Concrete Calculator: Sq Ft to Cubic Yards

This calculator converts a square-foot area and a depth (in inches or feet) into the volume of concrete needed in cubic yards. Enter the area in square feet and the slab thickness, and you get the exact cubic yards to order, plus the same volume in cubic feet and cubic meters for reference.

Formula

The core formula is simple volume calculation:

Cubic yards = (Area in sq ft × Depth in inches ÷ 12) ÷ 27

Where:

• Area = the footprint of the slab or pour, in square feet (sq ft)
• Depth = the thickness of the concrete, in inches (in)
• 12 converts inches to feet
• 27 = number of cubic feet in one cubic yard (3 ft × 3 ft × 3 ft)

If your depth is already in feet, skip the ÷12 step and just divide by 27.

Worked Example

Using the default values from the calculator: area = 100 sq ft, depth = 4 inches.

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Find volume in cubic feet: 100 sq ft × 0.3333 ft = 33.33 cu ft
3. Convert to cubic yards: 33.33 cu ft ÷ 27 = 1.2346 cu yd

Rounded to two decimals, the calculator shows 1.23 cubic yards. That matches the output above.

Why Use Cubic Yards for Concrete?

Ready-mix concrete is sold by the cubic yard in the United States. A typical concrete truck carries 9 to 11 cubic yards. Ordering by the yard lets you match your pour to a truckload, avoiding both shortages and costly overage. Even for small DIY projects where you buy bagged concrete, the cubic yard is the standard unit for comparing prices and estimating total weight.

One cubic yard of concrete weighs about 4,000 pounds (roughly 2,000 kg), so the volume directly tells you how much material you are handling. For example, the 1.23 cubic yards in the example above would weigh roughly 4,900 lb – a load that likely needs a truck or several strong helpers.

How to Measure the Area Correctly

For a simple rectangle, multiply length by width. For irregular shapes, break the area into rectangles and triangles, then add them. For a circle, use π × r² (r = radius in feet).

Common shapes and their area formulas:

• Rectangle: length × width
• Triangle: (base × height) / 2
• Circle: Ï€ × radius²
• L-shape: divide into two rectangles and add

Always measure in feet for area. If you measured in inches, convert to feet first (divide by 12). Mixing units is the most frequent error in concrete estimates.

Choosing the Right Depth

The depth depends on what you are building. Here are typical thicknesses for common projects:

• Sidewalk or patio: 4 inches
• Driveway (cars): 4–5 inches
• Driveway (heavy trucks): 6 inches
• Foundation footing: 8–12 inches
• Garage floor: 4–6 inches

These are general guidelines. Local building codes may require specific depths, especially for footings or structural slabs. Check with your local authority before pouring.

Common Mistakes and How to Avoid Them

Even with a calculator, people make errors. Watch for these:

• Unit mix-ups: Entering depth in feet when the calculator expects inches (or vice versa) changes the result by a factor of 12. Always confirm the unit.
• Forgetting to include waste: Concrete shrinks slightly, and forms are rarely perfect. Order 5–10% extra to cover spills, uneven subgrade, or over-excavation. For a 1.23 cu yd slab, add 0.06–0.12 cu yd.
• Rounding too early: If you round the cubic feet to a whole number before dividing by 27, you can lose accuracy. Keep at least two decimal places until the final step.
• Ignoring the thickness variation: If your subgrade is uneven, the actual depth varies. Use the average depth, but add extra for safety.

Limits of the Formula

This calculator gives the theoretical volume of a uniform slab. It does not account for:

• Reinforcement steel (rebar) displacing a small amount of concrete
• Waste or spillage
• Irregular subgrade that makes the depth vary
• Footings or thickened edges that add volume

For complex projects, it is wise to break the pour into sections and calculate each separately, then add them together. For a slab with a footing, calculate the slab volume and the footing volume separately and sum them.

Frequently Asked Questions

How many square feet does a cubic yard cover at 4 inches?

One cubic yard covers 81 square feet at 4 inches thick. That comes from 27 cubic feet ÷ (4/12 feet) = 81 sq ft. At 6 inches, it covers 54 sq ft; at 3 inches, 108 sq ft.

Can I use this calculator for bagged concrete?

Yes. The volume in cubic yards is the same regardless of how you buy the concrete. A 60-lb bag makes about 0.45 cubic feet, and an 80-lb bag makes about 0.6 cubic feet. Divide your total cubic feet by that number to get the bag count.

What if my depth is in feet, not inches?

If your depth is already in feet, simply multiply the area by the depth and divide by 27. For example, 100 sq ft × 0.5 ft = 50 cu ft ÷ 27 = 1.85 cu yd.

Should I order extra concrete?

Yes, most professionals add 5–10% to the calculated volume. This covers uneven ground, spillage, and the fact that forms may be slightly deeper than planned. For a small slab, rounding up to the next quarter yard is often enough.

How do I convert cubic yards to cubic meters?

Multiply cubic yards by 0.764555 to get cubic meters. For example, 1.23 cu yd × 0.764555 = 0.94 m³. The calculator provides this conversion in its third output.

Use this calculator before any concrete project, whether it is a small stepping stone or a large driveway. Accurate volume means fewer trips to the store, less waste, and a smoother pour.

Related

• Board Foot Calculator Formula
• Board Foot Calculator: Lumber Volume
• Board Foot Calculator From Log: How to Estimate Lumber

in area | Area | sq ft | 100 | 0
in depth | Depth | in | 4 | 0
out yards | Concrete volume | cu yd | (area * depth / 12) / 27 | 2
out cubic_ft | Volume in cubic feet | cu ft | area * depth / 12 | 1
out cubic_m | Volume in cubic meters | m³ | (area * depth / 12) * 0.0283168 | 2