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.