Slab 1
Length: — ft · Width: — ft · Thickness: — in
Slab and patio planner
Concrete Slab Calculator
Calculate concrete for rectangular, round, L-shaped, and thickened-edge slabs without hiding waste or rounding.
Project
Formula
Length, width, and thickness stay separate
Rectangular slab volume = length × width × thickness. Thickness is converted from inches to feet before cubic feet are divided by 27. A circle uses π × radius² × thickness. A thickened edge adds the extra edge volume instead of averaging it into the slab.
Common mistake
Measuring the excavation instead of the concrete
Gravel, sand, and overdig are not part of the slab volume. Measure the inside of the forms and the finished concrete thickness. If the edge is thicker, enter that edge on its own rather than increasing the whole slab.
Worked quantities
Common slab sizes at the default allowance
Waste is shown as its own yardage, then the order rounds up to the next 0.25 yd³. A 10 × 10 ft, 4 in slab is the small patio case; the last row adds a thickened edge to that same footprint.
| Slab | Measured yd³ | Waste yd³ | Order yd³ |
|---|---|---|---|
| 10 × 10 ft, 4 in | 1.23 | 0.12 | 1.50 |
| 12 × 12 ft, 4 in | 1.78 | 0.18 | 2.00 |
| 12 × 24 ft, 4 in | 3.56 | 0.36 | 4.00 |
| 20 × 20 ft, 4 in | 4.94 | 0.49 | 5.50 |
| 12 ft diameter circle, 4 in | 1.40 | 0.14 | 1.75 |
| 10 × 10 ft with a thickened edge | 2.12 | 0.21 | 2.50 |
Field check
Do not average a changing thickness
A thickened edge or a slab with distinct depth zones should be entered as separate geometry. Averaging depths can hide where the extra concrete comes from and makes the estimate harder to verify.
Field check
Check the formed dimensions
Measure the inside length and width of the forms after they are set. For a circular slab, use the finished diameter rather than the circumference or the size of the excavated area.
Field check
Keep base material out of the slab volume
Compacted stone, sand, insulation, and vapor barriers affect preparation but are not concrete. Enter only the finished concrete thickness required for the project.
Questions
What the slab calculator answers
Which slab shapes are supported?
Rectangle, circle, L-shape, and a thickened-edge estimate.
Why is the order number higher than the calculated volume?
The result shows waste and supplier rounding as separate additions.
Does the calculator decide slab thickness?
No. Enter the thickness required for your project and local conditions.
Next step
Count bags or price the ready-mix order
Bag count for this slab · Strip footing calculator · How to order ready-mix
Bag yields follow the cited manufacturer data. See sources and review dates.