Three dimensional objects include depth. For example, if you have a bowl, then from the top of the bowl to the bottom of the bowl is the depth of the bowl. If you have several objects that have a depth, then you can calculate the average depth. The average depth looks at how deep all the objects are when considered together. The formula for average is the sum of all the depths divided by the number of depths you have.
Measure all of your depths. For example, you measure five bowls with depths of 5 inches, 9 inches, 3 inches, 7 inches and 11 inches.
Add together the depths. In the above example, 5+9+3+7+11 = 35.
Divide the sum of the depths by the number of items you measured. In the example, 35 divided by 5 equals an average depth of 7 inches.