How to Measure the Inside of a Box: A Simple Guide to Calculating Volume. If you're in need of a quick calculation for volume, this guide will teach you how to measure the inside of a box. You know that when you multiply length times width times height, you get the volume in cubic inches. But if a box is a different shape than a cube, the values in each dimension won't be equal. To find the volume in cubic inches, start by finding the dimensions of your box in inches and convert them to feet. If you have a 12"x6"x3" box, your dimensions are 18", 15",...