How do you get class interval?

Answer 1

You don't GET class intervals. The person analysing the data chooses what class interval gives the best summary of the data. Ideally you are looking for class intervals that have

• a reasonable number of observations in each class (5 or more),
• a reasonable number of classes,
• (sometimes) observations within the same class are as similar to one another as possible while observations between classes are as different as possible (maximum discrimination),
• a nice round numbers as their midpoints.

The first three are important for some kinds of statistical analyses. The last is to make calculations simpler and interpretation of the results easier.

Answer 2

By doing same things you like to do