Concrete Calculator: Yards by Inches

This concrete calculator converts your project’s length, width, and thickness (in inches) into the volume of concrete needed in cubic yards. You enter the three dimensions, and it returns the cubic yards, along with the equivalent cubic feet and cubic meters for reference.

Formula

The core formula is: Volume (cubic yards) = (Length × Width × Thickness) / 324, where Length and Width are in feet and Thickness is in inches. This works because you first convert the thickness from inches to feet (divide by 12), calculate the volume in cubic feet (Length × Width × Thickness in feet), and then convert cubic feet to cubic yards (divide by 27, since 1 yard = 3 feet, so 1 cubic yard = 27 cubic feet).

In symbols: V_yd = (L × W × T_in / 12) / 27, where L is length in feet, W is width in feet, and T_in is thickness in inches. If you measure length and width in inches instead, the combined formula becomes V_yd = (L_in × W_in × T_in) / 46656 (since 36 × 36 × 36 = 46656).

Worked Example

Using the calculator’s default values: a slab 10 feet long, 10 feet wide, and 4 inches thick.

1. Convert thickness to feet: 4 inches ÷ 12 = 0.3333 feet.
2. Calculate volume in cubic feet: 10 ft × 10 ft × 0.3333 ft = 33.33 cubic feet.
3. Convert to cubic yards: 33.33 ÷ 27 = 1.2346 cubic yards.

The calculator rounds to two decimals, giving 1.23 cubic yards. The cubic feet output is 33.33, and the cubic meters output is 33.33 × 0.0283168 = 0.94 m³.

Why Inches Matter

Most DIY concrete projects—slabs, footings, patios, walkways—are specified in inches for thickness. A typical residential slab is 4 inches, a driveway might be 5 or 6 inches, and a footing can be 8 to 12 inches. The formula above directly handles inches, so you don’t have to convert to feet first, which is where many people make errors. For example, a 10×10 slab at 4 inches is 1.23 yards, but at 6 inches it’s 1.85 yards—a 50% increase in concrete for just 2 extra inches.

How to Measure for a Slab

Accurate measurements are critical. Follow these steps:

• Measure length and width in feet, from the outside edges of the form. For irregular shapes, divide into rectangles and calculate each separately, then add.
• Measure thickness in inches at several points. Ground is rarely perfectly level; use the average thickness. If you’re pouring on grade, the thickness is the distance from the bottom of the form to the top of the finished slab.
• If you have a slope, measure the surface distance, not the horizontal distance, since concrete follows the ground.
• Always round up your final yardage by 5–10% to account for spillage, uneven subgrade, or over-excavation. For a 1.23-yard slab, order 1.35 yards.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units. For instance, measuring length in inches but width in feet, or entering thickness in feet instead of inches. Always double-check that all dimensions are in the same unit before calculating. Another common issue is forgetting that ready-mix concrete is sold by the cubic yard, not by the cubic foot. A 10×10×4-inch slab is 33 cubic feet, but that’s only 1.23 yards—a truck typically carries 9 to 10 yards, so you’d need a fraction of a load. Also, be aware that the formula assumes a perfectly flat, rectangular prism. If your slab has a thickened edge or a stepped footing, you need to calculate those volumes separately and add them.

Ordering Concrete: What the Number Means

The cubic yard figure is what you’ll use when ordering ready-mix concrete. Most suppliers have a minimum delivery charge (often 1 to 2 yards), so for small projects, mixing bags yourself might be more economical. A 60-pound bag of concrete mix yields about 0.45 cubic feet, and an 80-pound bag yields about 0.6 cubic feet. To convert your cubic yards to bags: multiply cubic yards by 27 (to get cubic feet), then divide by the yield per bag. For 1.23 yards, that’s 33.33 cubic feet, so you’d need about 74 60-pound bags or 56 80-pound bags. This is a lot of mixing, so for anything over 2 yards, ready-mix is usually the better choice.

Cost Considerations (Rough Estimates)

While we can’t give exact prices, you can estimate costs by checking local ready-mix prices per cubic yard (often $100–$150 per yard) and bag prices at home centers. Add delivery fees, which are typically a flat charge per load. For a 1.23-yard slab, you’d pay for the minimum delivery, which might be 2 yards, so the cost is higher per yard than a larger pour. For a 10×10 slab, bagging it yourself might cost $200–$300 in materials, while ready-mix with minimum delivery might run $250–$400. For larger slabs, ready-mix is clearly cheaper per yard.

Safety and Practical Tips

Concrete is heavy—a cubic yard weighs about 4,000 pounds (1,800 kg). Never attempt to lift or move more than you can handle. When mixing bags, wear a dust mask and gloves; cement dust can irritate skin and lungs. For ready-mix delivery, ensure the truck can access the site and that the chute can reach the form. Have a wheelbarrow and friends ready to move concrete quickly, as it sets in 30–60 minutes. Also, check local building codes for required thickness and reinforcement; a 4-inch slab is fine for a patio, but a driveway may need 5–6 inches with rebar or wire mesh.

Frequently Asked Questions

How many 80-pound bags do I need for 1 cubic yard?

One cubic yard is 27 cubic feet. An 80-pound bag yields about 0.6 cubic feet, so you need 27 ÷ 0.6 = 45 bags. For a 60-pound bag (0.45 cubic feet), you’d need 60 bags. Always buy a few extra bags to cover spillage.

What if my slab is not a perfect rectangle?

Break the area into rectangles, calculate each volume separately, and add them. For a circle, use the formula π × (radius in feet)² × thickness in feet, then divide by 27. For irregular shapes, use the average thickness and measure the area using a grid method.

Can I use this calculator for footings or columns?

Yes, but you need to adjust the shape. For a footing, enter the length, width, and thickness as above. For a round column, use the formula for a cylinder: π × (radius in feet)² × height in feet, then divide by 27. The calculator here is for rectangular slabs; for columns, use a dedicated cylinder calculator.

How much should I order to avoid running short?

Add 5–10% to your calculated volume. For a 1.23-yard slab, order 1.3 to 1.4 yards. For ready-mix, you can’t order a partial yard easily, so round up to the next half-yard. It’s better to have a little extra than to run out mid-pour.

Does the thickness include gravel base?

No. The thickness in the formula is only the concrete layer. A gravel base is separate and is typically 4–6 inches of compacted gravel. You’ll need to excavate to the combined depth of gravel and concrete. For example, for a 4-inch slab on 4-inch gravel, dig 8 inches deep.

Related

• Board Feet Calculator Formula & How to Use It
• Board Feet Calculator: Lumber Volume Explained
• Board Foot Calculator: How to Figure Lumber

in l | Length | ft | 10 | 0
in w | Width | ft | 10 | 0
in d | Thickness | in | 4 | 0
out yards | Concrete needed | cu yd | (l * w * (d / 12)) / 27 | 2
out feet | Cubic feet | cu ft | l * w * (d / 12) | 2
out meters | Cubic meters | cu m | (l * w * (d / 12)) * 0.0283168 | 2