Dear oakie,
There is no specific 'formula' to calculate , how much water a body will displace.

By Arch. principle for a floating body
weight of the body = weight of water displaced.

if you know the weight of the body, you can calculate how much volume it displaces due to its own weight

volume displaced x density of fluid= weight

if you take a box of length=l, breadth=b and depth=d for example, it will displace a volume of l x b x d , when it is fully immersed.
If for unit depth it displaces l x b,  you can see that it immerses (volume of water displaced)/(lxb)

If it is a complex body, you have to do some integration to findout the volume it displaces at each value of d.

Hope this will answer your question