How Much Concrete Do I Need for a Slab? Formula, Examples and Free Calculator

Last reviewed: October 9, 2026. If you are pouring a slab, patio, driveway, footing or small foundation, one of the first questions is simple: how much concrete do I actually need? The math is not difficult, but small mistakes in thickness, unit conversion or waste allowance can leave you short during the pour.

The basic rule is to calculate the volume of the space you want to fill, convert that volume into cubic yards or cubic meters, then add a reasonable contingency for uneven subgrade, spillage and variations in the forms.

If you want the fastest answer, use our free Concrete Calculator. Enter the length, width and thickness and the tool will estimate concrete volume plus material quantities for common project types.

The basic concrete volume formula

For a rectangular slab, the formula is:

Volume = Length × Width × Thickness

All three dimensions must use compatible units before you multiply them. If length and width are in feet but thickness is in inches, convert the thickness to feet first.

Example: 12 ft × 20 ft slab at 4 inches thick

4 inches ÷ 12 = 0.333 ft

12 × 20 × 0.333 ≈ 80 cubic feet

One cubic yard equals 27 cubic feet, so:

80 ÷ 27 ≈ 2.96 cubic yards

With a 10% planning allowance:

2.96 × 1.10 ≈ 3.26 cubic yards

So a practical estimate is roughly 3.3 cubic yards, subject to the actual site conditions and your supplier’s delivery rules.

How much extra concrete should you order?

Ordering exactly the theoretical volume is risky. Real sites are rarely perfect: the base may be slightly low in places, forms can move, and some material is lost during placement.

The National Ready Mixed Concrete Association recommends allowing for waste, extra thickness, form deflection and other contingencies, with guidance in the range of roughly 4% to 10%. Cemex gives a broader general recommendation of approximately 10% to 15% for factors such as spillage and uneven subgrade.

For a straightforward, well-prepared residential slab, 10% is a useful planning starting point. Complex forms, rough excavation or uncertain depth may justify more. For structural or large pours, confirm the final quantity with the contractor or ready-mix supplier.

Quick concrete estimates for common slab sizes

• 10 ft × 10 ft × 4 in: about 1.23 yd³ theoretical, or about 1.36 yd³ with 10% extra.
• 12 ft × 20 ft × 4 in: about 2.96 yd³ theoretical, or about 3.26 yd³ with 10% extra.
• 20 ft × 24 ft × 5 in: about 7.41 yd³ theoretical, or about 8.15 yd³ with 10% extra.
• 6 m × 4 m × 100 mm: 2.40 m³ theoretical, or about 2.64 m³ with 10% extra.

These are quantity estimates only. The correct slab thickness, reinforcement, base preparation and concrete specification depend on the project, loads, soil and local requirements.

How to calculate concrete in cubic yards

1. Multiply length in feet by width in feet.
2. Convert slab thickness from inches to feet by dividing by 12.
3. Multiply area by thickness to get cubic feet.
4. Divide cubic feet by 27 to get cubic yards.
5. Add your waste or contingency allowance.

Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27

Multiply the result by 1.10 if you want to add a 10% allowance.

How to calculate concrete in cubic meters

Metric calculations are simple when all measurements are in meters. A 6 m × 4 m slab at 100 mm thick uses a thickness of 0.10 m.

6 × 4 × 0.10 = 2.4 m³

With 10% extra: 2.4 × 1.10 = 2.64 m³.

If thickness is in millimeters, divide by 1,000 to convert it to meters. If it is in centimeters, divide by 100.

Why slab thickness changes the answer quickly

Thickness is where many estimates go wrong. A slab that is only one inch thicker than expected can use a surprisingly large amount of extra concrete.

For a 20 ft × 20 ft slab:

• At 4 inches thick: about 4.94 cubic yards.
• At 5 inches thick: about 6.17 cubic yards.
• At 6 inches thick: about 7.41 cubic yards.

That is why checking elevations and preparing a consistent base before ordering matters. QUIKRETE also notes that bag-yield calculations are approximate and do not account for uneven subgrade or waste.

How much concrete do I need for a driveway?

The formula is still length × width × thickness. What changes is the design requirement.

A driveway normally carries heavier loads than a light patio or walkway, so thickness and reinforcement should be chosen for the expected vehicles, soil conditions and local requirements.

Do not choose slab thickness only because it reduces the concrete order. Quantity should follow the design, not determine it.

Once you know the specified thickness, enter the dimensions into the Horus Valley Concrete Calculator to get the volume quickly.

How much concrete do I need for a patio or shed base?

For a simple rectangular patio or shed base, measure the finished inside dimensions of the forms. If the project includes steps, thickened edges, grade beams or separate pads, calculate those sections separately and add them together.

A slab can look rectangular from above while hiding a deeper perimeter. Treat each depth as a separate volume so the extra concrete is not missed.

How to calculate concrete for footings

A rectangular footing uses the same formula: Length × Width × Depth.

If several footings are the same size, calculate one and multiply by the number of footings.

For more complicated foundation layouts, our Foundation Calculator can help estimate concrete for isolated and strip footings.

How to calculate concrete for circular columns or holes

For a cylinder, use: Volume = π × radius² × height. The radius is half the diameter.

This formula is useful for round columns, piers and post holes. If a post occupies part of the hole, the true concrete volume is the volume of the hole minus the volume occupied by the post.

What if the slab is an irregular shape?

Break the project into simple shapes instead of forcing one complicated formula. Divide the area into rectangles, triangles or circles, calculate the volume of each section, then add them together.

If one section has a different thickness, calculate it separately. This is generally more reliable than using one average depth for the whole project.

Ready-mix concrete vs bagged concrete

For very small jobs, packaged concrete may be practical. For larger slabs, ready-mix delivery is usually easier than mixing dozens of bags by hand.

Bag requirements depend on the exact product and its stated yield. Do not assume every 40 lb, 60 lb or 80 lb bag produces the same volume across all brands and mixes.

QUIKRETE’s official calculator provides separate estimates for different bag sizes and explicitly notes that yields are approximate.

If you are using bags, calculate the total volume first, then compare it with the yield printed on the product you intend to buy.

Common mistakes that make concrete estimates inaccurate

1. Forgetting to convert inches to feet

Multiplying a 4-inch thickness directly by dimensions measured in feet gives the wrong answer. Divide inches by 12 first.

2. Confusing square feet with cubic feet

Square feet measure area. Concrete is a volume, so thickness must be included.

3. Ordering the exact theoretical amount

A calculated 3.00 cubic yards does not guarantee that exactly 3.00 cubic yards will fill a real excavation.

4. Measuring the plan instead of the actual forms

Before ordering, recheck the forms. The finished dimensions may differ slightly from the original plan.

5. Ignoring deeper sections

Thickened edges, beams, steps and footings can add significant volume.

6. Using an assumed thickness

If the base is uneven, average thickness may be greater than the intended design thickness.

7. Treating a calculator as a structural design tool

A calculator estimates quantity. It does not determine reinforcement, concrete strength, joints, curing or structural safety.

A simple checklist before ordering concrete

• Confirm the final inside dimensions of the forms.
• Confirm the specified thickness or depth.
• Check for thickened edges, beams, steps or footings.
• Make sure the base is reasonably level and compacted.
• Calculate each different shape separately.
• Convert all units correctly.
• Add an appropriate contingency.
• Ask the supplier about minimum loads and delivery charges.
• Confirm the required concrete specification with the designer or contractor when applicable.

Use the free Concrete Calculator

You do not need to repeat the arithmetic every time. Open the free Concrete Calculator on Horus Valley, enter your dimensions and calculate the required volume instantly.

It is especially useful when comparing several slab sizes or thicknesses because you can change the inputs quickly and see how the required quantity changes.

You can also explore the other tools in our Engineering & Construction section for related project estimates.

Frequently asked questions

How many cubic feet are in one cubic yard of concrete?

One cubic yard equals 27 cubic feet.

How many cubic meters are in one cubic yard?

One cubic yard is approximately 0.765 cubic meters.

Should I add 10% extra concrete?

Ten percent is a useful planning allowance for many straightforward jobs, but the right margin depends on the site. NRMCA guidance discusses roughly 4% to 10%, while Cemex advises a broader 10% to 15% general allowance.

How thick should a concrete slab be?

There is no single correct thickness for every project. It depends on use, loads, soil, reinforcement and local requirements. Use the thickness specified for your project, then calculate the quantity from that value.

Can I calculate concrete from square footage alone?

Not accurately unless you also know the thickness. Concrete quantity is volume, not area.

Is a concrete calculator exact?

No calculator can know every variation in your excavation or forms. It produces an estimate from the measurements you enter, so field measurements and a contingency still matter.

The bottom line

To estimate concrete for a rectangular slab, multiply length × width × thickness, convert the result into cubic yards or cubic meters, and add a realistic allowance for site variation.

The biggest sources of error are incorrect unit conversion, underestimated thickness and ordering with no contingency.

For a faster estimate, use the Horus Valley Concrete Calculator and verify the final quantity against the actual site before ordering.

Sources

National Ready Mixed Concrete Association — concrete yield and quantity guidance.

Cemex USA — ready-mix concrete calculator and quantity guidance.

QUIKRETE — concrete calculator and packaged concrete yield estimates.