Concrete Calculator

How the concrete calculator works

For a slab, path, footing or pad with straight sides, the volume of concrete is simply length × width × depth:

Volume (m³) = length (m) × width (m) × depth (m)

Every input is in metres, and that includes the depth. A 100 mm slab is 0.1 m, and a 75 mm path is 0.075 m. Entering "100" for depth is the most common mistake on this calculator, and it makes the answer 1,000 times too big.

Worked example: a 6 m × 4 m shed slab

1. Length 6 m, width 4 m, depth 100 mm = 0.1 m
2. 6 × 4 × 0.1 = 2.4 m³
3. Add a waste allowance of about 5–10% for uneven sub-grade, formwork bulging and what stays in the barrow: 2.4 × 1.08 ≈ 2.6 m³

I would order 2.6 m³. Running short halfway through a pour leaves a cold joint, which is far worse than paying for a little extra. Most suppliers deliver in 0.2 m³ steps and charge a short-load fee under a minimum (often 3 m³), so it's worth asking about that when you book.

Ready-mix truck or bags?

As a rough guide, a 20 kg bag of premix makes about 0.01 m³ once mixed, so you need roughly 100–110 bags per cubic metre. Check the yield printed on your brand. Beyond about 0.5 m³, bags become hard work and a small truck or a mini-mix delivery usually makes more sense.

Choosing the depth

• Garden paths and light shed slabs are often 75–100 mm.
• Driveways for cars are commonly 100–125 mm with reinforcing mesh.
• House slabs, footings and anything structural should be designed by an engineer to your local standard (in Australia, AS 2870 for residential slabs and footings). Don't size those from a calculator.

Other shapes

Break an irregular area into rectangles, work out each one, and add them together. For round columns or piers, use volume = π × radius² × height. Our round column concrete calculator does that for you. For a strip footing, use the trench length × width × depth.

Common mistakes

• Entering depth in millimetres instead of metres.
• Measuring to the outside of the formwork when the boxing is set inside the area, or the other way round.
• Forgetting thickenings: edge beams and internal beams under a slab can add a lot of volume.
• Not allowing for a sub-grade that dips below the design level.

Frequently asked questions

How many cubic metres of concrete do I need for a 10 m² slab?

At 100 mm thick it is 10 × 0.1 = 1.0 m³, so order about 1.1 m³ with waste.

How do I convert cubic metres to cubic yards?

Multiply by 1.308. For example, 2.4 m³ ≈ 3.14 yd³.

Does reinforcement reduce the concrete volume?

Not in any way that matters. Steel mesh or bar takes up well under 1–2% of the volume, so ignore it for ordering.

Written by Sam Alex, civil engineer. About the author