I occasionally use Betfair.
Here's a quite rundown of how decimal odds are converted to fractional:
Fractional Odds to Implied Probability
Say you have the odds of 9/4 or 4/5 what would be the implied probability of
To convert take the denominator and divide it by the numerator plus the
denominator. Then multiplying by 100. So:
9/4 = 4/(4+9) = 4/13 = .3077 = 30.8%
4/5 = 5/(4+5) = 5/9 = .5556 = 55.56%
Note the book percentage is the sum of these implied probabilities. In
this case it's 106.32%.
Implied Probability to Decimal Odds
The decimal odds are calculated by dividing 1 by the implied probability (as
So for 9/4 (an implied probability of .3077) the decimal odds would be;
1/.3077 = 3.25
and for 5/9 (an implied probability of .5556)
1/.5556 = 1.80