Concrete Calculator
Work out how much concrete you need for a slab, footing, round column or set of stairs — in cubic yards, cubic feet and bags of every common size, with a waste allowance.
How much concrete do I need?
Concrete is ordered by the cubic yard and sold in bags by the cubic foot, so every estimate starts with volume. Multiply length × width × thickness to get cubic feet, then divide by 27 to get cubic yards. The only trap is the thickness: it is given in inches while everything else is in feet, so it has to be divided by 12 first.
cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
A 20 × 10 ft slab poured 4 inches thick works out to 20 × 10 × 0.333 = 66.7 cubic feet, which is 2.47 cubic yards. Add the 10% waste allowance the calculator uses by default and you are ordering 2.72 cubic yards — or 123 bags of 80 lb mix if you are mixing on site.
Bags or ready-mix?
The crossover comes sooner than most people expect. A single cubic yard takes 45 bags of 80 lb mix (27 ÷ 0.60), which is 3,600 lb of material to carry, open and mix by hand. Below about half a yard, bags win on convenience and cost; above one yard, a ready-mix truck is almost always cheaper and much faster — and it delivers one continuous pour instead of a series of batches.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 90 lb | 0.675 ft³ | 40 |
Those yields are the manufacturer's published figures, not rules of thumb — they come from the QUIKRETE Concrete Mix data sheet. Mixes from other makers use the same nominal yields, but it is worth a glance at the bag if you are ordering a large quantity.
Columns and stairs
A round footing or column is a cylinder: π × radius² × height. A 12-inch tube 4 ft deep holds 3.14 cubic feet, so four of them come to 12.6 cubic feet — under half a yard, which is squarely bag territory.
Stairs are the shape people most often get wrong, because each step sits on the ones below it. The calculator treats step n as a block n rises tall, so a four-step flight at 11-inch treads and 7-inch risers, 4 ft wide, adds up to 2.1 + 4.3 + 6.4 + 8.6 = 21.4 cubic feet rather than four identical steps.
Working out the area first? The square footage calculator handles irregular shapes. For the base underneath, see the gravel calculator, and for a paved surface instead of a poured one the asphalt calculator covers tonnage. More in the home and construction hub.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet (10 × 10 × 0.333). At 0.60 cubic feet per 80 lb bag that is 56 bags, or about 61 once you add a 10% waste allowance. In 60 lb bags it is 75 bags, and in 40 lb bags 112. At this size many people still mix on site, but it is close to the point where ready-mix is easier.
How many 80 lb bags of concrete make a cubic yard?
It takes 45 bags. A cubic yard is 27 cubic feet and an 80 lb bag yields 0.60 cubic feet, so 27 ÷ 0.60 = 45. The same yard needs 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb. Once you are past roughly one cubic yard, ordering ready-mix usually costs less than the bags and far less effort.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways and shed bases. Driveways and anything carrying vehicle weight are normally poured 5 to 6 inches, and the thickness matters more than the mix strength for cracking. Thickness drives the volume directly, so changing 4 inches to 6 inches adds 50% to the concrete you need.
How much does a cubic yard of concrete weigh?
About 3,780 lb, or just under 2 tons. That comes from a unit weight of roughly 140 lb per cubic foot (ASTM C138) × 27 cubic feet per yard. It is worth knowing before you plan how to move it — a single cubic yard is well beyond what a wheelbarrow run can handle comfortably.
How much extra concrete should I order?
Add 5 to 10% for spillage, uneven subgrade and over-excavation. The calculator defaults to 10%, which is the safer figure for a hand-dug slab or footing. Running short mid-pour creates a cold joint, so the extra is cheap insurance compared with the alternative.