The universal way of mentioning a box's dimensions, or on how to measure boxes, is in this order:  

3 measurements:

Length x Width x Depth
 

               

Length - The box's base longest side. 

Width - The box's base shortest side. 

Height (Depth) - From the base to the top,

Remember:
If you need the dimensions to pack some fragile goods or valuables, be sure to add some 2 to 3 inches more to account for the packing material you will need. Better be a couple of inches bigger than too small and chance to damage the goods inside.