Common question
How much concrete for a slab?
Order concrete from the inside of the forms. Length and width in feet and thickness in inches become cubic yards, which is the number a ready-mix plant expects.
Four inches is a common patio thickness. It is not a driveway design. If the slab will carry vehicles, confirm a thicker section before you trust a 4-inch estimate. The result is an estimate from the size you enter.
About 5.43 cubic yards for this slab.
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How we calculated this
Volume is length times width times thickness. Thickness is converted from inches to feet. Waste is added before converting to cubic yards (divide cubic feet by 27). Bag counts round up.
What does this mean?
A slab of this size needs about 5.43 cubic yards. Four inches is a common patio thickness; driveways are often thicker. Check the local requirement.
Your assumptions
Edit assumptions- Length
- 20.0 ft
- Width
- 20.0 ft
- Thickness
- 4.00 in
- Waste
- 10%
This calculator provides an estimate for planning. Material yield, waste, prices, and local requirements may vary.
How the estimate is calculated
Volume is length times width times thickness. Thickness is converted from inches to feet. Waste, often about 10%, is added for an uneven base and for concrete left in the chute.
Cubic yards are the cubic feet divided by 27. Bag counts use about 0.60 cubic feet for an 80 lb bag and 0.45 cubic feet for a 60 lb bag, then round up. A larger slab is usually ready-mix, so the yard quantity is the number to take to the plant.
Open the Concrete Slab Calculator · How this calculator works · Estimating a concrete slab
Questions
Does the thickness include the gravel base?
No. Thickness is concrete only. The pad calculator adds a gravel base if you need that too.
How much waste should I add?
The slab calculator starts near 10%. You can lower it. A small, even pad may need less. An uneven grade may need more.
Can I calculate a round slab here?
Not directly. This page uses a rectangle. A square that covers the circle will overstate the order.
Are wire and rebar included?
No. The result is concrete volume only.