Ordering concrete is one of those jobs where guessing wrong is expensive. Order too little and you’re scrambling mid-pour. Order too much and you’re paying for concrete you can’t use. The math is simpler than it looks once you break it into steps.
Step 1: Measure length, width and thickness
Measure the length and width of your project in feet. Then decide how thick it will be. Common thicknesses are:
- 4 inches for patios, walkways and shed floors
- 5 to 6 inches for driveways and areas that carry vehicles
Convert the thickness to feet by dividing inches by 12. Four inches is 0.333 feet.
Step 2: Find the volume
Multiply length × width × thickness (in feet) to get cubic feet. Concrete is sold by the cubic yard, and there are 27 cubic feet in a cubic yard, so divide by 27.
Example: a 10 × 10 foot patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 27 and you get about 1.23 cubic yards.
Step 3: Add extra for waste
Ground is never perfectly level and some concrete always gets left in the wheelbarrow. Add 5 to 10 percent. For the patio above, 1.23 yards plus 10 percent is about 1.35 cubic yards.
Our Concrete Calculator handles all three steps for you.
Bags or a truck?
For small jobs, bagged concrete mix works well. As a rough guide, an 80-pound bag makes about 0.6 cubic feet and a 60-pound bag makes about 0.45 cubic feet. That 10 × 10 patio would take about 81 of the 60-pound bags, which is a lot of mixing by hand.
For anything over about one cubic yard, ready-mix delivered by truck is often easier and can be cheaper. Ask local suppliers about their minimum order and any short-load fee for small deliveries.
Other things to plan for
- Base material. Most slabs sit on a compacted gravel base, often around 4 inches deep.
- Forms. You’ll need lumber and stakes to hold the edges.
- Reinforcement. Wire mesh or rebar helps control cracking, especially for driveways.
- Weather. Avoid pouring when freezing temperatures are expected unless you take cold-weather precautions.
- Permits and utilities. Call 811 before digging, and check with your city about permit rules for larger slabs and driveways.
Curing time
Concrete keeps getting stronger for weeks. You can usually walk on a new slab after a day or two, but wait about a week before driving on it. Keeping it damp for the first several days helps it cure evenly and reduces cracking.
Measure carefully, add a little extra, and you’ll have the right amount on pour day.
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