- Cement
- 1,270 kg
- Cement bags
- 26 × 50 kg · 51 × 25 kg
- Sand
- 1.323 m³
- Coarse aggregate
- 2.646 m³
Dry volume = 1.54 × wet volume; cement at 1440 kg/m³. Nominal mixes are for small works — use a designed mix for structural concrete.
How the Concrete Calculator works
Concrete is ordered by volume. This calculator works out the volume of a slab, footing or wall (length × width × thickness) or a round column (π/4 × diameter² × height), multiplies it by the quantity and adds a waste allowance for uneven ground, spillage and formwork bulge.
In US units the result is in cubic yards, with the number of premixed bags needed: an 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. Bags are always rounded up.
In metric units the result is in cubic metres, with the cement, sand and coarse aggregate for site mixing at a nominal ratio by volume. The dry materials take about 1.54 times the wet volume, because the sand fills the gaps in the aggregate and the cement fills the gaps in the sand; cement is taken at 1440 kg/m³.
- Slab, footing or wall
- V = L × W × t
- Round column
- V = π/4 × D² × H
- With waste
- V_order = V × n × (1 + waste/100)
- Cement for a 1 : s : a mix
- C = 1.54 × V × 1/(1 + s + a) × 1440 (kg)
Worked example: concrete and site-mix materials for a 5 m × 4 m slab, 150 mm thick
Given
- Length 5 m, width 4 m, thickness 150 mm
- Waste 5%
- Mix 1 : 1.5 : 3 (M20 nominal)
Steps
- Volume: 5 × 4 × 0.150 = 3.000 m³; with 5% waste, 3.150 m³ (4.12 yd³).
- Dry volume: 1.54 × 3.150 = 4.851 m³, split 1 : 1.5 : 3 over 5.5 parts.
- Cement: 4.851 / 5.5 = 0.882 m³ × 1440 = 1,270 kg, or 26 bags of 50 kg.
- Sand: 4.851 × 1.5 / 5.5 = 1.323 m³. Coarse aggregate: 4.851 × 3 / 5.5 = 2.646 m³.
Result: 3.150 m³ of concrete: 1,270 kg of cement (26 × 50 kg bags), 1.323 m³ of sand and 2.646 m³ of aggregate.
Enter the same values in the calculator above to check each step.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, or 1.23 cubic yards. With 5% waste that is 1.30 yd³, about 59 bags of 80 lb premix.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 ft³, so it takes 45 bags of 80 lb (0.60 ft³ each), 60 bags of 60 lb or 90 bags of 40 lb premixed concrete.
How many bags of cement are needed for 1 m³ of M20 concrete?
For a 1 : 1.5 : 3 nominal mix, the dry volume is 1.54 m³ and cement is 1/5.5 of it: 0.28 m³ × 1440 kg/m³ = 403 kg, about 8 bags of 50 kg. Sand is 0.42 m³ and aggregate 0.84 m³.
How much waste should I allow for concrete?
Allow 5 to 10%. Use the higher end on uneven or soft ground, for thin slabs where a small depth error matters, and for hand-mixed work.
How do I convert cubic metres to cubic yards?
Divide by 0.7646: one cubic yard is 0.7646 m³, so 3.15 m³ is 4.12 yd³. One cubic metre is 1.308 yd³ or 35.3 ft³.