Probability that a particular event will occur is the ratio of the number of ways the event can occur to the total number of possible outcomes.
Odds are the number of ways an event can occur divided by the number of ways an event cannot occur.

An illustrative example is given below:

Q. Find the probability of randomly selecting the letter p in the word "Mississippi."

There are 11 letters in the word Mississippi. 2 letters are p's and 9 letters are not p's.

Hence probability would be: 2/11 = 0.18

The formula used is:
Probability = Odds / (1 + Odds)

Odds are entered as a proportion into this equation; however, if a % is used, this is divided by 100 to calculate a proportion.