How do you determine class width?

It is the number of 'units' between the lower and upper boundaries.

For example you have the weight of from 10 to 20 kg, then from 20 to 30, then from 30 to 40, then form 40 to 45 kg.

The class width is 10 for the first ones, but for the 40 to 45 group, the class width is 5.

Ok? :)