From area to cubic yards and tons
Gravel is sold by volume and by weight. Turn the area into a volume by multiplying by the depth, then divide by 27 to convert cubic feet into the cubic yards used for bulk delivery. Multiplying cubic yards by the material's density gives the weight in tons, which is how many suppliers price and deliver gravel.
Ordering the right amount
Round up and add a little extra for compaction and uneven ground so you do not run short partway through. For driveways, build the depth over a firm sub-base and compact in layers. Comparing the per-yard and per-ton price from your supplier helps you choose the cheaper option for the quantity you need.