How to use the Concrete Volume
- Choose the shape — slab, footing, column or steps.
- Enter length, width and depth in consistent units.
- Read the volume, and add the suggested waste allowance.
- Convert to bags only if the pour is small; anything over about a cubic metre is cheaper ready-mixed.
How the calculation works
Volume is length × width × depth for a slab, and π r² × height for a column. The arithmetic is trivial; the errors come from units. A 100 mm slab depth entered as 100 (metres) inflates the order by a thousand, and a depth in inches multiplied by dimensions in feet is out by twelve. Convert everything to one unit before multiplying.
Order 5 to 10% over the calculated volume. Sub-base surfaces are never perfectly level, so the actual depth exceeds the nominal figure in places, and formwork flexes outwards under the weight of wet concrete. A shortfall part-way through a pour creates a cold joint — a structural weakness that a small surplus would have avoided entirely — while surplus costs only the price of the extra material.
Slab: V = L × W × D ; Column: V = π × (d/2)² × H ; order quantity = V × 1.05 to 1.10Source: Geometric volume formulas; waste allowance per standard ready-mix ordering practice. Bag yields vary by product — check the bag.
Worked example
A patio slab 5.4 m × 3.2 m at 125 mm thick.
- Convert depth: 125 mm = 0.125 m.
- Volume = 5.4 × 3.2 × 0.125 = 2.16 m³.
- Add 8% for uneven sub-base and formwork spread: 2.16 × 1.08 = 2.33 m³.
Order about 2.35 m³ of ready-mix — roughly 108 twenty-kilogram bags if mixed by hand, which is why ready-mix is the sensible choice at this size.
Frequently asked questions
How many bags of concrete make a cubic metre?+
Roughly 108 bags of 20 kg or 55 bags of 40 kg, though yield varies by product — the bag states its yield in litres. Beyond about one cubic metre, ready-mix is cheaper and far less work.
How much extra should I order?+
Between 5 and 10%. Sub-base irregularity and formwork movement both consume more than the nominal volume, and running short mid-pour causes a cold joint that weakens the slab.
What thickness does a slab need?+
Around 100 mm for foot traffic and patios, 150 mm or more for driveways and vehicle loads, on a compacted sub-base. Structural slabs and anything carrying a building must be designed to code.
Does the calculator include reinforcement?+
No, it gives concrete volume only. Mesh or rebar displaces a negligible amount of concrete but must be specified separately according to the load the slab will carry.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.