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Fractional or Decimal?


It is a common problem that many people are unable to understand what exactly betting odds mean. When you back a football team to win at 9/4 from a high street bookmaker, what exactly are you paying for. Is 9/4 good value?

Ideally whenever you place a bet you should be aware what it is "telling you". If you think an event has a better than 50/50 chance then any odds over 3/1 represents "value". So, how do you convert from one set of odds to another and into implied probability?

Converting fractional odds to implied probability

In the dim and distant past people used to use fractional odds. Fractional odds are quoted, not suprisingly in fractions. A football (soccer) match offering odds of 4/5 for the Home win, 9/4 for the draw and 4/1 for the away win would translate to a total book of 106%

However what exactly is 4/5 or 9/4 and how does it translate to 106%?

In order to convert fractional odds you simply divided the first fraction by the sum of the first and last. For example.

  • Odds of 9/4 = 4/(9+4) = 4/13 = .3077 = 30.8% - The implied probability you are being offered is 30.8%
  • Odds of 4/5 = 5/(4+5) = 5/9 = .5556 = 55.56%
  • Odds of 4/1 = 1/(1+4) = 1/5 = .20 = 20.00%
  • The sum of all those odds = .3077+.5556+.20 = 106.32%

Converting fractional odds to decimal odds

It is becoming more and more common for people to quote decimal odds - such as seen on the exchanges. For example a evens chance (1/1) would be 2.00 decimal odds. If you have fractional odds and wish to convert them into decimal odds then simply divide one by the result of the conversion from fractional to implied probability. An example: -

Fractional odds are 9/4

Stage one - Convert to implied probability

Odds of 9/4 = 4/(9+4) = 4/13 = .3077

Stage two - Convert to decimal odds

1/.3077 = 3.25

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