THERE IS A FORMULA AND NOBODY HAS MENTIONED IT YET.

Not even me in my first post!

As long as you work in metric this works for all builds near as dammit.

Measure your boat as a rectangle, forget the pointy bits - thats why I said near as dammit, you can get it perfect if you want to triangulate as well but its not worth it.

So bow to stern the ship is 800mm long and lets say 150mm wide. Now you need to know how high the waterline is, lets say 60mm.

So you cube 800 x 150 x 60 = 7200000, then divide by a 1000 = 7200. The 7200 figure is the weight of water in millilitres that it would take to fill the ship with water to float it to the waterline.

Now because of the way the metric system works, they said 1 square metre of water weighs 1 metric tonne, so you can extrapolate that the measurement above 7200 millilitres must weigh 7.2 kilo's, simple

This works for all measurements, I just used simple figures for length and width here.