Formula
How this estimate works
cubic feet = length x width x thickness in feet; cubic yards = cubic feet / 27
A 24 ft by 12 ft slab at 4 in thick with 10% waste needs about 3.91 cubic yards.
Worked example
Plan a 24 ft by 12 ft slab
The slab calculator is tuned for flat pours where thickness drives volume. It is useful for patios, shed slabs, walkways, and similar projects.
- Enter the finished slab length, width, and thickness.
- Add waste if the subgrade is uneven or the slab has thickened edges.
- Compare the rounded bag count with local ready-mix minimums before buying.
Concrete slab estimate basics
A slab estimate starts with the flat footprint and finished thickness. Convert the thickness from inches to feet, multiply by length and width, then divide by 27 for cubic yards.
When slab thickness changes the order
Small changes in thickness can add a surprising amount of concrete. A 4 inch slab and a 6 inch slab with the same footprint are very different orders, so measure the planned finished thickness carefully.
Common estimating mistakes
- Measuring excavation depth instead of finished concrete thickness.
- Leaving out thickened edges, turndowns, steps, or grade beams.
- Ordering exactly the calculated amount with no buffer for subgrade variation.
Assumptions used
- Normal-weight concrete is estimated at 150 lb per cubic foot.
- Premix bag yield is estimated from common 40, 50, 60, and 80 lb bag sizes.
- Waste factor is applied after the base volume is calculated.
Before you order materials
- Confirm finished slab thickness before ordering.
- Account for thickened edges or grade beams separately.
- Round up for uneven subgrade and form variation.
Frequently asked questions
How thick should a concrete slab be?
Many patios and walkways use around 4 inches, while driveways and structural slabs may require more. Confirm the design before ordering.
Should I add waste for a slab?
Yes. A 5% to 10% buffer is common for uneven subgrade, spills, and measurement uncertainty.
Can I use bags for a slab?
You can, but large slabs often require many bags. Compare bag count with local ready-mix delivery options.