"How much concrete do I need?" is the first question of every slab, patio and footing project — and getting it wrong is expensive in both directions. This concrete calculator turns your slab dimensions into cubic yards, pre-mixed bags and cost, with a live preview that reshapes as you type.
How to calculate concrete for a slab
Multiply length × width × thickness (all in the same unit) for the volume, then convert: 1 cubic yard = 27 cubic feet. Worked example: a 10 × 12 ft patio at 4 inches thick is 10 × 12 × 0.333 = 40 ft³ ≈ 1.48 cubic yards. Enter dimensions in feet, inches, meters or centimeters — the calculator converts everything for you.
Bags or ready-mix truck?
Bagged concrete yields approximately: 80 lb bag ≈ 0.60 ft³ (about 45 bags per yard), 60 lb ≈ 0.45 ft³ (60/yard), 40 lb ≈ 0.30 ft³ (90/yard). For that 1.48-yard patio you'd be mixing ~67 eighty-pound bags — hard work. As a rule of thumb, beyond about 1 cubic yard, a ready-mix delivery is usually cheaper and far easier than bags.
Always add a waste allowance
Add 5–10% extra for spillage, uneven subgrade and over-excavation — the waste slider handles this. Running short mid-pour is the costliest mistake in concrete work: a second batch creates a cold joint and a weaker, uglier slab.
Slab thickness guidelines
Common practice: 4 inches for patios, walkways and shed bases; 4–6 inches for driveways (6 where vehicles are heavy); footings per local code. Thickness drives volume linearly, so going from 4 to 6 inches costs 50% more concrete. Planning the base layer too? Estimate it with the gravel calculator, and get the slab's area with the square footage calculator.
Estimates only — confirm quantities with your supplier before ordering.