Concrete Calculator

The amount of concrete you need depends on the shape and size of what you're pouring. A flat slab is calculated differently from a round column or a rectangular footing, so it helps to know your project type before you start.

A few things to keep in mind as you estimate:

• Always add 5–10% extra to account for spillage, uneven subgrades, and waste during the pour.
• Measure twice. An error of even a few inches in length or depth can meaningfully change the volume, especially on larger pours.
• Concrete is sold by the cubic yard for ready-mix deliveries, but bags at the hardware store are measured by weight (typically 40 lb, 60 lb, or 80 lb bags).

Once you have your dimensions locked in, the calculator does the rest. The sections below break down the math for each project type.

A concrete volume calculator converts your length, width, and depth measurements into a cubic volume. Most online calculators let you enter dimensions in feet or inches and then output results in cubic feet, cubic yards, or the number of bags needed.

Here's the general process:

1. Select your project shape (slab, footing, column, etc.).
2. Enter the dimensions in your preferred unit.
3. Review the cubic yard total and the recommended bag count.
4. Add your waste buffer (typically 10%) before ordering.

The calculator is only as accurate as the numbers you feed it, so take your measurements carefully before you start. A tape measure and a notepad go a long way.

Slabs are the most common residential concrete pour. Driveways, patios, garage floors, sidewalks — they're all essentially flat rectangles (or close to it) with a consistent thickness.

To calculate the volume for a slab, you need three measurements: length, width, and thickness (depth). Standard residential slabs are usually 4 inches thick, though driveways that need to support heavier vehicles are often poured at 6 inches.

For example, a 12-foot by 20-foot patio poured at 4 inches thick works out to 80 square feet of surface area. Convert the thickness to feet (4 inches = 0.333 feet), multiply everything together, and you get roughly 26.7 cubic feet, or about 0.99 cubic yards. In practice, you'd order 1.1 cubic yards to cover waste.

Odd shapes? Break them into rectangles, calculate each section separately, and add the volumes together.

Footings and foundations are typically rectangular trenches or continuous strips of concrete that support walls, posts, or structures above grade. They're usually deeper and narrower than slabs, and they need to extend below the frost line in colder climates.

The calculation is the same basic formula: length × width × depth. But footings often run in a continuous loop around a structure, so you'll need to calculate the total linear footage of the footing before multiplying by the cross-section dimensions.

For a simple rectangular foundation that's 30 feet × 40 feet with a 12-inch wide by 18-inch deep footing, the total length of footing is 140 linear feet (the perimeter). Multiply 140 × 1 foot wide × 1.5 feet deep and you get 210 cubic feet, or roughly 7.8 cubic yards. Add 10% and you'd order around 8.6 cubic yards.

Always check local building codes for minimum footing dimensions. The calculator gives you the volume, but code requirements dictate the minimum size.

Columns and piers are cylindrical (round) in most cases, which means the volume formula is a little different. You're working with a circle instead of a rectangle, so the diameter of the column matters as much as its height.

The formula uses the radius (half the diameter) and the height of the column. Typical applications include deck piers, fence posts set in concrete, and structural support columns.

For a round pier that's 12 inches in diameter and 36 inches deep, the radius is 0.5 feet and the depth is 3 feet. Multiply pi (3.14159) by the radius squared (0.25) by the depth (3), and you get about 2.36 cubic feet, or roughly 0.09 cubic yards per pier. If you're setting 10 piers, that's 0.9 cubic yards total before the waste buffer.

Pre-mixed bags work well for small column pours. An 80 lb bag yields about 0.6 cubic feet, so a single 12-inch pier at 3 feet deep would need around 4 bags.

The formulas vary by shape, but they all follow the same logic: calculate the cross-sectional area, then multiply by the depth or height to get volume.

All of these formulas give you a result in cubic feet. To convert to cubic yards, divide by 27 (since there are 27 cubic feet in one cubic yard). That's the number your ready-mix supplier will quote you on.

Unit conversions trip people up more often than the actual math. Here's a quick reference so you can move between units without second-guessing yourself.

If you're buying bags instead of ordering a truck, figure out your total cubic feet first, then divide by the yield per bag. Round up to the nearest whole bag, and add a couple extra for good measure.

Let's run through a few quick examples to show how this all comes together in practice.

Example 1: Backyard patio slab
Size: 16 ft × 12 ft, 4 inches thick
Volume: 16 × 12 × 0.333 = 63.9 cubic feet ÷ 27 = 2.37 cubic yards
With 10% waste buffer: order 2.6 cubic yards

Example 2: Rectangular footing
Footing: 60 linear feet, 10 inches wide, 12 inches deep
Volume: 60 × 0.833 × 1 = 50 cubic feet ÷ 27 = 1.85 cubic yards
With 10% buffer: order 2.05 cubic yards

Example 3: Round deck piers
8 piers, each 10 inches diameter, 42 inches deep
Per pier: 3.14159 × (0.417)² × 3.5 = 1.92 cubic feet
Total: 8 × 1.92 = 15.36 cubic feet ÷ 27 = 0.57 cubic yards
Using 80 lb bags: 15.36 ÷ 0.60 ≈ 26 bags

These examples cover the most common scenarios. Your actual project may vary, but the math stays the same. Measure carefully, use the right formula for your shape, and don't skip the waste buffer.