Concrete Calculator
Work out how much concrete you need for a slab, footing or round column. Get the volume in cubic yards and cubic meters, plus how many 40, 60 or 80 lb bags, with a waste allowance built in.
Concrete needed
How to calculate concrete volume
Concrete is ordered by volume, so every estimate starts by working out how much space the pour will fill. For a rectangular slab or footing you multiply length by width by thickness. The one thing to watch is units: length and width are usually in feet while thickness is in inches, so you convert the thickness to feet first by dividing by 12. The formula is volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12. The calculator above does this for you and also handles round columns and circular slabs, which use the area of a circle (π × radius²) instead of length times width.
Concrete in the United States is sold by the cubic yard, and one cubic yard is 27 cubic feet. To convert, divide your cubic-foot figure by 27. For a metric order, one cubic foot is about 0.0283 cubic meters. A typical 4-inch residential slab uses roughly one cubic yard for every 80 square feet of surface, which is a handy sanity check against the exact number above.
A worked example
Say you are pouring a 10 ft by 12 ft patio slab at 4 inches thick. The volume is 10 × 12 × (4 ÷ 12) = 40 cubic feet, which is 40 ÷ 27 = 1.48 cubic yards. Adding a 10 percent waste allowance brings it to about 1.63 cubic yards, so you would order 1.75 cubic yards of ready-mix (suppliers sell in quarter-yard steps). If you were bagging it instead, 40 cubic feet ÷ 0.60 ft³ per 80 lb bag is about 67 bags, which is exactly the point where a ready-mix truck becomes the easier choice.
Bags or ready-mix?
Pre-mixed bags are convenient for small jobs like a few footings, fence-post bases or a small pad. As a rule of thumb, bags make sense up to about one cubic yard; past that the bag count, the mixing time and the risk of a cold joint between batches tip the balance toward truck-delivered ready-mix. The calculator shows bag counts for the three common sizes so you can compare: an 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³. Always round up, since a partial bag still has to be bought whole.
Why you should add a waste allowance
Ordering the exact calculated volume almost always leaves you short. Subgrade is rarely perfectly level, excavation tends to run slightly deep, some concrete sticks to the mixer or wheelbarrow, and bagged yields vary. A 5 to 10 percent allowance covers this, and running a little over is far cheaper than stopping a pour to mix more, which creates a weak seam. The waste field above defaults to 10 percent; lower it for tightly formed work and raise it for rough or sloping ground.
Planning the rest of the job
Most slabs sit on a compacted base of crushed stone or sand, so once you know the concrete volume you will usually need to work out the sub-base too. Use the gravel calculator for a crushed-stone base and the sand calculator for a bedding or leveling layer under pavers. If the project is a garden feature rather than a structural pour, the topsoil calculator and mulch calculator cover the landscaping side. Every tool here runs in your browser, keeps your numbers on your own device, and needs no account.