The correct and standard order to measure a box is length × width × height. Changing the order can result in incorrect box sizes. How are boxes measured for packaging? Boxes are usually measured from the inside. Understanding box dimensions is essential whether you're involved in shipping, moving, or custom product packaging. In this ultimate guide, I'll walk. This guide covers standard box sizes across every major industry, explains the difference between internal and external dimensions, breaks down carrier-specific size requirements for USPS, UPS, and FedEx, and gives you practical tips to find the right fit for any product. I've watched a full run get scrapped because the carton was a quarter inch short. Five quiet minutes with a tape measure would've saved a week of fire-drills.