How concrete volume is calculated for a slab
Knowing how much concrete a slab takes helps you understand an estimate and talk with your contractor and supplier. The math is simple; the key is using the right units.
The formula
Volume = length × width × thickness, with all three measurements in the same unit.
- In feet: thickness is usually given in inches, so divide it by 12.
- From cubic feet to cubic yards: divide by 27 (1 yd³ = 3 × 3 × 3 ft).
- In meters: m² × thickness in meters = m³.
Example: a 20 × 30 ft slab, 4 inches thick
- Area: 20 × 30 = 600 ft².
- Thickness: 4 ÷ 12 = 0.3333 ft.
- Volume: 600 × 0.3333 = 200 ft³.
- Yards: 200 ÷ 27 = 7.4074 yd³.
- With 5% waste: 7.4074 × 1.05 = 7.7778 yd³.
These are the same numbers the estimator gives, and it also shows the formulas on screen.
Footings and columns
Same method, multiplied by the quantity. Six 2 × 2 ft footings, 1 ft deep: 2 × 2 × 1 × 6 = 24 ft³ ≈ 0.89 yd³. Four 1 × 1 ft columns, 9 ft tall: 1 × 1 × 9 × 4 = 36 ft³ ≈ 1.33 yd³.
Why add waste?
The ground is never perfectly level, forms bulge slightly and some concrete always stays in the truck or pump. That’s why you order a little more. The estimator’s 5% is an editable default, not a rule: your contractor adjusts it to the job.
What this calculation does NOT decide
When you place the order
Ready-mix suppliers usually sell by the cubic yard. Confirm with your supplier the unit, the minimum order, the specified strength and whether the price includes pumping.
The online estimate is free and preliminary. The official quote, with a visit from a Cruz General Contractors professional to measure and inspect, costs $25. That $25 is credited to the job total when the work is done with Cruz General Contractors. Nothing is charged on this site.