This page explains the measurement or planning assumption behind the result. When you need the project quantity itself, use Concrete Calculator.
Why this planning detail matters
A slab estimate is a three-dimensional measurement. Floor square footage gives only the footprint; concrete quantity also depends on thickness. Treating thickness as a separate measured or specified dimension makes the calculation auditable and helps prevent the common error of ordering from square footage alone.
How to measure or choose the starting value
Measure the inside dimensions of the forms or the planned slab boundaries. Record the specified thickness. For L-shaped slabs, split the footprint into rectangles. For thickened edges, steps or pads, calculate the extra volumes as separate prisms and add them. Avoid multiplying the maximum overall length and width when that rectangle includes areas that will not contain concrete.
Adjustments that change the estimate
Ground irregularity, form movement, slope and excavation tolerance can change actual volume. Penetrations and embedded items may displace some concrete, but small displacements are often minor compared with field variation. Structural features such as footings should be included only through their actual geometry and project design.
How to use the number before you buy
After the geometric volume is complete, convert to the supplier’s order unit. For ready-mix, that is often cubic yards in the US. For bagged concrete, use bag yield. Keep any order allowance explicit so the base geometry can still be checked later.
Planning workflow
Sketch the slab footprint
Break irregular outlines into simple shapes.
Measure length and width
Use formed or planned dimensions.
Enter specified thickness
Convert thickness to compatible units.
Add separate thickened sections
Avoid double-counting overlapping volume.
Example: 10 × 6 × 4 in slab
Convert 4 in to 0.333 ft, then 10 × 6 × 0.333 ≈ 20 cu ft. Dividing by 27 gives about 0.74 cu yd before any ordering allowance.
Reference table
| Planning item | Useful starting point | What to verify |
|---|---|---|
| Rectangular slab | L × W × T | Use compatible units. |
| L-shaped slab | sum rectangles × T | Avoid overlapping rectangles. |
| Thickened edge | additional cross-section × length | Subtract overlap with base slab if necessary. |
Frequently asked questions
Can I order concrete from square footage alone?
No. Volume requires thickness in addition to floor area.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard.
Should footings be included with the slab?
Include them as separate designed volumes if they are part of the pour; do not guess structural dimensions.
Planning note: Product coverage, yield, package size, installation instructions and project requirements take priority over generic examples. Verify the exact product and project before purchasing.