Whenever a corrugated box is measured for sizing purposes, it is industry standard to go by the internal dimensions of the box. For example, a 12 x 10 x 8" box means, the box has an inside length of 12, inside width of 10, and inside height of 8 inches. If you measure the box from the outside, it will be a fraction of an inch to an inch bigger, to accommodate the width flaps and length flaps on top and bottom.
If you are measuring a box to calculate its shipping costs, only then do you use the outside dimensions. For example, if you are shipping a box through ups and on their screen, you can select the outside dimensions. Also, if you want to see the pallet configuration of a particular box to see how many fit on a pallet, you can use outside dimensions.