Concrete Calculator

TL;DR: To calculate concrete volume in cubic yards, multiply the length (ft) by the width (ft) by the thickness (in) divided by 12 to get cubic feet, then divide that result by 27; or use the formula: cubic yards = (length × width × (thickness ÷ 12)) ÷ 27.

What Is the Concrete Calculator?

The Concrete Calculator is a practical tool designed to estimate the volume of concrete needed for a slab, driveway, patio, foundation, or any flat structural element. It takes three key dimensions—length (in feet), width (in feet), and thickness (in inches)—and instantly returns the required concrete volume in both cubic yards and cubic meters. This eliminates guesswork and helps prevent costly over-ordering or under-ordering of ready-mix concrete.

Anyone involved in concrete work needs this calculator: DIY homeowners pouring a backyard patio, contractors estimating material for a garage floor, landscapers planning a walkway, or construction project managers ordering precast concrete for a building foundation. Even small projects benefit, because concrete is typically sold by the cubic yard, and mixing by hand without a volume estimate leads to waste or shortage.

Using the calculator ensures your measurements are converted correctly between different units (feet to inches, inches to feet, cubic feet to cubic yards) without manual error. It is especially valuable because concrete orders are placed in cubic yards, but most people measure dimensions in feet and inches.

How to Use the Calculator

Follow these steps to get an accurate concrete volume estimate in seconds.

1. Enter the slab length: In the field labeled ‘length’, input the measurement in feet. For example, if your slab is 20 feet long, type ‘20’.
2. Enter the slab width: In the field labeled ‘width’, input the measurement in feet. For a 12-foot-wide driveway, type ‘12’.
3. Enter the slab thickness: In the field labeled ‘thickness’, input the measurement in inches. A standard 4-inch patio slab would be entered as ‘4’.
4. Verify units: Ensure that length and width are entered in feet, and thickness is entered in inches. Do not mix feet and inches for the same dimension type.
5. Click calculate: Press the calculate button to process your inputs.
6. Read the results: The calculator displays two outputs: cubic yards (cu yd) and cubic meters (m³). Use the cubic yards value when ordering ready-mix concrete from a supplier.

If you need to re-calculate for a different slab, simply change any input field and click calculate again. The result updates instantly.

Formula and Calculation Method

The calculator works using a three-step mathematical process. First, it converts the slab thickness from inches to feet (since length and width are in feet). Second, it calculates the volume in cubic feet by multiplying the three dimensions together. Third, it converts cubic feet into the standard concrete ordering unit—cubic yards—and also into cubic meters for international use.

The complete formula is:

• Thickness in feet = Thickness in inches ÷ 12
• Volume in cubic feet = Length (ft) × Width (ft) × Thickness (ft)
• Volume in cubic yards = Volume in cubic feet ÷ 27
• Volume in cubic meters = Volume in cubic feet × 0.0283168

Worked example with real numbers:

Imagine you are pouring a concrete slab for a 10-foot by 10-foot shed base. You want the slab to be 6 inches thick.

1. Convert thickness: 6 inches ÷ 12 = 0.5 feet.
2. Calculate cubic feet: 10 ft × 10 ft × 0.5 ft = 50 cubic feet.
3. Convert to cubic yards: 50 ÷ 27 = 1.8519 cubic yards.
4. Convert to cubic meters: 50 × 0.0283168 = 1.4158 cubic meters.

The calculator performs these exact operations. If you enter length=10, width=10, and thickness=6, the output will show cubic_yards = 1.8519 and cubic_meters = 1.4158. This means you would need to order 1.85 cubic yards of concrete (typically rounded up to 2 cubic yards for safety) for a 10×10 shed slab that is 6 inches thick.

Practical Examples

Below are three realistic scenarios showing different dimensions and what the calculated volume means for ordering concrete.

In each case, note that the calculator gives precise decimal values. Concrete suppliers typically require you to round up to the nearest quarter or half cubic yard because they cannot deliver fractions of a yard. Always add 5–10% extra to your calculated order for spillage, uneven subgrade, or over-excavation.

Tips for Accurate Results

Accuracy starts with correct measurement. Use these tips to avoid common errors and ensure your concrete order is right the first time.

• Always keep units consistent: The calculator expects length and width in feet and thickness in inches. If you measure the slab length in inches, convert it to feet first (divide by 12). Do not enter thickness in feet, or the result will be wildly wrong.
• Measure the actual pour area: Do not use nominal building dimensions if the slab has curves, notches, or multiple sections. Break irregular shapes into rectangles, calculate each separately, then sum the cubic yards.
• Account for subgrade variation: If your site is not perfectly level, the thickness may vary. Measure thickness at several points and use the average. For a 4-inch design, but 5 inches in places, enter 4.5 or 5 to be safe.
• Do not forget waste and over-excavation: The calculator gives the theoretical volume for a perfect rectangular box. In practice, add 5–10% to the calculated cubic yards to account for concrete lost in mixing, forms that bulge, or a slightly uneven base.
• Use the cubic yard output for ordering: Most US concrete suppliers quote prices per cubic yard. The cubic meter output is useful for metric design requirements but not for ordering in the United States or Canada.
• Double-check thickness conversions: The most common mistake is entering thickness in feet instead of inches. If you enter a thickness of 0.33 (representing 4 inches), the calculator will interpret it as 0.33 inches, resulting in a tiny volume. Always enter whole numbers for thickness in inches.

Frequently Asked Questions

How many cubic yards of concrete do I need for a 20×20 slab?

For a 20-foot by 20-foot slab (400 square feet), the answer depends on the thickness. At a standard 4-inch thickness, the volume is (20×20×(4÷12)) ÷ 27 = 400 × 0.333 ÷ 27 = 4.94 cubic yards. At 6 inches thick, it becomes 400 × 0.5 ÷ 27 = 7.41 cubic yards. You would order 5 cubic yards for 4-inch thickness or 7.5 cubic yards for 6-inch thickness, plus 5–10% waste. Always confirm the intended thickness before ordering.

Why is concrete measured in cubic yards and not cubic feet?

Concrete is heavy and delivered by truck. A cubic yard of concrete weighs approximately 4,000 pounds (about 2 tons). Ready-mix concrete trucks typically carry between 8 and 10 cubic yards per load. Measuring in cubic yards keeps the numbers manageable—a 10-foot×10-foot×4-inch slab is about 1.23 cubic yards versus 33.3 cubic feet. The construction industry standardizes on cubic yards for quoting, ordering, and billing, so all concrete calculators should output this unit for practical use.

Can I use this calculator for circular or irregularly shaped concrete areas?

This specific calculator is designed for rectangular slabs with flat, uniform thickness. For circular areas (like a round patio or column footing), you would need a different formula: π × radius² × thickness, then divide by 27 to get cubic yards. For L-shaped or irregular areas, break them into multiple rectangles, calculate each using this calculator, and sum the results. For complex shapes (triangles, trapezoids, or curves), consult a structural engineer or use a specialized concrete volume calculator that accepts non-rectangular geometries.

Related Calculators

• Concrete Column Calculator
• Concrete Mix Design Calculator
• Concrete Rebar Spacing Calculator
• Concrete Slab Calculator
• Concrete Steps Calculator