How to calculate concrete volume
Concrete is ordered and priced by volume. For any rectangular element, multiply its three dimensions in the same unit, then multiply by the number of identical elements.
Circular column: V = π × (d ÷ 2)² × H × n
Add a wastage allowance for spillage, uneven sub-base and over-break. Most site engineers use 3–5% for slabs and beams and up to 10% for footings cast against excavated ground.
How cement, sand and aggregate are worked out
Wet concrete loses volume when the voids between particles fill with paste. To get the dry materials, multiply the wet volume by a dry volume factor of 1.54, then split it by the mix ratio.
- Dry volume = wet volume × 1.54
- Sum of ratio parts, for example M20 = 1 + 1.5 + 3 = 5.5
- Cement volume = dry volume × 1 ÷ 5.5, then × 1,440 kg/m³ for mass
- Bags = cement mass ÷ 50 kg
- Sand and aggregate follow the same split using their ratio parts
Worked example
A 10 m × 6 m slab, 200 mm thick, in M20 with 5% wastage: 10 × 6 × 0.2 = 12 m³, plus 5% = 12.6 m³. Dry volume is 12.6 × 1.54 = 19.40 m³. Cement is 19.40 ÷ 5.5 × 1,440 = 5,080 kg, or about 102 bags. Sand is 5.29 m³ and aggregate 10.58 m³.
Typical thicknesses
| Element | Typical depth | Common grade |
|---|---|---|
| Blinding / lean concrete | 50–75 mm | M10 |
| Ground floor slab on grade | 100–150 mm | M20 |
| Suspended floor slab | 150–250 mm | M25 and above |
| Isolated footing | 300–600 mm | M25 and above |
Structural grades above M25 are designed mixes (by weight) from a ready-mix supplier, not nominal volume ratios. Always follow the project specification and the structural engineer's drawings.
Frequently asked questions
How many bags of cement are in 1 m³ of M20 concrete?
About 8 bags of 50 kg. The dry volume is 1.54 m³, cement is one part in 5.5, giving 0.28 m³ × 1,440 kg/m³ ≈ 403 kg.
Why multiply by 1.54?
Loose dry materials contain air voids and bulk when stacked. Around 54% extra dry material is needed to produce one cubic metre of compacted wet concrete.
How do I convert m³ to cubic yards?
Multiply cubic metres by 1.308. One cubic yard is 0.7646 m³ or 27 ft³.
How much wastage should I add?
3–5% for formed slabs and beams, 5–10% for footings and pile caps cast against earth, and more for small, scattered pours.