How to Calculate Concrete Volume for a Slab

The requirement

Concrete is sold by volume — the cubic metre (m³). So before you order a truck or count bags, you need one number: how much space are you filling?

For a slab, that space is a box. Length, width, and depth. Multiply the three together and you have your volume:

Volume = Length × Width × Depth

The only trick is units. Do the whole sum in metres and the answer comes out in cubic metres — nothing to convert. Slab thickness is nearly always given in millimetres, so that’s the one you change: divide by 1000. A 100 mm slab is 0.1 m deep. A 150 mm slab is 0.15 m.

Length (L) — the longer side, in metres.
Width (W) — the shorter side, in metres.
Depth (D) — slab thickness, in metres (mm ÷ 1000).

Slab volume — length × width × depth.

That’s the whole method. Every slab, footing pad, path or driveway is the same three numbers — the shapes just get bigger or thicker.

Work it out by hand

Say you’re pouring a slab for a garden shed. It measures 4 m long × 3 m wide, and you’re going 100 mm thick.

Convert the depth to metres. 100 mm ÷ 1000 = 0.1 m.
Multiply the three dimensions. 4 × 3 × 0.1 = 1.2 m³.

So the slab itself needs 1.2 cubic metres of concrete.

Now the part that catches people out: order a bit more than that. The ground under a slab is never perfectly flat, the formwork bends, some concrete slops off the barrow, and a smear always stays stuck in the mixer or the chute. Add 5–10% on top:

1.2 m³ + 5% = 1.26 m³
1.2 m³ + 10% = 1.32 m³

Round up to what you can actually order. With ready-mix, call it 1.3 m³ and you won’t run short halfway through. That matters: a slab poured in two loads an hour apart has a weak line — a cold joint — through the middle of it. Running short is a far worse problem than a little left over.

If your slab isn’t a plain rectangle — a thick edge, a step, an L-shape — split it into rectangular blocks, work out each block the same way, and add the volumes together.

Do it in 5 seconds

Pillar: Concrete Calculations: The Complete On-Site Guide — every concrete sum in one place.
Next step: How Many Bags of Concrete Per m³? — once you know your volume, turn it into 20 kg, 25 kg or 40 kg bags (the amount each bag makes varies by product).

Sources

Slab volume is just sums, not rules — Volume = Length × Width × Depth, worked in metres for an answer in cubic metres. There’s no code number to cite here. The amount each bag makes, and mix ratios, do change by product and brand — check the bag for the yield before you turn cubic metres into bags.

Frequently asked questions

How do I calculate concrete for a slab?

Multiply length × width × depth. Use metres for all three and the answer comes out in cubic metres. A 4 m × 3 m slab at 100 mm (0.1 m) thick is 4 × 3 × 0.1 = 1.2 m³. Then add 5–10% for spillage before you order.

How much concrete do I need for a 4 m × 3 m slab?

At 100 mm thick it needs 1.2 m³ of concrete (4 × 3 × 0.1). If you order ready-mix, add 5–10% for spillage and uneven ground, then round up — call it about 1.3 m³ — so you don't run short partway through the pour.

How do I convert slab thickness in millimetres to metres?

Divide the millimetre number by 1000. So 100 mm is 0.1 m, 125 mm is 0.125 m and 150 mm is 0.15 m. Get every measurement into metres before you multiply, and your volume comes out in cubic metres with nothing left to convert.

How much extra concrete should I order for wastage?

Add about 5–10% on top of the volume you worked out. Uneven ground, digging too deep, spillage, and what sticks in the barrow and chute all eat into an exact order. Concrete waits for no one, so it's better to have a little spare than to run short partway through and end up with a weak line — a cold joint — through the slab.

Does the calculation change for a thicker slab or footing?

No — it's always length × width × depth. A thicker slab or footing just uses a bigger depth number; the method is the same. For a stepped, L-shaped or thick-edge slab, split it into rectangular blocks, work out each one, and add the volumes together.