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How to calculate concrete volume and formwork area.
Concrete is measured in cubic metres. Formwork, the temporary mould, is measured in square metres of the concrete surface it touches. Both come straight from the drawing dimensions.
Formulas by member
| Member | Concrete volume | Formwork area |
|---|---|---|
| Slab | length × width × thickness | underside (length × width) plus exposed edges (perimeter × thickness) |
| Beam | width × depth × length | soffit (width × length) plus two sides ((depth − slab thickness) × length × 2) |
| Column | width × depth × height | perimeter × height |
| Strip footing or ground beam | width × depth × length | two sides (depth × length × 2) |
| Pile cap | length × width × depth | four sides (perimeter × depth) |
| Bored pile | π × diameter² ÷ 4 × length | none, cast against the ground |
Worked example
Slab 6 m × 6 m × 0.20 m thick: volume 7.2 m³. Underside 36.0 m². Edges, if all four are exposed: 4 × 6 × 0.2 = 4.8 m².
Beam 0.30 m wide × 0.60 m deep × 6 m long, framing into the slab: volume 1.08 m³. Soffit 0.3 × 6 = 1.8 m². Two sides below the slab: 2 × (0.6 − 0.2) × 6 = 4.8 m². Formwork 6.6 m².
Column 0.30 m × 0.30 m × 3 m high: volume 0.27 m³. Formwork 4 × 0.3 × 3 = 3.6 m².
Where take-off mistakes happen
- Double counting. Where a beam meets a slab, or a column meets a beam, measure the concrete once and deduct the overlap.
- Wrong beam depth. Use the depth below the slab for side formwork, since the slab already forms the top.
- Missing edges. Slab edges, upstands and parapets need their own formwork.
- Mixed units. Convert millimetres to metres before multiplying.
From concrete to steel
Once volumes are known, the reinforcement tonnage follows from kilograms per cubic metre. See the rebar weight chart for bar weights and planning figures.
Guides by TapeLine, quantity take-off software from Natha & Co, founded by Natha Mohamed M.M.
Try Free Beta Version See the 3D take-off
Keep reading
Rebar weight chart: kg per metre and metres per tonne
Weight per metre for 6 to 40 mm bars, the d² ÷ 162.2 formula and a worked conversion to tonnes.
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