Formulas
Rectangular (slab, footing, beam, wall): V = L × W × T × n
Round column or pile: V = π/4 × D² × H × n
Volume to order: V × (1 + wastage % ÷ 100)n is the number of identical elements. Keep every dimension in the same unit before multiplying: this is where most errors happen.
Worked example
A 10 m × 10 m ground slab, 150 mm thick:
- Thickness = 150 mm ÷ 1,000 = 0.15 m
- V = 10 × 10 × 0.15 = 15.00 m³
- With 5% wastage = 15.00 × 1.05 = 15.75 m³
- Weight ≈ 15.75 × 2,400 = about 37,800 kg (37.8 t)
Six pad footings, each 1.5 × 1.5 m and 600 mm deep: V = 1.5 × 1.5 × 0.6 × 6 = 8.10 m³.
Concrete per area at common thicknesses
Use this to sense-check a result.
| Thickness | m³ per 10 m² | Thickness | yd³ per 100 ft² |
|---|---|---|---|
| 100 mm | 1.00 | 4 in | 1.23 |
| 125 mm | 1.25 | 5 in | 1.54 |
| 150 mm | 1.50 | 6 in | 1.85 |
| 200 mm | 2.00 | 8 in | 2.47 |
| 250 mm | 2.50 | 10 in | 3.09 |
Common mistakes
- Mixing mm and m. Entering 150 as metres gives 1,000 times the volume. The calculator warns you if a slab looks thinner than 75 mm, which usually means centimetres were typed.
- Ignoring thickenings. Slab edges, drop panels and thickened sections under walls add volume. Calculate them separately and add them.
- Deducting nothing for the beam–slab overlap. When you calculate beams and slabs separately, take the beam depth below the slab only, or you count the overlap twice.
Assumptions
- Dimensions are the finished concrete sizes; the calculator does not deduct reinforcement volume (it is under 2% and is normally ignored).
- Weight uses 2,400 kg/m³ (about 150 lb/ft³) for normal-weight concrete.
- To turn the volume into truck loads, use the concrete truck calculator.