staked — a profit of ₦2,250, or 4.5% of turnover. Win 52% instead and the same 100 bets return ₦49,400: a loss of ₦600. Three percentage points of accuracy is the difference between profit and loss.
FAQ
Does this work for fractional and American odds?
Yes, after converting them to decimal. Fractional 5/2 becomes 5 ÷ 2 + 1 = 3.50, so the implied probability is 1 ÷ 3.50 = 28.6%. American +150 becomes 150 ÷ 100 + 1 = 2.50 (40%), and −200 becomes 100 ÷ 200 + 1 = 1.50 (66.7%).
Why do two bookmakers show different probabilities for the same match?
Because they hold different opinions and different mark-ups. That is exactly why comparing prices is worth the minute it takes: the same bet at 2.30 instead of 2.15 changes your expected return by roughly seven percentage points.
Should I use the raw or the cleaned probability?
Cleaned, always, when judging whether a price is fair. The raw figure includes the mark-up, so comparing your estimate against it makes almost every bet look bad — including the good ones.
How accurate is the simple cleaning method?
Good enough for practical use, but be aware that bookmakers load more margin onto outsiders than on favourites. That means the cleaned price of a long shot is usually a little more generous than this method suggests, and the cleaned favourite a little less.
This article is for information only and is not an inducement to gamble. Betting involves the risk of losing money — never stake more than you can afford to lose. 18+. If gambling stops being entertainment, read our responsible gambling guide and seek help.
