How Much Concrete Do I Need?

Concrete is unforgiving: too little mid-pour leaves a weak seam, and too much is money down the chute. A quick volume calculation gets the order right. Here is how to work it out by hand, with a worked example and the details that matter on site.

The short version: turn the thickness into feet, multiply by the length and width for cubic feet, then convert to cubic yards or bags. Order a little extra, because the ground is never as flat as the plan.

Concrete is a volume, not an area

Paint and tile cover a surface, but concrete fills a space, so you need three measurements: length, width, and thickness. The catch is that thickness is given in inches while length and width are in feet, so you have to match the units first.

Divide the thickness in inches by 12 to put it in feet. A 4-inch slab is 4 / 12, which is 0.333 feet. Now all three measurements share the same unit and you can multiply them.

A worked example, step by step

Say you are pouring a shed base that is 8 feet by 6 feet, 4 inches thick. Work the volume in order.

1. Thickness in feet: 4 / 12 = 0.333.
2. Volume in cubic feet: 8 × 6 × 0.333 = 16.
3. Volume in cubic yards: 16 / 27 = 0.59.

So the base needs about 16 cubic feet, or 0.59 cubic yards, of concrete. The cubic yard figure is the one to quote a ready-mix supplier, since that is how delivered concrete is sold.

Why divide by 27

A cubic yard is a cube three feet on each side. Three times three times three is 27, so one cubic yard holds 27 cubic feet. That is why you divide cubic feet by 27 to reach cubic yards.

The unit matters because it changes who you call. Anything under about a cubic yard is usually a bag job, while larger pours go to a ready-mix truck that quotes by the yard.

Turning the volume into bags

For a small pour, premixed bags are simpler than a truck. Each bag size yields a set volume of mixed concrete: an 80 pound bag makes about 0.6 cubic feet, a 60 pound bag about 0.45, and a 40 pound bag about 0.30.

Divide the volume by the yield. For the 16 cubic foot base in 80 pound bags: 16 / 0.6 = 26.7, which rounds up to 27 bags. In 60 pound bags it would be 16 / 0.45, about 36 bags, since each holds less.

Always order a little extra

The calculated figure is the bare minimum, never the order. Forms bow out, the subgrade dips, and some concrete is always lost to spillage. Adding about 10 percent keeps a pour from coming up short.

Running out mid-pour is the worst case. Fresh concrete against a set edge forms a cold joint, a weak line that can crack later. The cost of a few extra bags is nothing against the cost of a failed slab.

Pick the right thickness

Four inches handles foot traffic and light loads: a patio, a walkway, a shed base, or an air conditioner pad. A driveway that carries vehicles usually steps up to five or six inches to spread the weight.

Thickness drives the volume directly, so a thicker slab needs more concrete in proportion. A solid, compacted base under the slab matters just as much, since soft ground lets concrete flex and crack no matter how much you pour.

Common mistakes to avoid

• Leaving thickness in inches. Convert it to feet first, or the volume comes out far too large.
• Treating concrete as an area. It is a volume, so length and width are not enough; include the thickness.
• Ordering the exact figure. Add about 10 percent for an uneven base, form flex, and spillage.
• Mixing up the bag yields. An 80 pound bag yields more than a 60, so use the right figure for the bag you buy.

Frequently asked questions

How do I find the volume? Convert the thickness to feet, then multiply length, width, and thickness. The result is cubic feet; divide by 27 for cubic yards.

How many cubic feet are in a cubic yard? Twenty-seven. A cubic yard is a cube three feet on each side, and 3 × 3 × 3 is 27.

How much does a bag of concrete yield? About 0.6 cubic feet for an 80 pound bag, 0.45 for a 60 pound bag, and 0.30 for a 40 pound bag. Divide your volume by the yield.

Should I order extra? Yes. Add about 10 percent for an uneven base and spillage. Coming up short during a pour leaves a weak cold joint.

Do I need to do this by hand? No. The method is good to understand, but a tool is faster. Use the one below to enter the slab size and thickness and see the yards and bags in a second.