Concrete Slab Volume Calculator
Estimate how much concrete you need for a rectangular slab. Enter the width and length in feet and the thickness in inches to get the pour volume in cubic yards.
Calculate Concrete Volume for Slab Pour
Concrete Slab Volume Formula
Where:
- \(w\) — width of the slab (ft)
- \(l\) — length of the slab (ft)
- \(t\) — thickness of the slab (in)
- \(V\) — required concrete volume (yd³)
The width and length are multiplied to get the slab's surface area, the thickness is converted from inches to feet by dividing by 12, and the resulting volume in cubic feet is divided by 27 (the number of cubic feet in one cubic yard) because ready-mix concrete is typically ordered by the cubic yard.
Concrete Slab Pour Volume – Calculation Example
Example: a patio slab is 10 ft wide, 20 ft long, and 6 inches thick.
\[ V = \frac{10 \times 20 \times (6/12)}{27} = \frac{100}{27} \approx 3.7037 \]
Result: about 3.70 yd³ of concrete is needed for the pour.
How to Use the Calculator
- Enter the Width of the slab in feet (default 10 ft).
- Enter the Length of the slab in feet (default 20 ft).
- Enter the Thickness of the slab in inches (default 6 in, a common slab thickness).
- Read the Concrete Volume in cubic yards — the amount you should order.
What Is a Concrete Slab Pour Volume?
A concrete slab is a flat, horizontal layer of concrete used for patios, driveways, shed bases, garage floors, and building foundations. Before pouring, you need to know exactly how much concrete the job requires, because ready-mix trucks deliver concrete by the cubic yard and a shortage mid-pour can ruin the finish and weaken the slab.
The math is simple volume geometry: the slab is a rectangular box, so its volume equals the surface area multiplied by the thickness. The only catch is unit conversion — width and length are usually measured in feet, thickness in inches, and concrete is sold in cubic yards. This calculator handles all three units and returns the answer directly in cubic yards.
Interpreting the result: the figure is the net volume of the slab itself. In practice, most contractors add a 5–10% margin for spillage, over-excavation, and uneven subgrade, so a result of 3.70 yd³ usually means ordering 4 yd³. If your result falls between whole yards, always round up — it is cheaper than having a truck come back for a small top-up, and a cold joint between two pours weakens the slab.
Assumptions and Tips
- Rectangular shape only: the calculator assumes a flat, rectangular slab of uniform thickness. For L-shaped slabs, split the area into rectangles, calculate each one separately, and add the results.
- Thickness matters a lot: because thickness is multiplied across the whole surface, a 5-inch slab needs about 17% less concrete than a 6-inch slab over the same area. Measure the actual planned thickness, not a nominal one.
- Add a waste margin: order 5–10% more than the calculated volume to cover uneven ground, spillage, and slight over-depth of the excavation.
- Common thicknesses: patios and walkways typically use 4 in, driveways 5–6 in, and slabs that will carry vehicle loads or heavy equipment often need 6 in or more with reinforcement.
- Volume, not weight: the result is a volume estimate. To find the amounts of cement, sand, and gravel for a mix, see the Concrete Ingredients and Concrete Mix Ratio calculators.
Related Calculators
For jobs beyond a simple flat slab, the Concrete Yardage calculator covers general pour shapes, the Concrete Footing/Slab Pours calculator handles combined footing and slab work, and the Concrete Wall calculator estimates vertical pours. If you are checking whether a slab can carry a load, try the Concrete Slab Maximum Wall Load calculator.
Frequently Asked Questions
How do I calculate concrete volume by hand without a calculator?
Multiply width by length by thickness (in feet) to get cubic feet, then divide by 27. For example, a 12 ft × 12 ft slab at 4 inches thick: thickness in feet is 4/12 = 0.333 ft, so 12 × 12 × 0.333 ≈ 48 ft³, and 48 ÷ 27 ≈ 1.78 yd³. A quick shortcut: one 80 lb bag of concrete yields about 0.6 ft³, so small pours are often easier to estimate in bags.
How many square feet does a yard of concrete cover?
One cubic yard is 27 ft³, so coverage depends on thickness. At 4 inches thick, a yard covers about 81 ft²; at 5 inches, about 65 ft²; at 6 inches, about 54 ft². A handy rule of thumb: divide 324 by the thickness in inches to get the square feet one yard covers. So at 4 in: 324 ÷ 4 = 81 ft².
How much extra concrete should I order?
Order 5–10% more than the calculated volume and always round up to the nearest whole or half yard, since most suppliers sell in those increments. Uneven subgrade, spillage, and slightly over-deep excavation all eat into your estimate. Running short mid-pour forces a second delivery and creates a cold joint — a weak seam between two pours — which usually costs far more than the extra concrete.
How do I convert the result to cubic meters or bags of concrete?
Multiply cubic yards by 0.7646 to get cubic meters (1 yd³ ≈ 0.765 m³). For bagged concrete, one 80 lb bag yields roughly 0.6 ft³ and one 60 lb bag about 0.45 ft³. So a 3.7 yd³ pour equals about 100 ft³, which would take around 167 eighty-pound bags — a strong argument for ordering ready-mix on anything larger than a small slab.
How do I calculate concrete for an L-shaped or irregular slab?
Split the shape into rectangles, calculate each rectangle's volume separately with the calculator, and add the results. For a circular pad, use the area of a circle \( \pi r^2 \) instead of width × length, then multiply by thickness in feet and divide by 27. For triangular sections, use half the base times the height. Measure each section's thickness separately if it varies.
What is a common mistake when estimating a slab pour?
The most frequent error is mixing up units — entering thickness in feet (e.g., 6 instead of 0.5) inflates the result twelvefold, while measuring length in inches but entering it as feet understates it. Also avoid using nominal thickness: if plans call for 4 inches but the ground is uneven, the average pour depth may be closer to 5 inches, which is 25% more concrete. Always measure the actual excavation depth.
How much does a cubic yard of concrete weigh?
A cubic yard of standard ready-mix concrete weighs roughly 4,000 lb (about 2 tons, or 2,400 kg per m³), depending on the mix design. This matters for transport: a 10 yd³ truck load weighs around 40,000 lb, so check driveway and soil bearing capacity before the truck arrives. Note that concrete volume and weight are different measures — order by volume, but the weight affects delivery logistics.