How Much Concrete Do I Need?

A patio is a flat box. A fence post hole is a cylinder with a post standing in it. A set of steps is a staircase of boxes that get taller as they go back. All three are poured from the same truck, but each one needs a slightly different calculation. This guide shows how much concrete you need for the projects homeowners take on most often, with the formula for each shape and a worked example you can adapt to your own measurements.

The Short Answer

Work out the volume of each part of the project in cubic feet, add the parts together and divide by 27. That gives you cubic yards, the unit ready-mix suppliers use. If you are buying bags instead, divide the cubic feet by the yield printed on the bag. The Concrete Calculator does both for slabs, footings and round columns.

Concrete Volume Formulas by Shape

Measure everything in feet, or convert inches by dividing by 12, and these formulas give you cubic feet:

Then divide by 27 for cubic yards. If the reasoning behind that number is new to you, our guide to calculating cubic yards explains it.

How Much Concrete for Common Projects

A driveway

A single-car driveway 14 ft wide and 40 ft long covers 560 square feet. Poured 5 inches thick, it needs 560 × 5 ÷ 12 = 233.3 cubic feet, which is 8.64 cubic yards. Driveway thickness is often set by local rules and by the vehicles it will carry, so confirm yours before ordering. Longer driveways are often poured in sections separated by joints, and each section can be calculated and ordered on its own. Most driveways also sit on a compacted gravel base, which our gravel guide helps you work out.

A sidewalk or garden walkway

A path 3 ft wide and 60 ft long, 4 inches thick, comes to 3 × 60 × 0.333 = 60 cubic feet, or 2.22 cubic yards. Long, narrow pours like this are easy to underestimate because the area does not look large, but the length adds up quickly.

Fence post holes

Post holes are cylinders, and the post itself takes up some of the space. Say each hole is 10 inches across and 3 ft deep, holding a 4×4 post, which actually measures 3.5 inches square.

• Hole: 3.1416 × (5 ÷ 12)² × 3 = 1.64 cubic feet
• Post: (3.5 ÷ 12) × (3.5 ÷ 12) × 3 = 0.26 cubic feet
• Concrete per hole: 1.64 − 0.26 = 1.38 cubic feet

For a 20-post fence, that is 27.6 cubic feet, almost exactly one cubic yard. Fence jobs are nearly always done with bags, since each hole is filled separately. With 80 lb bags yielding about 0.6 cubic feet, you need roughly 46 bags, so buy 47.

Concrete steps

Solid steps poured against the ground are best treated as a stack of boxes. Picture three steps, each 4 ft wide with a 7-inch rise and an 11-inch tread. The bottom step is one rise tall, the second is two rises tall and the top one is three. Add those up and you get:

Rise and tread dimensions for steps are often covered by building codes, so check the figures that apply where you live before you build the forms.

A low garden wall and its footing

A freestanding garden wall 20 ft long, 8 inches thick and 3 ft tall holds 20 × 0.667 × 3 = 40 cubic feet. It also needs a footing underneath. A footing 18 inches wide and 10 inches deep along the same length adds 20 × 1.5 × 0.833 = 25 cubic feet. Together that is 65 cubic feet, or 2.41 cubic yards. Retaining walls that hold back soil are a different matter and usually need an engineered design.

Concrete Yardage per 100 Square Feet

For flat work, it helps to know how much concrete every 100 square feet takes at common thicknesses. Multiply by your area in hundreds of square feet for a fast estimate.

The driveway above is 5.6 hundred square feet at 5 inches, so 5.6 × 1.54 = 8.6 cubic yards, which matches the full calculation. For a single slab traced from measurement to final order, including thickened edges and an allowance, see how to estimate concrete for a slab.

Calculating Concrete in Metric

Many product sheets, and most suppliers outside the United States, measure concrete in cubic meters. The method is identical: multiply length, width and thickness, all in meters. A garden slab 5 m by 4 m and 100 mm thick is 5 × 4 × 0.1 = 2 cubic meters. If your supplier works in yards, multiply by 1.308 to get about 2.62 cubic yards.

Watch the thickness. A 100 mm slab is 0.1 m, and a slipped decimal point is the metric version of mixing inches with feet. The Concrete Calculator accepts millimeters, centimeters and meters directly, so you can enter measurements exactly as you took them.

Ordering Ready-Mix: What to Tell the Plant

Volume is only part of the order. When you call a ready-mix supplier, be ready to tell them:

• How many cubic yards you need, including whatever allowance you have decided on.
• What the concrete is for. The supplier can suggest a suitable mix strength for a patio, a driveway or a footing, though plans or local codes may specify one.
• How the truck will reach the pour. Mention narrow gates, soft lawns, slopes and overhead wires. If the chute cannot reach, you may need a pump.
• How quickly you can place it. A small crew working with wheelbarrows needs more time than a crew pouring straight from the chute.
• The weather on the day. Hot or cold conditions can affect how fast concrete sets, and suppliers may offer admixtures to help.

Ask about delivery fees, minimum loads and how long the driver can wait on site before extra time is charged. These details often make a bigger difference to the final bill than an extra quarter yard of concrete.

Common Concrete Calculation Mistakes

• Treating steps as one big box. The three steps above, measured as a single block 4 ft wide, 2.75 ft deep and 1.75 ft tall, come to 19.25 cubic feet. That is 50 percent more than they really need.
• Forgetting the footing. Walls, columns and some slabs sit on footings that hold a lot of concrete out of sight.
• Assuming everything is 4 inches thick. Driveways, equipment pads and footings often need more. Use the right thickness for each part.
• Ignoring the post in a post hole. On a single hole it is a small difference, but across a long fence it adds up to several bags.
• Rounding each piece before adding. Keep each part in cubic feet, total them and convert once at the end.

Frequently Asked Questions

How much concrete fits in one ready-mix truck?

Many full-size mixer trucks carry somewhere around 8 to 10 cubic yards, though capacity varies with the truck and local weight limits. Larger pours are delivered in several loads, so ask the supplier how they will schedule them.

How do I calculate concrete for a round slab?

Use 3.1416 × radius² × thickness. A round pad 10 ft across has a 5 ft radius, so at 4 inches thick it needs 3.1416 × 25 × 0.333 = 26.2 cubic feet, or about 0.97 cubic yards.

What happens if I order too much concrete?

The driver usually cannot dump leftover concrete just anywhere, and some suppliers charge to take it back. Having a spare form or two ready, such as a stepping stone mold, gives the surplus somewhere useful to go.

How long can a ready-mix truck wait on site?

Suppliers typically include a set amount of unloading time in the price and charge for extra waiting beyond that. Concrete also needs to be placed promptly once it is mixed, so have your forms, tools and helpers ready before the truck arrives.

Does the Concrete Calculator handle steps and walls?

Walls and footings are rectangular, so enter them like a slab using height or depth as the thickness. For steps, enter each step as its own block and add the results, or use the stacked-box formula above.

Final Thoughts

Concrete projects rarely come as a single shape. Break each job into slabs, cylinders and steps, work out the cubic feet of every part and convert the total to cubic yards. With that number in hand, you can order with confidence and plan the pour around it. The Concrete Calculator covers slabs, footings and round columns in yards and bags, and the CY Calculator handles any other shape. To find out how heavy your concrete will be, see our cubic yards to tons guide.