Calculate cubic yards of concrete for a slab, patio or pad from length, width and thickness. Add a practical allowance and estimate 80-lb bag quantity.
Enter your measurements and material details.
Verify finished slab thickness after base preparation; an extra inch over a large slab materially changes volume.
A slab is a rectangular prism. The calculator converts thickness from inches to feet, multiplies length by width by thickness, converts cubic feet to cubic yards and adds the selected allowance.
Measure inside the forms after the base is prepared whenever possible. If the slab has thickened edges or grade beams, calculate those extra volumes separately rather than averaging them into the slab.
There are 27 cubic feet in one cubic yard.
A 5–10% allowance is common for many small slabs, but form and subgrade accuracy matter.
Yes for rectangular sections with a consistent thickness.
No. Add those as separate volumes.
Roughly 45 at 0.6 cubic feet per bag, but verify the package yield.
Using the example inputs shown in the calculator — Slab length (ft) = 20, Slab width (ft) = 12, Thickness (in) = 4, Extra allowance (%) = 8 — the result is 3.2 cubic yards. This demonstrates the formula with a complete set of numbers rather than a hidden assumption.
The result breakdown also shows Raw slab volume: 2.96 yd³; 80-lb bags: 145. Replace the example values with field measurements and the exact product coverage, yield or package size you plan to buy.
For this concrete slab calculator, the inputs with the greatest practical effect are slab length, slab width and thickness. Recalculate when any of those change; do not reuse a result from a different room, run, layer depth or product specification.
Concrete Slab Calculator is intended to help with quantity planning and arithmetic. It does not inspect the site, choose structural dimensions, determine code compliance or replace manufacturer installation instructions. Conditions such as slope, substrate quality, moisture, soil, framing, access and workmanship can change the final amount required.
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