We recently moved into a new home and completely redid the existing driveway.
We used a special geotextile grid which we then filled with gravel. The amounts used were specified on the manufacturer's website. If you choose to use a similar form of plastic ground reinforcement, then the amount of gravel required might vary, but in our case we used the following supplier and referred to their detailed DIY page: http://www.gridforce.co.uk/diy-driveways.html.
This details the amount of materials (including gravel) required for making a gravel driveway. However, to be honest we simply approximated the amount we needed (much like the fellow who responded to your question above), purchased what we believed to be a sufficient amount of gravel, and followed the instructions given by our grid supplier. On the webpage above, it does specify that you should not use a gravel smaller than 10mm in size. There is a video on the page that details it further.
The amount of gravel needed is dependant on the size of your driveway. Larger driveways will need more, and smaller drives will need less.
The depth of the gravel should be at least 2 times the length of the stones you are using, and no more than approximately 3 times the stone size.
Most gravel driveways use stones approximately 10-20mm in diameter. So if you choose 10mm stones, the minimum depth should be 20mm and the maximum depth should be 30mm. As such, for 10mm gravel stones it is a good idea to aim for a depth of around 25mm. Too few stones and you will see the ground underneath, but too many stones and your driveway will act more like the gravel pits along the outside of racetracks.
The next step is to calculate the size of your driveway. If your driveway is 100 square metres, you will need enough gravel to cover this space to a depth of 25mm (0.025m). To calculate this, times the area of your driveway by the required depth of the coverage, which in this case is 100 (area) x 0.025 (depth) = 2.5 cubic metres. So, if your driveway is 100 square metres, and you require a depth of 25mm, you will need 2.5 cubic metres of gravel.
1 cubic metre is approximately 1.92 tonnes, so for the job above you will require approximately 4.8 tonnes of gravel