Concrete Calculator: Cubic Yards, Bags and Cost

Worked examples

10 ft by 10 ft slab, 4 in thick

100 sq ft × 0.333 ft = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards (about 0.94 cubic meters). That is roughly 56 of the 80 lb bags.

A driveway section

A 20 ft × 10 ft driveway at 5 in (0.417 ft) thick is 200 × 0.417 = 83.3 cubic feet, or 3.09 cubic yards. Order ready-mix rather than bags.

A round footing (column)

A 12 in (1 ft) diameter footing 8 ft tall is π × 0.5² × 8 = 6.28 cubic feet, about 0.23 cubic yards, or roughly 11 of the 80 lb bags.

Adding waste to the order

A job that calculates to 1.23 cubic yards, plus a 10% waste allowance, means ordering about 1.35 cubic yards so you are not left short.

How the concrete calculator works

Concrete is ordered and priced by volume, so the whole job is a volume calculation followed by a couple of unit conversions. You measure the space you are filling, work out its volume in cubic feet, then convert to cubic yards (for ready-mix) or to a bag count (for small pours).

The calculator handles two common shapes: a rectangular slab or footing, and a round column (post footing or pier). Enter your dimensions in feet, inches, yards, or meters and it converts everything to feet before doing the math, so you can mix units freely.

Volume of a slab or footing

A slab, footing, or pad is a rectangular box, so its volume is a single multiplication:

Volume (cubic feet) = length × width × thickness

The catch is that slabs are thin, so the thickness is usually in inches while length and width are in feet. Convert the thickness to feet first: divide inches by 12. A 4 inch slab is 4 ÷ 12 = 0.333 ft thick; a 6 inch slab is 0.5 ft.

Converting to cubic yards

Ready-mix concrete is sold by the cubic yard, and one cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft). To go from your cubic-feet figure to the number you actually order:

Cubic yards = cubic feet ÷ 27

If you prefer metric, cubic meters = cubic feet × 0.0283168 (or cubic yards × 0.7646). The result card shows all three so you can match whatever your supplier quotes in.

Volume of a round column

A round column, pier, or tube footing is a cylinder. Its volume uses pi and the radius (half the diameter):

Volume (cubic feet) = π × (diameter ÷ 2)² × height

For a 12 inch (1 ft) diameter footing that is 8 ft tall: π × 0.5² × 8 = π × 0.25 × 8 ≈ 6.28 cubic feet, or about 0.23 cubic yards. Column diameters are often given in inches, so let the unit selector convert for you.

Estimating bags, and why bags are only for small jobs

Pre-mixed bags list a rough yield of set concrete:

80 lb bag ≈ 0.60 cubic feet
60 lb bag ≈ 0.45 cubic feet
40 lb bag ≈ 0.30 cubic feet

Divide your volume in cubic feet by the bag yield and round up. Because a cubic yard is 27 cubic feet, a single yard is roughly 27 ÷ 0.6 = 45 of the 80 lb bags. That is why bags make sense only for small pours (post holes, a small pad, repairs). For anything larger, ordering ready-mix by the truck is cheaper and much less mixing.

Adding a waste allowance

Real pours never match the math exactly: the subgrade is uneven, forms bow slightly, and some concrete is lost to spillage. Add a 5-10% waste allowance so you are not left short mid-pour, when there is no time to mix more. The calculator adds your waste percentage (default 10%) to the recommended order and to the bag counts, while still showing the exact base volume.

A fully worked example

Say you are pouring a 10 ft × 10 ft slab at 4 inches thick:

Thickness in feet: 4 ÷ 12 = 0.333 ft.
Volume: 10 × 10 × 0.333 = 33.3 cubic feet.
Cubic yards: 33.3 ÷ 27 = 1.23 cubic yards.
Cubic meters: 33.3 × 0.0283168 ≈ 0.94 cubic meters.
Bags (80 lb, no waste): 33.3 ÷ 0.6 ≈ 56 bags, which already shows why you would order ready-mix instead.
With a 10% waste allowance: order about 1.23 × 1.10 = 1.35 cubic yards.

A note on accuracy

These figures are estimates for planning and budgeting. Measure carefully, round up when ordering, and confirm the exact quantity, mix, and pricing with your concrete supplier before you pour.

Frequently asked questions

How much concrete do I need?

Multiply length × width × thickness (all in feet) to get the volume in cubic feet, then divide by 27 to get cubic yards, which is how ready-mix is sold. A 10 ft × 10 ft slab that is 4 inches (0.333 ft) thick is 100 × 0.333 = 33.3 cubic feet, or 33.3 ÷ 27 = 1.23 cubic yards. This calculator does the conversion and adds bag counts for you.

How many 80 lb bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes roughly 27 ÷ 0.6 = 45 of the 80 lb bags. A 60 lb bag yields about 0.45 cubic feet (60 bags per yard) and a 40 lb bag about 0.3 cubic feet (90 bags per yard).

Should I order extra concrete?

Yes. Add a 5-10% waste allowance for spillage, uneven subgrade, and over-excavation. Concrete sets fast, so running short mid-pour is far worse than having a little extra. The calculator has a waste field (default 10%) that it applies to the recommended order.

How do I convert cubic yards to cubic meters?

Multiply cubic yards by 0.7646, or work from cubic feet: 1 cubic foot = 0.0283168 cubic meters. So 33.3 cubic feet is about 0.94 cubic meters, and 1.23 cubic yards is roughly the same figure. The calculator shows cubic yards, cubic feet, and cubic meters at once.

Should I use bagged concrete or ready-mix?

Bagged concrete is practical only for small jobs (a few post holes, a small pad, repairs) because the bag count climbs fast: even a single cubic yard is around 45 of the 80 lb bags. For anything over about half a cubic yard, ordering ready-mix delivered by the truck is usually cheaper and far less work. These figures are estimates for planning; confirm quantities and pricing with your supplier.