Concrete Calculator
Concrete volume, cement bags, sand and aggregate
Concrete Needed
Working
- Wet volume = 2.5 m³ × 1 + 5% wastage = 2.625 m³
- Dry volume = 2.625 × 1.54 = 4.043 m³
- Mix 1 : 1.5 : 3 (5.5 parts): cement = 4.043 × 1/5.5 = 0.735 m³ ÷ 0.0347 = 21.2 bags
- 1Choose the shapeSlab, footing, beam or column
- 2Enter the sizeIn metres, feet, mm or inches
- 3Get the materialsVolume, cement bags, sand, aggregate
How is concrete quantity calculated?
Work out the wet volume, convert to dry volume (materials bulk up and shrink when mixed), then split it by the mix ratio:
Wet volume = length × width × thickness
Dry volume = wet volume × 1.54
Cement = dry volume × cement share ÷ 0.0347 m³ per bag
1 m³ = 35.31 cft
Which grade to use
M20 is the usual minimum for RCC slabs, beams and columns in homes. M10 and M15 are for plain concrete (PCC) such as levelling beds under footings and floors. For big or high-rise work, use a design mix from a structural engineer.
Ready-mix concrete
For volumes over about 3 m³, ready-mix (RMC) is often more consistent and less work. Order the wet volume in m³ plus 5–10% extra.
Related calculators
Frequently asked questions
How many cement bags are needed for 1 m³ of M20 concrete?
About 8 bags (50 kg each), with roughly 15 cft of sand and 30 cft of 20 mm aggregate.
Why multiply by 1.54?
Dry cement, sand and aggregate have air gaps that fill up when mixed with water, so you need about 54% more dry material than the finished wet volume.
How thick should a house roof slab be?
Commonly 125–150 mm (5–6 inches) for residential RCC slabs, but always follow your structural drawings.
How much cement is needed for a 1,000 sq ft slab?
A 5-inch (125 mm) M20 slab of 1,000 sq ft takes about 11.8 m³ of concrete, or roughly 95 bags of cement, plus 5–10% extra for wastage.
Sources
Last reviewed: October 2026