How Many Bags of Concrete Do I Need?

Concrete Bag Yields

A bag lists how much mixed concrete it makes, called its yield. These figures come from the Quikrete Concrete Mix data sheet, and they are approximate.

Bags by Slab Size, 4 Inches Thick

These counts are rounded up for each slab, with nothing added for waste.

Bags by Slab Size, 6 Inches Thick

A 6 inch slab takes one and a half times the concrete of a 4 inch slab.

The Formula

Find the volume in cubic feet, then divide by the yield of your bag. Round up, since you cannot buy part of a bag.

Length and width are in feet, and thickness is in inches.

Worked Example

Here is the math for a 10 by 10 foot slab, 4 inches thick, using 80 lb bags at 0.60 cubic feet each.

1. 10 ft x 10 ft = 100 sq ft
2. 100 sq ft x 4 in / 12 = 33.33 cu ft
3. 33.33 cu ft / 0.60 cu ft = 55.6, rounded up to 56 bags
4. 33.33 cu ft x 1.10 = 36.67 cu ft with 10 percent extra
5. 36.67 cu ft / 0.60 cu ft = 61.1, rounded up to 62 bags

Plan the Job, Not Just the Bags

At 62 bags, that example is 4,960 pounds of dry mix. Plan how you will move and mix it before you buy. The Quikrete data sheet warns against partial depth fills, which could lead to cold joints, so have enough help to work continuously. For a job this size, it is also worth pricing ready-mix delivery.

A Quick Estimate by Square Foot

For a 4 inch slab, Sakrete gives a shortcut that already includes waste. Multiply the square footage by the number of bags per square foot.

For a 100 square foot slab, that is 60 bags of 80 lb mix. That is a little under the 62 bags in the example above, since the shortcut rounds its waste allowance, so do not add another 10 percent on top. Sakrete says it only works for 4 inch slabs. Use the full formula for any other thickness.

Round Footings and Post Holes

For a round hole, find the area with pi times the radius squared, then multiply by the depth. This example is a hole 12 inches across and 3 feet deep, with nothing in it.

1. 12 in / 2 = 6 in, or 0.5 ft radius
2. 3.14 x 0.5 ft x 0.5 ft = 0.785 sq ft
3. 0.785 sq ft x 3 ft = 2.36 cu ft
4. 2.36 cu ft / 0.60 cu ft = 3.9, rounded up to 4 bags

That count has no extra for waste. A post in the hole takes up some of the space, so check it against your own hole size.

Do Not Forget the Base

Bags of concrete are only part of the job. For a 4 inch slab, the Quikrete how-to says to excavate about 7 inches, leaving 3 inches for a gravel base and 4 inches for concrete.

For the same 10 by 10 foot slab, a 3 inch gravel base is 25 cubic feet, or 0.93 cubic yards.

Mixing Water by Bag Size

The data sheet gives a starting and a maximum expected amount of water for each bag. Start with the lower figure, and add water only as needed. The sheet describes a stiff, moldable consistency as the goal for hand mixing, and says not to leave standing puddles of water.

Safety When Mixing

Quikrete warns that its Concrete Mix causes severe skin burns and serious eye damage. Its data sheet says to wear impervious gloves, such as nitrile, along with a mask and eye protection.

What Changes the Number

Thickness has a large effect, so measure your forms before you buy. Quikrete says its Concrete Mix is designed for pours 2 inches thick or more.

The ground is rarely flat. Quikrete notes that its calculator yields are approximate, and do not allow for uneven substrate or waste. Sakrete says it is a good idea to add 10 percent to every estimate.

The figures here are for Quikrete Concrete Mix, so check the yield printed on your own bag.

Frequently Asked Questions

These cover the questions that come up most often.