Concrete Slab Calculator

Trucks deliver concrete by the cubic yard, and one cubic yard holds 27 cubic feet. Bags come in weights such as 80 lb, but a slab is measured by volume, so the calculator converts each bag to the volume it makes once mixed. An 80 lb bag yields about 0.6 cu ft.

How to use this calculator

1. Choose Rectangle for a square or oblong slab, or Triangle for a corner pad, where length is the base and width is the height.
2. Enter the length and width in feet and the thickness in inches, the usual units for a slab.
3. Enter the thickness from your plans or from the guidance further down this page. A generator pad and a drive that trucks cross need different thicknesses.
4. Keep the default allowance for waste at 10 percent unless you have a reason to change it. It covers soft ground, flexing forms and spills.
5. Order cubic yards for a ready-mix truck, or use the bag count if you will mix by hand. Enter your supplier's price per cubic yard for an estimated cost.

Worked examples

Cubic yards for common slab sizes, before extra for waste

Cubic yards, exact volume without extra for waste.

Thickness drives the volume

A slab's volume is its footprint times its thickness, so thickness has as much say over the order as the footprint does. At 4 inches thick, one cubic yard covers 81 square feet of ground. At 6 inches thick, that same yard covers only 54 square feet, so a few extra inches of thickness add noticeably to the order.

Bags or ready-mix for a slab

Bags suit small jobs, such as a generator pad or a few post footings beside a slab. A cubic yard takes 45 bags of the 80 lb size or 60 of the 60 lb size, and each one is carried, mixed and placed by hand.

For a slab you plan to finish in one session, a truck is usually the easier choice, because the whole pour arrives at once and the crew can place it before it stiffens. Ask about minimum orders and short-load charges, since a small slab can cost more per cubic yard than a large one.

Bags still make sense for a shed base on a tight budget or a site a truck cannot reach. Buy the full count before you start.

Adding a thickened edge

A thickened edge is a deeper band of concrete around the outside of a slab, usually where a wall or heavy load sits. The calculator has no edge setting, so work it in two parts: the main slab at its normal thickness, and the edge as a separate strip.

For the strip, enter its length as the distance around the edge, its width as the width of the band, and its depth as the extra depth beyond the main slab. Use the rectangle shape for each part, then add the two cubic yard figures. Bags round up separately for each part, so expect a few more bags than a single entry would give.

Check the band's width and depth with your local building department, since edges that carry a wall can have their own rules.

Matching thickness to the use

• Passenger vehicles on a residential driveway need at least 4 inches.
• Where trucks or large RVs park or turn, plan on 5 to 6 inches.
• A garage floor that holds only passenger cars can follow the driveway minimum, but the base and your local code come first.
• Patios, walkways and shed bases have no fixed figure here, and many rest on a compacted gravel base. Check the shed kit's floor instructions, local building rules and the product label.

Small slab in bags: how many bags for a 4x8 slab?

For a small slab, find the volume in cubic feet, add the waste allowance, and divide by your bag's yield. An 80 lb bag makes 0.6 cu ft and a 60 lb bag makes 0.45 cu ft. Round up to whole bags.

Common questions