How to use the concrete calculator
- Choose slab for floors, patios, driveways, paths and footings, or round column for posts, piers and post holes.
- Pick metric or imperial units and enter the size. Add how many you need and an extra percentage for waste and spillage. 5% to 10% is common.
- You get the volume in cubic metres, cubic feet and cubic yards, the number of premix bags, and the cement, sand and aggregate for a site-mixed batch.
Concrete volume formulas
Slab volume = Length × Width × Thickness
Round column volume = π × (Diameter ÷ 2)² × Height
Cubic yards = Cubic feet ÷ 27
Always convert to the same unit first: a 10 cm slab is 0.10 m thick, and a 4-inch slab is 4 ÷ 12 = 0.333 ft thick.
Worked examples
Metric: a 4 m × 3 m patio, 10 cm thick, with 10% extra: 4 × 3 × 0.10 = 1.2 m³, plus 10% = 1.32 m³. As an M20 site mix (1 : 1.5 : 3) that's 11 bags of cement (10.64 bags exactly), 0.55 m³ of sand and 1.11 m³ of aggregate.
Imperial: a 10 ft × 10 ft slab, 4 inches thick: 10 × 10 × 0.333 = 33.33 ft³ = 1.23 cubic yards, or 56 bags of 80 lb premix (0.60 ft³ each).
Cement, sand and aggregate
For site-mixed concrete, the calculator uses the standard nominal-mix method used across India and South Asia: the dry materials take up about 1.54 times the volume of the finished concrete, which is split by the mix ratio. Cement is counted at 1,440 kg per m³ in 50 kg bags.
| Mix | Cement : sand : aggregate | Cement per m³ | Typical use |
|---|---|---|---|
| M25 | 1 : 1 : 2 | about 11.1 bags | Heavier-duty slabs and beams |
| M20 | 1 : 1.5 : 3 | about 8.1 bags | Slabs, beams and columns in houses |
| M15 | 1 : 2 : 4 | about 6.3 bags | Floors, paths and non-structural work |
| M10 | 1 : 3 : 6 | about 4.4 bags | Levelling and base (blinding) layers |
Frequently asked questions
How many bags of concrete do I need for a cubic yard?
About 45 bags of 80 lb premix (0.60 ft³ each), 60 bags of 60 lb or 90 bags of 40 lb. A cubic yard is 27 cubic feet.
How much extra should I order?
5% to 10% covers spillage, uneven ground and forms that bow slightly. Uneven sub-bases may need more.
Is this a structural design?
No. It estimates quantities. Foundations, beams and anything load-bearing should be designed by a qualified engineer, who will specify the mix and any reinforcement.
Sources
- Quikrete: product yields printed on bags
- Bureau of Indian Standards, IS 456:2000, Plain and Reinforced Concrete: Code of Practice
Formulas checked 29 October 2026. How we test our calculators. Measurements are estimates. Add a margin for waste and check with your supplier before you buy.