Concrete slab

Concrete Slab Calculator

Calculate concrete volume for patios, pads, sidewalks and other rectangular slabs using U.S. construction units.

Slab dimensions

Length (ft)

Width (ft)

Thickness (in)

Waste allowance (%)

Estimate

Before waste—

With waste—

80 lb bags—

Slab area—

Concrete slab formula

A slab is a rectangular prism. Convert thickness to feet, multiply the three dimensions, and divide cubic feet by 27 to get cubic yards.

cubic yards = length(ft) × width(ft) × thickness(in ÷ 12) ÷ 27

Example

A 12 ft × 12 ft slab at 4 in thick is about 1.78 yd³ before waste. Adding 10% gives a planning quantity of about 1.96 yd³.

Choosing a waste allowance

Waste helps cover small measurement differences, uneven excavation and placement loss. The right allowance depends on the project, so verify dimensions and site conditions before ordering.

Concrete Slab planning guide

For a slab, small changes in thickness can noticeably change total concrete volume. Re-measure the inside of the forms, check depth at several points and account for uneven subgrade before ordering; the calculator gives a planning quantity, while the final order should reflect field measurements and a sensible allowance for placement variation.

Turning the estimate into an order

Calculate the base requirement first, then convert it into cubic yards or whole bags, depending on how the concrete will be supplied. Before ordering, verify finished slab dimensions, thickness, thickened edges and any grade beams. Round only at the final purchasing step so intermediate rounding does not quietly inflate the estimate.

What changes the field quantity

A slab that looks uniform on plan can contain turned-down edges, depressions, pads or uneven subgrade. Those details can add significant volume.

Handling a non-rectangular project

Divide L-shaped or stepped slabs into rectangles. Calculate thickened edges and isolated pads as separate prisms, then add everything before applying the waste allowance.

Mistakes worth checking

Entering inches as feet for slab thickness
Ignoring thickened perimeter edges
Using outside form dimensions when the plans specify another reference
Applying waste before adding all slab components

Concrete slab FAQ

What slab thickness should I enter?

Enter the thickness required by your project design or local requirements. This calculator does not determine structural thickness.

Does the calculator include reinforcement?

No. Reinforcement, base material, forms and labor are separate from the concrete volume estimate.

Can I use this for irregular slabs?

Break an irregular layout into simple rectangles, calculate each section, and add the quantities together.

Slab Concrete ordering notes

For slabs, thickness errors matter quickly because they affect the entire area. Recheck the finished thickness at the formwork, add any turned-down sections, and round the final ready-mix order in the increment accepted by the local supplier.

Use the calculator result as a planning quantity, then compare it with the packaging, minimum order, delivery rules and technical data for the exact slab concrete you intend to use. Where drawings, codes or manufacturer instructions conflict with a general estimate, follow the project-specific requirement.

Before placing the order

Recheck the final measured dimensions after the project scope is fixed.
Confirm finished slab dimensions, thickness, thickened edges and any grade beams.
Convert the calculated requirement into the supplier’s actual selling units and round up only where whole units are required.
Keep labor, delivery, tax, equipment and site-specific requirements separate unless the calculator explicitly includes them.