Figure driveway 14' x 20' and your estimate will be good. Here is why: The driveway is 16' at the top and 12' at the bottom. Through the center the width averages 14'.
It can be upsetting to end up with any extra concrete. You are having to pay for this concrete after all. However, realize you are doing your job a service by ordering enough concrete - which means you are going to have a little concrete left.
After you have chosen a Ready Mix Supplier, have a representative come to your site to give his/her opinion on the quantity required. Compare the figure with what you came up with. Discuss any variances with the supplier.
Estimating Concrete Steps Steps seem tricky to calculate but are not. If there are three steps leading up to a porch: Use the slab calculator to figure the concrete needed for the porch surface. Use the footing calculator to calculate the sides of the porch and the steps Here is an example: This porch has 9 sq. feet of porch surface, so enter in the slab calculator 4" thickness by 3' width by 3' length. This totals .11 cubic yards. The porch also has 9 linear feet of 6" step. So enter in the footing calculator, 6" depth by 12" width (always figure steps with a 12" width) by 9' length. This totals .17 cubic yards. The total concrete needed for the 3' by 3' porch will be .28 cubic yards. (.11 + .17 cubic yards = .28 cubic yards) Repeat this for added layers of steps.