Fill a kiddies swimming pool to the brim. Gently put the board in (being careful not to make waves) and push it below the surface. The amount of water that spills out onto the lawn will tell you the exact volume of the board. It works; I've done it many times.
Free R 298 63.5 9kg 137 litres 37cm Trim Box Freeride Fabien Vollenweider Slalom 6,0 - 8,3 Thermo Sandwich (CSC)