Concrete quantities: bags, cubic metres and tonnes

Ask three suppliers to price the same slab and you will be quoted in three different units. The ready-mix plant works in cubic metres, the aggregate yard in tonnes, the builders' merchant in 25 kg bags, and any American guide you read online is in cubic yards. They all describe the same hole in the ground. The only number you have to get right is the volume; everything else is a conversion.

Start with volume, always

Volume is length × width × depth, in metres. The mistake that costs money is depth: it is quoted in millimetres and everything else in metres, so a 100 mm slab is 0.1, not 100. A 4 × 3 m slab at 100 mm is 1.2 m³. That is the figure the whole order hangs off.

Add the waste allowance now, not later. At 10% the slab becomes 1.32 m³. Concrete is the one material where under-ordering is not a second trip — a pour that stops halfway sets a cold joint through the middle of your slab.

The three conversions

Once you have 1.32 m³, the rest is arithmetic the concrete calculator does for you:

Tonnes. Cured concrete is taken at 2,400 kg per cubic metre, so 1.32 m³ is 3,168 kg — call it 3.17 tonnes. Aggregate yards quote by weight and this is the number they want.

Cubic yards. One cubic yard is 0.7646 m³, so 1.32 m³ is 1.73 yd³. Useful only for reading US guidance, where slabs are almost always quoted this way.

Bags. This is where the numbers get ugly. A 25 kg bag of ready-to-use concrete yields roughly 0.0125 m³ once mixed — about 12.5 litres. At that rate 1.32 m³ needs 106 bags, which is 2.65 tonnes of bags to carry, open and mix by hand. Check the yield printed on the bag you are actually buying, because it varies with the mix.

Where the bag/ready-mix line sits

The honest threshold is not a volume, it is a morning. Somewhere around 0.5 m³ — forty-odd bags — hand mixing stops being cheaper and starts being a bad day. Below that, bags win on access and on not having to accept a minimum load. Above it, ready-mix is faster, more consistent, and pours as one continuous batch, which matters structurally.

Ready-mix plants also have a minimum charge, typically for a load well under a full truck, so a 1.2 m³ slab may cost the same as a 2 m³ one. Worth asking before you shave the design.

The number nobody converts correctly

Density. 2,400 kg/m³ is a reasonable figure for ordinary cured concrete, but it is not universal: lightweight mixes run nearer 1,800 and heavy aggregate higher still. If a supplier quotes you tonnes and you are converting back to volume to check the order, use their density, not a textbook one. The construction reference lists the density, coverage and deduction figures every tool here applies.

Why the mix ratio changes the bag count

A ratio like 1:2:4 describes cement, sand and aggregate by volume, and different jobs use different ratios: roughly 1:2:4 for general foundations and slabs, 1:1.5:3 where higher strength is wanted, 1:3:6 for mass fill and blinding. The proportions change how much cement a given volume of finished concrete needs, so two people can calculate the same slab correctly and order different quantities of cement — because they are mixing different concrete.

Dry volume also exceeds wet volume. Cement, sand and aggregate pack into each other's voids, so the loose dry material needed is conventionally taken as around 1.5 times the finished wet volume. Any calculation that skips that factor under-orders by a third, which is the single most common reason a mix runs out before the pour is finished.

Ready-mix is ordered differently

Once the volume passes roughly two cubic metres, bagged mixing stops being sensible and ready-mix takes over — but the ordering conventions change with it. Suppliers usually sell in quarter-cubic-metre increments with a minimum load, quote by strength class (C20/25, C25/30) rather than by ratio, and charge waiting time beyond an allowed discharge window. There is no partial return: concrete left on the truck is paid for.

That makes the volume figure the whole job. Measure each pour separately rather than as one lumped area, add a small allowance for uneven sub-base rather than a generous one, and have somewhere to put a modest surplus — a spare path bay or post base — so a small over-order is used rather than wasted. Work the volumes out with the concrete calculator, and if the pour is part of a wider job, the flooring calculator and construction reference cover the finishes that follow it.

A checklist before you order

Depth in metres, not millimetres. Waste added before conversion, not after. Formwork and over-dig included in the volume — ground is never as flat as the drawing. And the supplier's own unit confirmed in writing, because "one and a half" means something different in yards.