Measure the M2 of an object. First measure two of its dimensions, such as length and width, and then multiply the two together. For example, if a box has a length of 10 and a width of 10, its M2 equals 100 square meters.
Measure the third dimension of the object. This is the object's height or depth. Measure it in meters in order to keep your units consistent throughout your calculation. Using the previous example, assume a height of 10 meters.
Multiply M2 as you measured it in Step 1 by the height or depth you measured in Step 2. This new product is your M3. In the example, take 100 (the Step 2 M2 measurement) and multiply it by 10 (the height as measured in Step 2) to calculate 1,000 cubic meters as the M3 measurement.