I tend to agree with a post by Stuthepirate (Posted 20/11/2012) for a general guide to calculate the volume of a board. He suggested length x width x height then remove 30% however I would remove 40%. I tested his theory on a couple of my boards where I new the volume of each and removing 40% seemed to be a little more accurate. Either way it's really just a guide and the only way to accurately measure capacity is displacement, submerge the board in water and measure how much the water rises.

Calculation: length x height x width divide by 1000 x .6 (for 40%) = approx. volume
Calculation: length x height x width divide by 1000 x .7 (for 30%) = approx. volume