How is the volume calculated?

Length times width times thickness, with thickness converted from centimetres to metres. A 4 × 3 m slab 10 cm thick is 1.2 m³.

Why add a waste allowance?

Some concrete is lost to spillage, uneven ground, and over-filling forms. A 10% allowance is a common rule of thumb; increase it for rough sub-grade.

How many bags will I need?

The estimate assumes a 25 kg bag yields about 0.0114 m³. Brands vary, so treat it as a guide and check the yield printed on the bag.

Does it handle non-rectangular slabs?

It calculates a rectangle. For L-shapes, split the area into rectangles, calculate each, and add the volumes.

What thickness should a slab be?

It depends on the load: paths are often 10 cm, while driveways and heavier loads need more. Follow local building guidance.

Can I share a calculation?

Yes. Use Share to copy a link that reopens the calculator with the same dimensions.

Concrete calculator

Enter the slab length and width in metres and the thickness in centimetres. The calculator shows the concrete volume, the volume with a 10% waste allowance, the cubic-yard equivalent, and an estimated number of 25 kg bags.

Use it to estimate the concrete for a rectangular slab and how many bags of mix to buy.

Concrete volume

1.2 m³

With 10% waste: 1.32 m³
Cubic yards: 1.57 yd³
25 kg bags: 116

The result updates as you type. The headline is the raw volume; the waste figure is what to order, and the bag count rounds up to whole bags.

How does it work?

Thickness is divided by 100 to convert centimetres to metres. A 10% waste allowance covers spillage and uneven ground. Bag counts assume a 25 kg bag yields about 0.0114 m³.

Concrete volume formula

V = l \times w \times \tfrac{t}{100},\quad V_{order} = 1.1\,V

V: Concrete volume in cubic metres.
l, w: Slab length and width in metres.
t: Slab thickness in centimetres.
V order: Volume with a 10% waste allowance.

A 4 m × 3 m slab, 10 cm thick, is 1.2 m³; with 10% waste that is 1.32 m³, about 116 bags of 25 kg mix.

Method & sources

Length and width are in metres; thickness is in centimetres. A 10% waste allowance is added for spillage and uneven ground. Bag counts assume a 25 kg bag of mix yields about 0.0114 m³.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

Concrete volume estimation — Portland Cement Association, verified 2026-06-10

How we calculate

Length and width are in metres; thickness is in centimetres.
A 10% waste allowance is added for spillage and uneven ground.
Bag counts assume a 25 kg bag of mix yields about 0.0114 m³.
The slab is treated as a simple rectangular block of even thickness.

Rounding

Volumes are shown to three decimals and bag counts as whole bags. The calculation uses full precision.

What this calculator does

Concrete is ordered and mixed by volume. This calculator multiplies a slab's length, width, and thickness to give the volume in cubic metres, adds a 10% waste allowance for spillage and uneven ground, converts to cubic yards, and estimates how many 25 kg bags of dry mix that takes.

How to use it

Enter the slab length and width in metres.
Enter the thickness in centimetres (a path is often 10 cm).
Read the volume, the volume with waste, and the bag estimate.
Order the volume-with-waste figure to avoid running short.

A worked example

A 4 m × 3 m slab, 10 cm thick, is 12 m² × 0.1 m = 1.2 m³. With a 10% waste allowance that is 1.32 m³, roughly 116 bags of 25 kg mix or a small ready-mix delivery.

Bags or ready-mix?

Bags suit small jobs up to about 0.5 m³. Above that, ready-mix delivered by the cubic metre is usually cheaper and faster. The bag count here is a guide; yields vary by brand, so check the bag.

Common mistakes

Entering thickness in metres instead of centimetres.
Forgetting the waste allowance and ordering exactly the raw volume.
Treating a sloped or thickened-edge slab as a flat block.

When it's useful

Planning a patio, shed base, path, or footing — anywhere you need to know the concrete volume and roughly how much to buy before starting.

FAQ

How is the volume calculated?: Length times width times thickness, with thickness converted from centimetres to metres. A 4 × 3 m slab 10 cm thick is 1.2 m³.
Why add a waste allowance?: Some concrete is lost to spillage, uneven ground, and over-filling forms. A 10% allowance is a common rule of thumb; increase it for rough sub-grade.
How many bags will I need?: The estimate assumes a 25 kg bag yields about 0.0114 m³. Brands vary, so treat it as a guide and check the yield printed on the bag.
Does it handle non-rectangular slabs?: It calculates a rectangle. For L-shapes, split the area into rectangles, calculate each, and add the volumes.
What thickness should a slab be?: It depends on the load: paths are often 10 cm, while driveways and heavier loads need more. Follow local building guidance.
Can I share a calculation?: Yes. Use Share to copy a link that reopens the calculator with the same dimensions.

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