This is a simple method for specifying class brreaks. First find the minimum and maximum values in your distribution. Let's say that those values are 27 and 99. Then you find the range, which is simply the difference, in this case (99-27)=72. If we wish to have six classes, then we divide the range by six to get the class width: 72/6=12. This width is then added to the minimum value to get the first class: 27+12=39. The first class goes from 27 to 39. Then add 12 to 39 to get the upper limit on the second class (51). Continue for each of the six classes desired.
Note: It is wise to avoid overlapping classes such as 12 to 24, and 24 to 36. In this case the class in which 24 belongs is ambiguous. If we extend the number of places in our measurement, this ambiguity is resolved. For example, 12.0 to 24.0 and 24.1 to 36.0 would work well.