Concrete Quantity & Mix Calculator

Before pouring a slab, footing or column you need to know how much concrete to order, or how much cement, sand and aggregate to buy for site mixing. This calculator works out the wet volume from dimensions, converts it to dry material volume and splits it by the mix ratio.

How to use it

1. Enter the length, width and depth of the member, or the total volume.
2. Choose the grade, such as M15 (1:2:4), M20 (1:1.5:3) or M25 (1:1:2).
3. Read the cement in bags, sand and coarse aggregate in m³.

The method

Dry volume = wet volume × 1.54, an allowance for voids and shrinkage. For a mix of 1:a:b, total parts = 1 + a + b. Cement volume = dry volume / total parts, and bags = cement volume / 0.0347. Sand = cement volume × a, aggregate = cement volume × b.

Worked example

A slab 5 m × 4 m × 0.15 m = 3.0 m³ in M20 (1:1.5:3). Dry volume = 3.0 × 1.54 = 4.62 m³. Total parts = 5.5. Cement = 4.62 / 5.5 = 0.84 m³ = 24.2 bags. Sand = 0.84 × 1.5 = 1.26 m³. Aggregate = 0.84 × 3 = 2.52 m³.

Notes

• Nominal mixes are acceptable for small works. For M25 and above, design mixes based on trial batches are recommended and often required.
• Add 2 to 5% for wastage and uneven formwork.
• The water-cement ratio, typically 0.45 to 0.55, controls strength more than the cement quantity. Do not add extra water for workability.
• Ready-mix concrete is ordered by wet volume, so order the calculated volume plus a small margin.