Concrete Volume Slab Calculator
Find the cubic yards of concrete needed for a rectangular slab from length, width, thickness, and a waste allowance.
Cubic yards to order
1.358 cu yd
1.038 m³
What is concrete volume, and why does it matter?
Concrete volume is the amount of space inside the form that the mix must fill. For a rectangular slab, that is length times width times thickness. Because ready-mixed concrete is quoted and delivered in cubic yards, the industry standard is to express volume in that unit: one cubic yard equals 27 cubic feet.
Getting the order quantity right matters more than it might seem. Concrete cannot be stored once batched; if you order too little, the second delivery carries a minimum-order charge and may not match the first load’s color or workability. If you order too much, the surplus has no use and the driver charges for the excess. A 10 percent waste allowance is standard practice for residential flatwork and accounts for slight form deflection, uneven subgrade, and spillage during the pour.
This calculator starts from the dimensions you plan to build and gives you the volume to order. It does not account for reinforcing steel, which displaces a small fraction of the volume, or for changes in mix design.
How to calculate concrete volume for a slab
Formula
cubic yards = (length ft x width ft x thickness in / 12) / 27 x (1 + waste% / 100)
How the formula works
Cubic yards = (length ft x width ft x thickness in / 12) / 27. The thickness is entered in inches and divided by 12 to convert it to feet, giving volume in cubic feet. Dividing by 27 converts cubic feet to cubic yards. Adding a waste multiplier (1 + waste% / 100) scales the net volume up by the allowance percentage.
Unit conversions
If you enter length or width in inches, the calculator divides them by 12 before multiplying. All three dimensions must be in the same linear unit (feet or inches) before the formula applies; the interface handles the conversion when you switch units.
Waste allowance
The waste percentage covers concrete that does not end up in the finished slab: forms that bow slightly outward, a subgrade that has low spots, and concrete that spills or clings to chutes and wheelbarrows. Ten percent is the industry standard for a straightforward residential pour. Use 5 percent for a small, well-formed flatwork project and 15 percent for a complex or irregular shape.
Rounding and ordering
Concrete is sold in increments. Most batch plants round the order to the nearest quarter or tenth of a yard, and minimum orders are typically 1 cubic yard with a short-load fee below that. Treat the result from this calculator as the minimum order quantity before rounding up to the plant’s next increment.
Example
- 1Volume in cubic feet10 x 10 x (4 / 12) = 33.333 cu ft
- 2Convert to cubic yards33.333 / 27 = 1.235 cu yd
- 3Add 10% waste1.235 x 1.10 = 1.358 cu yd to order
FAQ
Why is concrete sold in cubic yards?
Ready-mix concrete in the United States is quoted and delivered by the cubic yard. One cubic yard is 27 cubic feet. Batch plants price by the yard, and trucks are rated by the yard, so the industry settled on that unit. If you have a volume in cubic feet, divide by 27. If you need a metric equivalent, 1 cubic yard is approximately 0.765 cubic meters.
Why add a waste percentage?
The formula gives net volume for a perfect rectangular form. In practice, forms bow slightly, the subgrade is not perfectly flat, and some concrete is lost to spillage during the pour. A 10% allowance is standard for residential flatwork; some contractors use 5% for small, well-formed pours and 15% for irregular or complex shapes. Ordering too little is costly because the second load often has a minimum charge and does not match the first load exactly.
What is the minimum ready-mix order?
Most batch plants have a minimum order, commonly 1 cubic yard, and charge a short-load fee for orders below their minimum. Confirm with your local supplier before ordering. For very small pours, bagged concrete (sold by the 60 lb or 80 lb bag) may be more practical: one 80 lb bag yields about 0.6 cubic feet, so you would need roughly 45 bags per cubic yard.
Does the calculation account for slump or mix design?
No. This calculator gives geometric volume only. Slump (how fluid the mix is), water-to-cement ratio, air entrainment, and admixtures affect workability and strength but do not change the volume you need to fill the form. Specify mix design separately when you order, based on the intended use and local conditions.
Should I enter nominal or actual thickness?
Enter the intended finished thickness at the top of the slab, not the thickness of the form boards. For a standard 4-inch residential slab, enter 4. If the site is sloped and the slab varies, use an average thickness. The result is the volume of concrete in the form, not accounting for reinforcing steel, which displaces a small but usually negligible volume.
Sources
1. Concrete in Practice: Ordering Ready Mixed Concrete
National Ready Mixed Concrete Association (NRMCA)
Describes how ready-mixed concrete is specified, ordered, and delivered in cubic yards; discusses short loads and ordering tolerances.
https://www.nrmca.org/concrete-in-practice/2. ACI 302.1R-15: Guide for Concrete Floor and Slab Construction
American Concrete Institute
Section 4 covers recommended slab thicknesses for residential and commercial floors; referenced for typical thickness values cited in FAQ.
https://www.concrete.org/store/productdetail.aspx?ItemID=302115&Format=DOWNLOAD&Language=English