I'm sure this has been asked before but how do the windsurfing board company's work out the volume of their boards ?

In Starboard's case, very badly. For many seasons, their 97 model was actually closer to 105 litres

Theres a lot of company's out there that state a lot lower than they actually are. Then when you get on them and sail them, they seem that little bit easier than you were expecting and hold a little larger sail than you were expecting too.

Some seem a lot floatier than they are due to width which displaces the water more and width gives stability too so seem a lot bigger.

Madge, Decrepit is spot on. Diplacement is directly related to volume not width and is really only relevant when you are off the plane. Volume distribution is also important, placing the volume where you stand and not in the nose has a big impact on the "perceived" volume of the board. An "S" shaped board will feel floatier than a board with a more contant volume distribution ( until you stand too far forward)

Back to the original question, I think you will find most if not all 'Brands" ( as opposed to custom builders) use CAD and will know exactly what the volume of the board is. Whether they chose to state the volume correctly is another subject.

^^ minus the weight of the board itself :)

I agree they tend to understate, as everyone likes an early-planer...