Cement calculator
Enter the size of your slab, footing or wall and pick a mix ratio. You’ll get the cement bags, sand and aggregate needed, in metric and imperial units.
How this cement calculator works
This tool estimates concrete material quantities the same way a site engineer would: it takes the wet volume of your slab, wall or footing, converts it to a dry volume, then splits that dry volume across cement, sand and aggregate according to your chosen mix ratio.
Step 1 — wet volume
All three dimensions are converted to meters internally, so you can mix units freely — length in feet, thickness in inches, it doesn’t matter.
Step 2 — dry volume
Concrete shrinks once cement, sand and aggregate are mixed with water, since dry materials settle into the gaps between each other. A multiplier of roughly 1.54 is the standard allowance used on site to account for this, and your wastage percentage is added on top of it.
Step 3 — split by ratio
For a ratio of 1 : 2 : 4, the total is 7 parts. Cement takes 1/7 of the dry volume, sand takes 2/7, and aggregate takes 4/7. Cement volume is then converted to weight using a density of 1440 kg per cubic meter, and divided by 50 kg to get bag count — the standard bag size in most markets. Sand and aggregate are converted to weight using bulk densities of 1600 kg/m³ and 1450 kg/m³.
Choosing a mix ratio
The right ratio depends on what you’re building. Higher-strength mixes use more cement relative to sand and aggregate.
| Grade | Ratio (C:S:A) | Typical use |
|---|---|---|
| M7.5 | 1 : 4 : 8 | Lean concrete, leveling course |
| M10 | 1 : 3 : 6 | PCC, flooring base |
| M15 | 1 : 2 : 4 | Footings, small slabs, driveways |
| M20 | 1 : 1.5 : 3 | General RCC — slabs, beams |
| M25 | 1 : 1 : 2 | Columns, load-bearing beams |
If your project specifies a design mix (common on larger jobs), use the exact ratio from your structural drawing rather than these general figures.
Why the numbers are an estimate, not a guarantee
Site conditions vary. Sand moisture content, aggregate size, and how tightly materials are packed during transport all shift the real quantity you’ll use. Builders commonly buy 5–10% more than the calculated figure, which is what the wastage field above accounts for. Treat the output as a solid planning number for ordering material, not a substitute for a quantity surveyor’s take-off on structural work.
Frequently asked questions
- How many bags of cement do I need per cubic meter of concrete?
- For an M20 mix (1:1.5:3), roughly 6.4 bags of 50 kg cement are needed per cubic meter of finished concrete. This number rises for richer mixes like M25 and falls for leaner mixes like M10.
- Why is dry volume larger than wet volume?
- Dry cement, sand and aggregate have air gaps between particles. Once mixed and compacted with water, those gaps close up and the volume reduces — so you need more dry material upfront than the final concrete volume suggests.
- Can I use this for plastering or brickwork instead of concrete?
- The same wet-to-dry volume logic applies, but plaster and mortar typically use a two-part ratio (cement:sand) rather than three, and a different thickness range. This calculator is built around a three-part concrete mix.
- What thickness should I use for a driveway or footpath slab?
- Most residential driveways use 100–150 mm (4–6 in), and footpaths 75–100 mm (3–4 in), though local codes and subgrade conditions can change this.