Concrete Calculator
Find out exactly how much concrete to buy for a slab, footing, column, or round pad — in cubic yards, bags of mix, or ready-mix to order — plus weight and a quick cost estimate.
A flat, solid pad of concrete — a patio, shed floor, walkway, or driveway. Measure how long and how wide it is, and how thick to pour it (4 inches is typical).
Bag & ready-mix counts include your 10% waste buffer.
Estimate only. These figures help you plan how much to buy — they are not engineering or structural advice. Actual needs vary with site conditions, waste, and installation method, so order a little extra and confirm final quantities with your supplier or contractor before you buy or start the job.
Popular concrete projects
Jump to a ready-made answer for a common size.
How the concrete calculation works
Every concrete estimate is the same two steps: find the volume, then convert it to whatever you're buying — cubic yards of ready-mix or bags of dry mix.
Length (ft) × Width (ft) × Thickness (ft) = cubic feet. Thickness in inches ÷ 12 gives feet. Divide cubic feet by 27 for cubic yards.
π × radius² × thickness. Radius is half the diameter (ft); thickness in feet.
π × radius² × height. Tube diameter is usually given in inches — convert to feet (÷ 12) before squaring.
Bags = volume (with waste) ÷ bag yield. An 80 lb bag yields ≈ 0.60 cu ft; 60 lb ≈ 0.45 cu ft; 40 lb ≈ 0.30 cu ft. Concrete weighs ≈ 150 lb/cu ft (≈ 2 tons per cubic yard).
Sources & assumptions
Constants are cross-checked against manufacturer spec sheets and standard references, so you can trust the numbers — and verify them:
Frequently asked questions
How much concrete do I need for a 10x12 patio at 4 inches?
A 10 ft × 12 ft slab at 4 inches thick is 120 sq ft × 0.333 ft = 40 cubic feet, or about 1.48 cubic yards. With a 10% waste buffer that's roughly 1.63 cubic yards — about 74 bags of 80 lb mix, though at this size ready-mix delivery is usually cheaper.
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag of concrete mix yields about 0.60 cubic feet. A cubic yard is 27 cubic feet, so you need about 45 bags of 80 lb mix per cubic yard (27 ÷ 0.60).
When should I use bags vs. ready-mix concrete?
Bagged mix is practical for small jobs — under about 1 cubic yard. Past that, mixing dozens of bags by hand is slow and often more expensive than ordering ready-mix delivered by the cubic yard.
How much does concrete weigh?
Standard concrete weighs about 150 pounds per cubic foot, so one cubic yard weighs roughly 4,050 pounds — about 2 tons.
Why add a waste percentage?
Real pours lose material to spillage, uneven sub-base, and over-excavation. A 5–10% buffer keeps you from coming up short mid-pour, which is far costlier than a little leftover.