Mark

I am a quantity surveyor its simple enough to do, you need to work out the centre line of the trench to give you a perimiter and then times it by width and depth of the trench to give you the conc. for the foundations.

So

How wide is the trench?
What are the measurements to the extremes of the trench?

Do you want the conc. required for the slab as well? Need the depth of the concrete as I can work the rest out from the other measurements.

Jake