Concrete Calculator: Cubic Yards & Bags for Slabs

How Many Cubic Yards of Concrete Do I Need?

For a flat slab it comes down to square footage and thickness. At 4 inches thick, one cubic yard covers 81 square feet, so you can divide the slab's area by 81 for a quick answer. At 6 inches, one yard covers 54 square feet. The table below runs the numbers for some common slab sizes.

Cubic yards of concrete for example slab sizes
Slab size	4 in thick	6 in thick
10 ft × 10 ft	1.23 yd³	1.85 yd³
12 ft × 12 ft	1.78 yd³	2.67 yd³
20 ft × 20 ft	4.94 yd³	7.41 yd³
24 ft × 24 ft	7.11 yd³	10.67 yd³

Those are exact volumes with nothing added. Your own slab will not match them, so use the concrete calculator above for the real measurements. For a worked walkthrough of a single slab, see our slab estimating guide.

How to Use This Concrete Calculator

Enter the length, width and thickness of the pour. It returns cubic yards for a ready mix order and the number of premix bags if you are mixing it yourself.

Choose the shape that matches the pour: Rectangle for slabs, footings and walls, Circle for piers and round pads, Annulus (Ring) for a hollow round section.
Enter the sizes and pick a unit for each. Feet, inches, yards, millimeters, centimeters and meters can be mixed freely.
Enter the thickness. For a column or pier, type its height here.
Open the optional section to pick a premix bag size, add a waste percentage of your own, or enter a price per cubic yard or per bag.
Press Calculate, then read the cubic yards for a ready mix order, or the bag count for the bag size you will use.

Change the thickness and press Calculate again to see what an extra inch does to the total.

Concrete Volume in Cubic Yards: The Formula

If you prefer to calculate concrete cubic yards by hand, work in feet and finish with one division by 27.

Slab or footing: cubic feet = length × width × thickness (ft)

Round pier: cubic feet = π × (diameter ÷ 2)² × height (ft)

Cubic yards = cubic feet ÷ 27

Bags = cubic feet ÷ bag yield (ft³), rounded up

Inches become feet by dividing by 12, so a 4 inch slab is 0.3333 ft thick. The calculator keeps full precision and rounds only the number it shows. For the method in more detail, read the cubic yard calculation guide. The cubic yard calculator uses the same sum for anything that is not concrete.

Concrete Slab Calculator: Slabs, Footings and Piers

The same sum applies to every pour, but each type has its own habits. Treat the thicknesses below as starting points rather than specifications.

Slabs and patios

For a slab, the calculator only needs length, width and thickness. These are common starting points, not a design:

Common slab thickness by use
Use	Typical thickness	Concrete per 100 sq ft
Patios, walkways and sidewalks	4 in	1.23 yd³
Shed and garden building floors	4 in	1.23 yd³
Driveways	5 to 6 in	1.54 to 1.85 yd³
Hot tub or heavy equipment pads (check the manufacturer)	6 in	1.85 yd³

Local building codes, soil and the loads a slab carries set the real figure, so confirm the thickness and any rebar or wire mesh with your building department or contractor before you order.

Footings and thickened edges

A footing is a long, narrow slab: enter its length, width and depth as a Rectangle. If a slab has a thickened edge, calculate the main slab and the edge as separate pieces and add the cubic yards together.

Round piers and columns

Volume (ft³) = Area × D ÷ 12

Cubic yards = Volume ÷ 27

Ø₁ = outer diameter (ft) · Ø₂ = inner diameter (ft) · D = thickness (in)

Example: Ø₁ = 5 ft, Ø₂ = 3 ft, D = 4 in. Area = 12.57 ft². Volume = 12.57 × 4 ÷ 12 = 4.19 ft³, which is 0.16 yd³.

Concrete Bags or Ready Mix?

Bag labels list volume in cubic feet while ready mix suppliers quote in cubic yards. The calculator converts between the two. Typical yields look like this, but the figure printed on your bag always wins.

Typical premix bag yields and bags needed per cubic yard
Bag size	Typical yield	Bags per cubic yard
40 lb	0.3 ft³	90
60 lb	0.45 ft³	60
80 lb	0.6 ft³	45

These yields match the manufacturer figures for a standard premix. For example, the QUIKRETE Concrete Mix data sheet lists about 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags. High strength and fast setting mixes can yield a little differently, so check your own bag.

Bags suit small jobs such as a step, a post footing or a small pad. Mixing dozens of them by hand is slow, heavy work, so many people switch to a delivered load once a pour passes a yard or so. Suppliers may have minimum loads or short load charges, so ask for a quote before you decide. The calculator above gives you the number to ask for.

Concrete Calculator Examples

Patio slab: 10 ft × 10 ft, 4 in thick

Four inches is 0.3333 ft, so the volume is 100 ft² × 0.3333 ft.

100 ft² × 0.3333 ft = 33.33 ft³ ÷ 27 = 1.23 yd³

About 56 bags of 80 lb premix at 0.60 ft³ each.

Footing: 30 ft long, 16 in wide, 8 in deep

Mixed units are fine here. The calculator converts 16 in to 1.3333 ft and 8 in to 0.6667 ft before multiplying.

40 ft² × 0.6667 ft = 26.67 ft³ ÷ 27 = 0.99 yd³

About 45 bags of 80 lb premix at 0.60 ft³ each.

Round pier: 12 in across, 4 ft tall

Choose Circle and type the height into the thickness field. The area is π × 0.5² ≈ 0.785 ft².

0.7854 ft² × 4 ft = 3.14 ft³ ÷ 27 = 0.12 yd³

About 6 bags of 80 lb premix at 0.60 ft³ each.

Garage pad with a 10% cushion: 12 ft × 12 ft, 4 in thick

The 10% is your own allowance on top of the exact volume. Leave the field blank and nothing is added.

144 ft² × 0.3333 ft = 48 ft³ ÷ 27 = 1.78 yd³

With 10% allowance: 1.96 cubic yards. About 88 bags of 80 lb premix at 0.60 ft³ each.

Should You Order Extra Concrete?

Many contractors add about five to ten percent. The subgrade is rarely perfectly level, forms bulge slightly and some mix is lost in handling. The bigger risk is running short halfway through a pour, because a second batch will not blend with the first. The waste field starts at 0%, so the result is the exact volume until you add an allowance of your own.

Tips for an Accurate Concrete Volume

Measure the depth of the excavation in several places, not just one corner.
Keep forms square and check the diagonals before you pour so the dimensions match the numbers.
Write the unit beside each measurement so inches and feet never get mixed up.
Add a gravel or sand base separately. The Gravel Calculator and Sand Calculator use the same measurements.
If you only know the sides of an odd shape, the Square Feet Calculator finds the square footage to type into Known Area.

Concrete Calculator FAQ

How many bags of concrete make a cubic yard?

At typical yields, a cubic yard takes about 90 bags of 40 lb, 60 bags of 60 lb or 45 bags of 80 lb mix. Yields differ between brands, so check the volume printed on your bag.

How many cubic yards of concrete do I need for a 10×10 slab?

A 10 ft by 10 ft slab that is 4 inches thick needs about 1.23 cubic yards. At 6 inches thick it needs about 1.85 cubic yards.

How do I convert cubic feet of concrete to cubic yards?

Divide the cubic feet by 27. For example, 54 cubic feet of concrete is 2 cubic yards. The calculator does the conversion for you, along with the bag count.

How much does a cubic yard of concrete weigh?

Ordinary concrete weighs roughly 4,000 pounds per cubic yard, which is about two tons. The exact figure depends on the mix, so treat it as a planning number.

Does the result include rebar, gravel or the base?

No. It calculates only the concrete volume you enter. A gravel base is a separate material, and the Gravel Calculator can work that out with the same measurements.

Can I enter concrete measurements in metric units?

Yes. Choose meters, centimeters or millimeters for any field and the volume is converted exactly. Cubic meters are shown next to cubic yards.

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