How Much Concrete for a 3 x 3 m Slab?
A 3 by 3 metre slab, 100 mm thick, is 0.9 cubic metres. That is 90 bags of 20 kg premix with no waste, 99 bags with 10% extra, or about 1 m³ of ready-mix.
Try your own numbers
Last checked 10/11/2026. Sources: Easy Mix Super Strength Concrete data sheet, Wilson Timbers (about 99 bags of 20 kg per m³); Before You Dig Australia.
Bags or ready-mix?
Volume = 3 × 3 × 0.1 = 0.9 m³. A 20 kg bag of premix makes about 0.01 m³ (the Easy Mix Super Strength data sheet lists about 99 bags per cubic metre), so 90 bags, or 99 with 10% waste. That is nearly two tonnes of bagged mix, which is heavy work to mix by hand and leaves joins between batches.
Ready-mix is sold by the cubic metre. For 0.99 m³ many suppliers apply a minimum load or a short-load fee, so ring a couple and compare; the saving on mixing time is often worth it at this size.
Thickness. Going from 100 mm to 125 mm adds a quarter more concrete. The table under the calculator shows 75, 100, 125 and 150 mm. A slab that carries a vehicle, or sits under a building, needs a design and approval: this is not structural advice. Request plans of underground services through Before You Dig Australia before any digging.
Common questions
How many 20 kg bags make a cubic metre?
About 100 at 0.01 m³ a bag. The Easy Mix data sheet lists 99.
Can I use a different bag size?
Yes. Enter the yield from the bag in the yield box.