Concrete calculator — estimate concrete volume

Choose slab, footing or column, enter the dimensions and get the concrete volume — plus how many bags of concrete mix you need and what the concrete weighs. Works in metric and US units.

m
m
mm

Common garage slab: 100–150 mm.

%

5–10% is typical to cover uneven subgrade and spillage.

Concrete volume incl. waste
2.1 m³
Volume without waste
2 m³
Dry concrete, 25 kg bags
168 pcs
approx. 12.5 L of mixed concrete per bag
Weight of mixed concrete
5 tonnes
approx. 2,400 kg/m³

For volumes above roughly 1 m³, ready-mix concrete (truck delivery) is usually cheaper and more consistent than bagged dry mix. Always state exposure and strength class when ordering.

The calculation is an estimate and does not replace engineering design or the supplier’s instructions. Always check against project documents, product data sheets and applicable codes.

How to work out the concrete volume

Concrete volume is always geometry: for a slab you multiply length × width × thickness, for a rectangular footing length × width × height, and for a round column π × radius² × height. Keep every measurement in the same unit and the answer drops straight out in cubic meters or cubic feet.

Always add a waste margin. The ground is rarely perfectly level, and ordering 5–10% extra is far cheaper than standing next to a half-poured slab when the concrete runs out.

Bagged mix or ready-mix truck?

As a rule of thumb: below half a cubic meter (about two-thirds of a cubic yard), bagged mix is the most convenient; between that and a full cubic meter it is a judgment call; and above roughly one cubic meter (1.3 yd³) ready-mix from a truck almost always wins — on price, labor and consistency. A cubic yard is about 45 bags at 80 lb, which is heavy work to mix by hand.

When ordering ready-mix you specify volume, strength class (for example C25/30 or a 4,000 psi mix), exposure conditions, slump and maximum aggregate size — you find these in the structural documents.

Frequently asked questions

How do I calculate how much concrete I need?
Multiply length × width × thickness (in the same unit) for a slab. A 16 × 13 ft slab at 4 inches thick needs 16 × 13 × 0.33 ≈ 69 ft³, or about 2.6 yd³. In metric: a 5 × 4 m slab at 100 mm is 5 × 4 × 0.10 = 2.0 m³. Add 5–10% waste margin for uneven ground.
How many bags of concrete make a cubic yard?
A 60 lb bag yields about 0.45 ft³ and an 80 lb bag about 0.60 ft³, so a cubic yard (27 ft³) takes roughly 60 bags of 60 lb or 45 bags of 80 lb. For volumes above about a cubic yard, ready-mix from a truck is usually cheaper and more consistent than bagged mix.
How much does a cubic meter or cubic yard of concrete weigh?
Normal-weight concrete is about 2,400 kg per cubic meter (roughly 150 lb per cubic foot, or about 4,000 lb per cubic yard). Reinforced concrete is often counted as 2,500 kg/m³.
What concrete strength should I choose?
It depends on exposure and load. For an outdoor garage slab, C25/30 (roughly 3,600 psi) is common in Europe; in the US, 3,500–4,000 psi mixes are typical for driveways and slabs. Environments with frost and de-icing salt need higher strength and air entrainment. Always check the structural documents or ask your supplier.
How thick should a concrete slab be?
A typical slab for a garage or shed is 100–150 mm (4–6 inches). Slabs carrying heavier loads are designed by a structural engineer and are often thicker with more reinforcement.

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