The calculation is Pi * radius^2 * length.

Where, Pi=3.141, radius=half the diameter and length (if you can't work this out, stop now).

Eg.. Log is 50cm diameter and 6mtrs long.
3.141*(0.25*0.25)*6
=1.178m3

This is really rough, but you can make it better by averaging the radius by measuring both ends of the log - if you want to do the same for a tree (standing), look for a volume formula for the species you have using a good search engine (the formulas vary a lot between species).