Concrete Calculator

Volume first, everything else second

A concrete order is quoted in cubic metres, but a slab is talked about in length, width and finger-widths ("thick as two fingers" often means 8 cm). This page does the one multiplication that mattered all along — length × width × thickness — and then does the two things people forget: it adds a wastage margin, and it converts volume to a rough weight, which is what bag-based purchasing and truck-load planning actually run on. Ground is rarely perfectly level, forms flex by a centimetre, and spillage is a fixed cost of every pour; the wastage field exists so the order is right on the first call.

The weight estimate uses the usual fresh-concrete figure of 2,400 kg per cubic metre. Real mixes range a few per cent around it, and supplier data sheets win over this page whenever they disagree — the point is to have a defensible order-of-magnitude before the phone call, not to skip the call.

A worked example, step by step

Problem: a patio slab 4 m by 3 m, 10 cm thick, with 10% allowed for wastage.

1. Plain volume: 4 × 3 × 0.10 = 1.2 m³ — the figure a plan would quote.
2. Wastage: 1.2 × 1.1 = 1.32 m³ — the figure to order.
3. Litres: 1,200 exact, a useful cross-check when a supplier talks in litres.
4. Weight at 2,400 kg/m³: 1.32 × 2,400 = 3,168 kg. If the supplier sells 20 kg bags of ready-mix at roughly 0.01 m³ each, that is the "bags vs truck" decision point — dozens of bags vs one small truck.
5. Bigger variant: 6 m × 4 m at 12 cm is 2.88 m³ plain, 3.168 m³ to order (the panel prints 3.168), about 7,603 kg.

Note what the panel does not pretend to know: your mix strength (C20/25 and friends), your reinforcement, and your supplier's batching rules. Those live in the phone call this page prepares you for.

Thickness makes the difference

The same 4 × 3 footprint at different depths, before wastage:

Thickness is the most expensive knob on the page: 10 cm to 20 cm doubles the order for the same footprint. The engineering rule of thumb is that stiffness grows with the cube of thickness, which is why doubling beats reinforcing on some projects — but that is a conversation for the building code, not for this calculator.

Ordering reality

Truck deliveries usually batch in 0.5 m³ increments and often carry a minimum-load charge; a 1.32 m³ order may be priced as 1.5 m³, and paying the shortfall beats paying the return trip. Bagged mix skips the minimum but scales brutally: at about 100 bags per cubic metre (a 20 kg bag of ready-mix yields roughly 0.01 m³), the patio example is 132 bags — usually the moment a delivery becomes obviously cheaper. Sub-base, formwork and cure time are outside this page's maths entirely; they are the other three quarters of a successful pour.

Mistakes that quietly short a pour

• Measuring in centimetres but entering metres. A 10 entered as metres makes a slab 1000 cm thick; keep the unit fields matched.
• Ordering exactly. Zero wastage is the classic short-pour; running out mid-pour leaves a cold joint that no marketing fixes.
• Counting the sub-base in the thickness. Sub-base is compacted stone below the pour, not concrete above it.
• Forgetting slope. A 2% fall over 4 m adds depth at one edge; the wastage margin usually absorbs it, but know why.

Keep exploring

• Square Footage Calculator — same footprint arithmetic when the quote is per square metre of finished surface.
• Gravel Calculator — the sibling calculator for the sub-base under the slab.
• Tile Calculator — the finish-layer arithmetic if pavers rather than poured slabs win the design.
• Guide: Unit Conversion Mistakes to Avoid.