To estimate concrete for a rectangular slab, multiply its length by its width by its thickness. The three measurements must use the same unit. Convert the result into cubic yards or cubic metres before ordering.
Quick answer: Slab volume = length × width × thickness. In US customary units, 1 cubic yard = 27 cubic feet.
Concrete slab volume formula
Volume (ft3) = length (ft) x width (ft) x thickness (ft)
If slab thickness is given in inches, divide it by 12 to convert it to feet. To convert cubic feet to cubic yards, divide by 27.
Worked example: a 20 ft by 12 ft slab, 4 inches thick
- Convert thickness: 4 inches ÷ 12 = 0.3333 feet.
- Calculate cubic feet: 20 × 12 × 0.3333 = approximately 80 ft³.
- Convert to cubic yards: 80 ÷ 27 = approximately 2.96 yd³.
The geometric volume is about 2.96 cubic yards, before any allowance for site conditions or waste.
Should you order extra concrete?
A job may need more than the geometric volume because of uneven subgrade, spillage, over-excavation, formwork variation, and other site conditions. An allowance such as 5% or 10% is sometimes used for estimating, but the correct amount depends on the project and should be confirmed with the concrete supplier or contractor.
For illustration, adding 10% to 2.96 yd³ gives about 3.26 yd³. This is an example allowance, not a universal ordering rule. The American Concrete Institute’s concrete guide explains the need to convert thickness to the same length unit, multiply the dimensions, divide cubic feet by 27 for cubic yards, and consider a suitable allowance for site conditions and waste. See the ACI concrete calculation guidance.
Metric example
For a slab 6 metres long, 4 metres wide, and 0.10 metres thick, volume = 6 × 4 × 0.10 = 2.4 m³. Keep all measurements in metres for a cubic-metre result.
Common mistakes when estimating concrete
- Mixing inches with feet without converting thickness.
- Using the slab’s area as if it were volume; thickness is essential.
- Dividing by 27 when the result is already in cubic metres.
- Forgetting steps, thickened edges, footings, openings, or irregular geometry.
- Treating a rough online estimate as an engineered quantity or a supplier order.
Calculate your slab volume
Use the Calclio Concrete Calculator for a rectangular slab estimate. Enter the slab length, width, and thickness, and confirm units before interpreting the result.
Frequently asked questions
How many cubic feet are in one cubic yard?
There are 27 cubic feet in one cubic yard.
Is 4 inches a standard slab thickness?
Thickness depends on the intended use, loads, soil, reinforcement, local codes, and design. Do not select slab thickness from this example; follow project specifications.
Can this formula calculate a circular slab?
No. A circular slab needs a different formula: pi × radius squared × thickness. The current Calclio tool is intended for a rectangular slab.
This is a quantity estimate, not construction or engineering advice. Confirm design, reinforcement, base preparation, local requirements, and order quantities with a qualified professional.