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A concrete driveway calculator sizes two things at once: the concrete for the driveway slab, and the compacted gravel sub-base underneath it. Enter the length, width, slab thickness and base depth, and it returns cubic yards of ready-mix plus the volume of gravel to order.
Driveways carry vehicle weight, so they are poured thicker than a patio — 5 inches is typical for cars, 6 inches if a heavy vehicle or RV will use it. A 4-inch compacted gravel base is standard and is what stops the slab from cracking as the ground moves.
The slab volume is length × width × thickness; the sub-base is the same area × the base depth, reported in cubic yards or metres. We add 10% waste to the concrete. Because a poured driveway is heavy work against the clock, this is tagged as a get-quotes job as well as a DIY estimate.
A 20 ft × 16 ft driveway at 5 inches needs 320 sq ft × 0.42 ft = about 133 cubic feet, or 4.9 cubic yards of concrete, plus roughly 4 cubic yards of gravel for a 4-inch base.