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How to Calculate Concrete Volume for a Slab

Work out how much concrete you need for a slab: length × width × depth in metres gives cubic metres. A worked example, plus how much to add for wastage.

Scope: Geometry and quantity estimates only; use the adopted code and approved design for requirements.

Worked arithmetic checked . Review method.

The calculation

Multiply length × width × depth, all in metres. The answer comes out in cubic metres (m³), which is how concrete is sold. Before you order a truck or count bags, that is the one number you need.

A slab is just 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 there is 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).
W = 3 mL = 4 mD = 0.1 mV = L × W × D = 1.2 m³
Slab volume — length × width × depth.

This formula covers a rectangular block with a uniform depth. For other shapes or depths, split the pour into parts and add their volumes. Count each part once so that edges and overlaps are not counted twice.

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.

  1. Convert the depth to metres. 100 mm ÷ 1000 = 0.1 m.
  2. 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 dead flat, and the formwork bends. Some concrete slops off the barrow, and a smear always stays stuck in the mixer or the chute. For illustration, here are 5% and 10% allowances:

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

A 10% allowance gives 1.32 m³. Confirm the final order, delivery increments and a suitable allowance with the supplier. Plan the pour with the responsible contractor; timing, conditions and the specified construction method affect joint formation.

Slab not a plain rectangle? A thick edge, a step or an L-shape works the same way. Split it into rectangular blocks, work out each block, and add the volumes together.

Calculate and keep your working

Use the free web tool or download the Chippy Tools printable reference.

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. Agree a suitable allowance with your supplier before ordering.
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). Allow for the actual excavation and losses. A 10% allowance gives 1.32 m³; agree the final order and delivery increments with your supplier.
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?
Agree an allowance with the supplier based on the excavation, formwork and likely losses. The 5% and 10% examples here show the arithmetic, not a fixed ordering rule. Confirm delivery increments and pour timing with the contractor.
Does the calculation change for a thicker slab or footing?
For a rectangular block, use length × width × depth. For a stepped, L-shaped or thick-edge slab, split it into parts and add their volumes. Avoid counting overlaps twice. Other shapes need the matching volume formula.