Cubic inches are the expression of the volume of a three-dimensional object. For typical square or rectangular areas, multiple the length of each side in inches, according to Math.com.
Measure each side
A three-dimensional object has three sides -- often referred to as the object's length, width and depth. Use a ruler to measure each side in inches.
Multiply the three lengths
For a square, all three sides are the same. If a rectangular area has sides of 7 inches, 4 inches and 10 inches, multiply these together. The answer is 280.
Record the result
The answer of 280 is recorded as 280 cubic inches.