Governing Formula
The concrete volume for any uniform rectangular slab or prism is the product of its three orthogonal dimensions. This forms the geometric basis for every ordering and batching decision on site.
V = L × W × H Where:
-
L= Slab length [m] -
W= Slab width [m] -
H= Slab depth (thickness) [m] -
V= Theoretical concrete volume (V = L × W × H) [m³]
Derived Equations:
V_net = SUM(item volumes) V_supply = V_net + V_net × (wastage / 100) How the Calculation Works
Each schedule row is measured from the element's own geometry. Slabs, beams, walls and curbs reduce to V = L × W × H; footings to V = L × W × D; rectangular columns to V = A × H; and circular columns to V = (π / 4) × D² × H. Every row is calculated by the same engine that runs the single-element calculators, so a row here and a standalone calculation always agree.
The worksheet sums the valid rows into a net volume and groups them into subtotals by element category. A wastage margin you enter is then added to produce the supply volume used for ordering. The margin defaults to 0% — nothing is added unless you ask for it.
Worked Engineering Example
Design Scenario: House Ground Slab
A resident engineer must order ready-mix concrete for a 6 m × 4 m ground slab with a design thickness of 150 mm, allowing a 5% wastage margin.
- Compute the net volume of the slab row:
V = 6 m × 4 m × 0.15 m = 3.60 m³ - Add wastage (5%):
V_supply = 3.60 × (1 + 5/100) = 3.78 m³ - Build the full schedule: four 1.2 m × 1.2 m × 0.3 m pad footings add
4 × 0.432 = 1.728 m³, giving a net schedule total of 5.328 m³ and, at the same 5% margin, a supply volume of 5.594 m³ to order.
Engineering Notes & Design Benchmarks
| Application | Typical Thickness | Typical Mix |
|---|---|---|
| House ground slab | 100 – 150 mm | 1:2:4 (C25 / M25 equivalent) |
| Driveways / yards | 150 – 200 mm | 1:2:4 reinforced mesh |
| Heavy equipment pads | 200 – 300 mm | Designed mix with admixtures |
| Footpaths | 75 – 100 mm | 1:3:6 (lean concrete) |
Assumptions & Limitations
Engineering Assumptions:
- Uniform dimensions across each individual element.
- Wastage is applied linearly on the net schedule total, and defaults to 0%.
Design Limitations:
- Intersections between members are not deducted. A beam meeting a column is counted twice, so the net total is an upper bound on placed concrete.
- Openings, recesses, formwork bulking, and reinforcement displacement are not modelled.
- Does not include formwork, blinding, or finishing operation quantities.
- For structural-critical pours, moisture content of aggregates and admixture dosages must be adjusted by the batching plant.
Frequently Asked Questions
Does this worksheet deduct where members intersect?
No. Each row is measured independently and the rows are simply added, so a beam that crosses a column is counted in both rows. Treat the net total as an upper bound and deduct intersections yourself if your specification requires a strict take-off.
What wastage margin should I allow when ordering ready-mix?
A common allowance is 5% to 10%. Small irregular pours, formwork tolerances, and slab thickening near edges justify the higher end. Ordering exactly the theoretical volume risks running short 8 m³ minimum-charge trucks. The worksheet defaults to 0% and only adds what you enter.
Should I subtract the volume of steel reinforcement?
For estimating purposes, reinforcement steel (typically 80–120 kg/m³) displaces a negligible percentage of concrete volume and is normally ignored. Structural detailing, however, must ensure bar clearance and cover whether or not the volume is adjusted.
Engineering Disclaimer
Engineering Note: This calculator provides simplified quantity estimates for preliminary planning and educational use. Final concrete supply quantities should be confirmed with the ready-mix supplier, which accounts for mix shrinkage, aggregate moisture, slump, and local batching practice.
Technical References
- ACI 211.1: Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, American Concrete Institute.
- ACI 117: Specification for Tolerances for Concrete Construction and Materials, American Concrete Institute.
- EN 206: Concrete – Specification, Performance, Production and Conformity, European Committee for Standardization.