Taking off concrete volumes without mis-ordering the pour
Where concrete volume actually goes — including the parts that are habitually forgotten — and how wastage and batching affect the order.
The engineering question
How much concrete does a structure really need, and which quantities are routinely missed between the drawing and the delivery ticket?
Worked example
The figures below were produced by running the Slab Concrete Volume Calculator with the same inputs .
These figures are worked by the published method rather than executed at build time, because this tool is a self-contained page calculator without a programmatic engine.
- Plan area multiplied by thickness gives the structural volume.
- Add the projections, thickening and wastage that the drawing does not draw.
- Order in whole delivery units and check a part-load is not being paid for at full-load rate.
Sanity check
A 12 × 8 m slab at 200 mm is 19.2 m³. If your takeoff says otherwise, the units have been mixed.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- BS 8500 — specification for the supply and placing of concrete
Related problems
Designing formwork for the pressure of fresh concrete, not hardened concrete
Why formwork is designed for placement rate and temperature rather than the weight of the finished slab, and where the hydrostatic cap applies.
Also: Civil & Structural
Staircase concrete volume and how it splits up
Waist slab, steps and landing as separate volumes, and why the pour is usually smaller than people expect.