Turn a slab plan into a ready-mix quantity.
Use this page when the project starts with a patio, floor, walkway, or foundation slab footprint and a specified uniform thickness.
Check the planned thickness before estimating. Loads, soil, frost, reinforcement, joints, and local code determine slab design—not this volume calculation.
Match the estimate to the purchasing decision.
Concrete Slab intentionally uses the same rectangular volume formula as the broad Concrete Calculator, but its workflow emphasizes area and slab thickness. It does not size a slab structurally, count bags, or estimate the full installed cost.
Formula and calculation method.
Slab order volume = length × width × thickness × (1 + waste percentage ÷ 100)The footprint area is length multiplied by width. Multiplying that area by slab thickness produces base volume; unit conversion and the waste allowance produce the order volume.
Follow realistic concrete takeoffs.
Length 20 ft; width 12 ft; thickness 4 in; waste 10%
Working: 20 × 12 × (4 ÷ 12) ÷ 27 × 1.10
Result: 3.26 yd³ to order (2.96 yd³ base)
Interpretation: The estimate models one uniform 4-inch slab and does not include thickened edges or footings.
Length 6 m; width 4 m; thickness 10 cm; waste 8%
Working: 6 × 4 × 0.10 × 1.08
Result: 2.59 m³ to order
Interpretation: Any steps, beams, edge thickenings, or openings should be calculated separately.
Turn the estimate into an order plan.
Small thickness changes affect the entire footprint, so verify thickness rather than using an informal guess.
Round only after combining slab sections, then follow the ready-mix supplier’s order increment.
Assumptions and common mistakes.
What the estimate assumes
- The slab has a rectangular footprint.
- Thickness is uniform across the modeled area.
- Dimensions describe finished concrete.
- The allowance covers volume variation only.
Mistakes to avoid
- Using excavation depth as slab thickness.
- Omitting thickened edges or grade beams.
- Entering inches in a feet field.
- Assuming the calculator recommends a safe slab thickness.
Handle unusual concrete plans explicitly.
Split the plan into non-overlapping rectangles and add their volumes.
Calculate the opening separately and subtract its base volume before deciding on waste.
Model the extra perimeter volume separately so the main slab is not double-counted.
What this estimate leaves out.
This is a volume takeoff, not structural advice. It excludes reinforcement, joints, sub-base, vapor barrier, finishing, pumping, labor, drainage, bearing capacity, and code requirements.
Concrete planning questions.
What slab thickness should I use?
Use the thickness on an approved project plan or confirmed for the site and loads. The calculator does not design thickness.
Why does slab thickness matter so much?
Thickness multiplies the full footprint, so a 25% thickness increase creates a 25% base-volume increase.
Can I calculate an L-shaped patio?
Yes. Divide it into non-overlapping rectangles, calculate each slab section, and add the quantities.
Does this include a thickened slab edge?
Not automatically. Calculate additional edge or beam volume separately.
Is slab volume different from general concrete volume?
The rectangular math is identical; this page provides slab-specific planning context around footprint and thickness.