What is probability ?
Probability is a quantitative measure of the likelihood of a given event. If we are certain that an event will occur it is assigned a probability of 100%. If we are certain that an event will not happen it is assigned a probability of 0%. Other events which are neither certain to occur nor certain not to occur are assigned probabilities between 0% and 100%. If an event has a probability of 50% then it is as likely to happen as not to happen. An event with a probability of 10% is not likely to happen while an event with a probability of 90% is likely to happen.
Example 1 Suppose you take a coin from your pocket and toss it in the air. Assuming that it is a fair coin, there are two possible outcomes, heads or tails, and each outcome is equally possible. Therefore, the probability of it being heads is 50% and the probability of it being tales is also 50%.
Example 2 Using a full deck of cards and drawing one card at random the probability of the card being a spade is 1 in 4, there are four suits in a deck of which one is spades. To express this as a percentage you divide the required outcome by the number of possible outcomes and multiply by 100%. So the probability of a spade being drawn at random is (1 / 4) X 100% = 25%
How do odds relate to probability ?
First of all let me explain that there are two ways of expressing odds, fractional odds and decimal (sometimes known as digital) odds.
Fractional odds look like this, 2/1 4/1 11/2
Decimal odds look like this, 3 5 6.5
Odds are simply a way of converting the probability of an event happening into the amount of money you will receive back if you bet on it.
Using the card example above, there is a 25% chance that a spade will be picked at random.
In fractional odds, there are 3 chances you will choose wrongly and 1 chance you will choose correctly, therefore the odds are 3/1. If you put £10 on at 3/1 and are correct you will win 3 X £10 so you will now have £40 as your original £10 is returned for winning bets. To convert a %age to fractional odds divide 100% by the probability %age and subtract 1. In this example,
100% / 25% = 4
4 – 1 = 3
The odds are 3/1
In decimal odds things are much simpler to work out. The odds are simply 100% divided by the probability %age. Using the same card example,
100% / 25% = 4
The odds are 4
Therefore, if you put £10 on at odds of 4 you will now have 4 X £10 = £40.
Below is a table of odds and percentages.
%age

Fractional Odds

Decimal Odds

%age

Fractional Odds

Decimal Odds


99.01

1/100

1.01

50

1/1

2

98.04

1/50

1.02

47.62

11/10

2.1

97.06

1/33

1.03

44.44

5/4

2.25

96.15

1/25

1.04

42.11

11/8

2.38

95.24

1/20

1.05

40

6/4

2.5

94.12

1/16

1.06

38.10

13/8

2.63

92.31

1/12

1.08

36.36

7/4

2.75

90.91

1/10

1.1

34.78

15/8

2.88

89.89

1/8

1.12

33.33

2/1

3

87.50

1/7

1.14

30.77

9/4

3.25

85.71

1/6

1.16

28.67

5/2

3.5

84.61

2/11

1.18

26.67

11/4

3.75

83.33

1/5

1.2

25

3/1

4

81.82

2/9

1.22

23.08

10/3

4.33

80

1/4

1.25

22.22

7/2

4.5

77.78

2/7

1.29

20

4/1

5

76.92

3/10

1.3

18.18

9/2

5.5

75

1/3

1.33

16.67

5/1

6

73.33

4/11

1.36

15.39

11/2

6.5

71.43

2/5

1.4

14.29

6/1

7

69.23

4/9

1.44

12.5

7/1

8

66.67

1/2

1.5

11.11

8/1

9

65.22

8/15

1.53

9.09

10/1

11

63.64

4/7

1.57

7.69

12/1

13

61.90

8/13

1.62

5.88

16/1

17

60

4/6

1.67

4.76

20/1

21

57.89

8/11

1.73

3.85

25/1

26

55.56

4/5

1.8

2.94

33/1

34

52.38

10/11

1.91

1.96

50/1

51

50

1/1

2

0.99

100/1

101

Please note that for betting purposes there are no odds for 0% or 100%. This is because if an event has a probability of 100% nobody would take your bet because they are guaranteed to lose. Likewise, if an event has a probability of 0% then you are guaranteed to lose.
There is a quick and simple online tool for converting moneyline, fractional and decimal odds at