I made the assumption that your avg thickness wouldn't change. Say your avg thickness is 2 in. Using the square and the rectangle in the previous post, the volumes would be 50 cubic inches and 32 cubic inches respectively.
It might be just as quick to measure the width of the board 10 times at equal intervals to get an average width, then multiply that by your avg thickness and then multiply by the length of the board. The more measurements you take the better the approximation will be.
Using 42's formula, my hpsb's volume is about 36.7 L or about 1.29 cu ft.
The Firewire volume calculator here: http://www.firewiresurfboards.com/qu...olume_calc.php says I need a minimum volume of 41 L (max 45) or 1.45 cu ft (max 1.59 cu ft). For the record, I entered 6'6x20x2.75 as my board dims (and that might even be slightly exaggerated), "intermediate/advanced" skill level (I'm way past my prime), average fitness at 49 years old, and 190lbs.
Just for the sake of discussion... My board is not undervolumed. It floats me fine and catches waves as easily, if not easier, than my 7'0 East Coast Gun. I'll admit it does not paddle like the 7'0... or like even like my fish (volume 38.4 L or 1.35 cu ft). My point to all this is volume is only one indicator of how well a board "fits" you and meets your performance needs. Rocker, foil, template, rail volume... and a bunch of other design elements, but these are the most important to me... all have to be factored in. Consider a wide nosed board with a lot of entry rocker... that board will push a lot of water and be more difficult to paddle. Pull in the nose template, or lower the entry curve and it makes a big difference. Consider a board that's too thin in the nose, lacking balanced volume in relation to the location of thick point and wide point (center of volume).
I think of overall board volume as most important when paddling, and not necessarily the most important factor in wave catching or overall performance. A balanced foil/rocker/template design matters at least as much. Calculating volume is relatively new in the surfboard industry, and it's getting a lot of attention... and that's good. It's an important element of design. But it's not a "magic number" by any stretch, and is sometimes over emphasized.