Concrete Calculator
Estimate cubic yards and premix bags (40, 60 or 80 lb) of concrete for slabs, footings, columns, tubes and stairs, with waste.
Concrete Needed
Volume, bags, and ordering by the yard
Concrete is ordered by volume, so the calculation is area times thickness with every measurement in the same unit. The usual slip is thickness: a 4-inch slab is 0.333 feet, not 4, and mixing inches with feet inflates the answer twelvefold. In the US, ready-mix is sold by the cubic yard, and one cubic yard is 27 cubic feet — not 3.
Add a waste allowance of 5-10%. Subgrade is never perfectly level, forms bulge slightly, and spillage is inevitable, so ordering exactly the calculated volume reliably leaves you short. Running out mid-pour is much worse than over-ordering: the joint between a first and second batch is acold joint and is a permanent structural weakness.
Bags versus ready-mix is a threshold decision. An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes roughly 45 of them — practical for post holes and small pads, and genuinely unwise much beyond that. Past about half a cubic yard, ready-mix is usually cheaper, faster and more consistent. Note that concrete cures chemically rather than drying, reaching most of its strength in about 28 days, and keeping it damp early produces a stronger result.
Frequently asked questions
How to calculate concrete volume?
Volume (Cubic Feet) = Length (ft) × Width (ft) × (Thickness (in) / 12). Divide by 27 to get Cubic Yards.
How much concrete do I need?
Multiply length by width by thickness in consistent units to get volume, then convert: one cubic yard is 27 cubic feet, and one cubic metre is roughly 1.31 cubic yards. Order 5-10% extra for spillage, uneven subgrade and over-excavation - running short mid-pour creates a cold joint that permanently weakens the slab.
How many bags of concrete are in a cubic yard?
About 45 80-lb bags, 60 60-lb bags or 90 40-lb bags. Bagged mix is economical for post holes and small pads, but beyond roughly half a cubic yard - about 20-25 bags - ready-mix delivery is usually cheaper and certainly less work, and it gives a consistent mix throughout.
How thick should a concrete slab be?
Four inches is typical for patios, walkways and residential floors; five to six inches for driveways carrying cars, and more with reinforcement for heavy vehicles. Thickness interacts with subgrade preparation - a well-compacted base under four inches outperforms six inches poured over soft ground.