How Much Concrete Do I Need?

What is the formula for concrete?

Volume in cubic feet = length × width × thickness, with all three in feet. Divide by 27 to get cubic yards, because a cubic yard is 3 × 3 × 3 = 27 cubic feet. Ready-mix concrete in the US is usually quoted by the cubic yard, while bags are usually compared by the cubic feet they yield.

The most common mistake is leaving the thickness in inches. Divide inches by 12 first. The concrete calculator lets you pick a unit for each dimension and does this conversion for you.

How much concrete do you need for a slab?

Multiply the slab's area in square feet by its thickness in feet, then divide by 27. Worked example: a 12 by 12 foot slab, 4 inches thick. The area is 144 square feet, and 144 × 0.333 gives 48 cubic feet, which is 48 ÷ 27 = 1.78 cubic yards. If you need help with the area of an irregular space, see how to calculate square footage and split it into rectangles first.

The table shows several sizes with no extra added. The cubic feet, cubic yards and 60-lb and 80-lb counts come from our calculator. It doesn't report 40-lb bags, so that column is our own arithmetic using a yield of 0.30 cubic feet per bag. The 4-inch thickness is only an example; take the thickness from your plans or local building code.

How much concrete do you need for footings or columns?

For a rectangular footing, use length × width × depth. For a round column or post hole, use π × (diameter ÷ 2)² × depth, then multiply by the number of holes. Measure the actual hole if you can, since a hole wider than planned takes more concrete than the formula predicts.

Worked example, round footings: six holes, each 12 inches across and 4 feet deep. Each holds 3.1416 × 0.5² × 4 = 3.14 cubic feet, so all six hold 18.85 cubic feet, or 0.70 cubic yards. The Hole, Column, or Round Footings form on the concrete calculator gives 32 80-lb bags or 42 60-lb bags. At 0.30 cubic feet per bag, our own arithmetic gives 63 40-lb bags.

Worked example, trench footing: 20 feet long, 12 inches wide and 18 inches deep. That is 20 × 1 × 1.5 = 30 cubic feet, or 1.11 cubic yards: 50 80-lb bags or 67 60-lb bags. These sizes only illustrate the math. Footing dimensions for a structure depend on the load, the soil and your local code, so confirm them with your building department or an engineer. For the geometry behind other shapes, see our guide to volume formulas.

How many bags of concrete are in a cubic yard?

It takes 45 80-lb bags, 60 60-lb bags or 90 40-lb bags to make one cubic yard, based on yields of 0.60, 0.45 and 0.30 cubic feet. Divide 27 cubic feet by the yield of the bag. Each size works out to 3,600 pounds of dry mix per yard.

The yields come from the data sheet for Quikrete Concrete Mix (No. 1101), which lists them as approximate. Our calculator uses the same 0.60 and 0.45 figures for its 80-lb and 60-lb counts. It doesn't offer a 40-lb count, so the 40-lb figures here are our arithmetic from Quikrete's stated yield. Other products and brands can yield different amounts, so check the bag you are buying.

How much extra concrete should you order?

Add 5 to 10 percent to the calculated volume. This is a common rule of thumb, not a figure from a standard or manufacturer, and our calculator's note uses the same range. It covers spillage, forms that bow and ground that isn't flat. Running short mid-pour is a bigger problem than having a little left over, because fresh concrete placed against concrete that has started to set can leave a weak seam called a cold joint.

Thickness has more effect than people expect. On a slab designed for 4 inches, an average depth of 4.5 inches is 12.5 percent more concrete, which is already above a 10 percent allowance. Measure the depth in several places before you order, and increase the allowance if the ground is uneven.

Should you use bags or ready-mix concrete?

Bags suit small pours, and ready-mix becomes worth pricing as the volume grows. There is no fixed cutoff, but about one cubic yard is a reasonable point to start comparing. Consider the labor: the 56 80-lb bags for a 10 by 10 foot slab weigh 56 × 80 = 4,480 pounds, all of which has to be moved and mixed before the first batch begins to set.

Ask local suppliers for a delivered price, including any fee for a small load, and compare it with the bag total. Quikrete's data sheet describes its Concrete Mix for pours 2 inches thick or more, so check the product's limits if your pour is thin.

How much gravel do you need under a concrete slab?

Use the same footprint and the depth of the base your plans call for. A 4-inch layer under a 10 by 10 foot slab is 33.33 cubic feet, or 1.23 cubic yards, the same volume as the slab above it. The gravel calculator converts that to weight: about 1.66 short tons at its default density of 1,600 kilograms per cubic meter. Check your supplier's density, since it varies by material.

Compacted gravel settles, and the calculator suggests adding 5 to 10 percent for a compacted base, also a rule of thumb. The same length, width and depth approach also applies to mulch.