Concrete Calculator

How It Works

The calculator finds the volume of your shape in cubic feet, then divides by 27 to get cubic yards. Inches are divided by 12 first, so every measurement is in feet. For bags, it adds the extra you picked, divides by the yield of one bag, and rounds up, since you cannot buy part of a bag.

A cubic yard is 27 cubic feet, per NIST Handbook 44. A footing here means a long strip, so enter its total length in feet. This tool works out the volume of a footing or column you have already sized. It does not size them, so take those dimensions from your plans or your local building department.

See Methods and Sources

Worked Example

Here is the math for a patio slab that is 16 feet long, 12 feet wide, and 4 inches thick, using 80 lb bags and 10 percent extra.

1. 16 ft x 12 ft = 192 sq ft
2. 192 sq ft x 4 in / 12 = 64 cu ft
3. 64 cu ft / 27 = 2.37 cu yd
4. 64 cu ft x 1.10 = 70.4 cu ft, or 2.61 cu yd with extra
5. 70.4 cu ft / 0.60 cu ft = 117.3 bags, rounded up to 118 bags

Four tubes 12 inches across and 4 feet tall come to 12.57 cubic feet, or 0.47 cubic yards. With 10 percent extra, that is 13.82 cubic feet, or 24 bags of 80 lb mix.

Bag Yields and Coverage

A bag lists how much mixed concrete it makes, called its yield. These are approximate figures from the Quikrete Concrete Mix data sheet, and the coverage column is the yield spread 4 inches thick.

For more bag counts by slab size, see How Many Bags of Concrete Do I Need?

Cubic Yards by Slab Size and Thickness

Use this table for a quick ready-mix estimate. A 400 square foot slab at 4 inches thick takes 4.94 cubic yards, with nothing added for waste.

Typical Slab Thickness

Thickness has a large effect on your total, so settle it before you buy. These are Quikrete’s figures for common jobs. They are not code values, and your plans or local rules may call for something else.

Quikrete says sections thinner than about 3 1/2 inches crack too easily, though it calls 2 inches adequate for smaller sections, such as those made with its Walk Maker. It designs its Concrete Mix for pours 2 inches thick or more. With 2 by 4 forms, its patio instructions allow 3 1/2 inches for the concrete, so enter 3.5 in the calculator.

Round Tubes by Diameter

For round tubes, volume depends on the square of the diameter, so a 12 inch tube holds 2.25 times as much as an 8 inch tube of the same height. The bag counts below have nothing added for waste.

The 6 to 12 inch bag counts match the capacity table on the Quikrete QUIK-TUBE data sheet.

Why You Add Extra

The volume from your plan is the space to fill, and real jobs take more. The National Ready Mixed Concrete Association lists waste and spillage, over-excavation, and spreading forms among the usual reasons. It says to order 4 to 10 percent more than the volume worked out from plan dimensions, and Sakrete says to add 10 percent to every estimate.

Small errors add up. The association notes that a slab just 1/8 inch thicker than planned at 4 inches would leave you 3 percent short, so measure your forms in several spots and check the base before you order.

Bags or a Ready-Mix Truck?

Bags suit small jobs, and the weight is the thing to plan for. The patio above takes 118 bags of 80 lb mix, which is 9,440 pounds of dry mix to move and mix. Sakrete says a pallet of bags is about a cubic yard as a rough rule, and that it takes 45 bags of 80 lb mix to reach a full yard.

Ready mix is sold by the cubic yard, and truck mixers hold 8 to 12 cubic yards, according to the National Ready Mixed Concrete Association. It says to place your order well in advance, give the producer the address and the nature of the job, and avoid small clean-up loads. Ask your local supplier about any minimum order or delivery fee before you pick a method.

Frequently Asked Questions

These cover the questions that come up most often.