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.