In general, all boxes and specifically corrugated boxes are measured from the inside. We always recommend measuring dimensions from inside because the corrugated board thickness can vary. For example, a box made of B-flute will not have the same outer dimensions as a box made from E-flute. With the box open toward you, here is how to get the correct measurements for the inside of the box:
The length is measured from the inside edge on the left to the inside edge of the right side of the box.
The width is measured from the front inside edge to the back inside edge of the box.
The height is measured from the top inside edge to the bottom inside edge of the box.
In addition, it is always helpful to measure and provide the dimensions of the product that is to be enclosed in the box in order to assure a good fit