Why 1 m³ of concrete needs 1.54 m³ of dry ingredients
This is the bit that trips everyone up, and it is the reason a mix worked out on the back of a docket always comes up short.
Loose dry sand and aggregate are full of air. Tip a bucket of aggregate out and look at it — the stones only touch at a few points and the gaps between them are a large fraction of the bucket. When you add water and cement and compact the mix, the cement paste flows into those gaps and the whole lot closes up. The finished concrete takes up substantially less room than the pile of ingredients did.
1.54 is the standard allowance for that shrinkage. Measure out one cubic metre of dry material and you will end up with about 0.65 m³ of concrete. Work backwards and a cubic metre of concrete needs 1.54 m³ of dry material. Every quantity on this page is derived from that one figure.
A worked example — half a cubic metre of N20 for a footpath
- Dry volume: 0.5 × 1.54 = 0.77 m³
- N20 is 1:2:4, so seven parts. One part is 0.77 ÷ 7 = 0.11 m³
- Cement: 0.11 m³ × 1440 kg/m³ = 158 kg, which is 8 × 20kg bags
- Sand: 2 parts = 0.22 m³, or 0.35 tonnes
- Aggregate: 4 parts = 0.44 m³, or 0.66 tonnes
- Water: 158 × 0.5 = 79 litres, added a bit at a time, not all at once
The four grades, per cubic metre of finished concrete
| Grade | Ratio | Cement (20kg bags) | Sand | Aggregate | Water |
|---|---|---|---|---|---|
| N20 | 1 : 2 : 4 | 16 | 0.44 m³ (0.70 t) | 0.88 m³ (1.32 t) | 158 L |
| N25 | 1 : 2 : 3 | 19 | 0.51 m³ (0.82 t) | 0.77 m³ (1.16 t) | 185 L |
| N32 | 1 : 1.5 : 3 | 21 | 0.42 m³ (0.67 t) | 0.84 m³ (1.26 t) | 202 L |
| N40 | 1 : 1 : 2 | 28 | 0.39 m³ (0.62 t) | 0.77 m³ (1.16 t) | 277 L |
Read across and you can see what a grade actually costs you: going from N20 to N40 nearly doubles the cement. That is the whole trade-off — strength is bought with cement.
More water is not more concrete
The water-cement ratio is the weight of water divided by the weight of cement. At 0.5, a 20kg bag of cement wants about 10 litres of water. That is a lot less than most people tip in.
Extra water makes the mix easier to place, and it makes the concrete weaker — not slightly, badly. Only some of the water reacts with the cement. The rest sits in the mix, bleeds to the surface and evaporates, and every drop of it leaves a capillary void behind. A batch mixed sloppy can lose a large share of its strength compared with the same ingredients mixed stiff.
Two practical consequences on site:
- Add water last and add it slowly. Get the dry ingredients turning first, then bring the water in until the mix holds a ridge off the shovel. You can always add. You cannot take it out without adding more cement and sand.
- Damp sand already carries water. Sand out of a yard after rain can be carrying several per cent by weight, which on a big batch is buckets. Start dry and judge by the mix.
Never trowel water into the surface to make it work up. All that does is thin the paste at the top and give you a slab that dusts, scuffs and pops in a couple of summers.
Batching on site without a scale
Use one bucket for everything and count. A 1:2:3 mix is one bucket cement, two sand, three aggregate — buckets, not shovels. A shovel of damp sand and a shovel of dry cement are not the same volume, and the difference shows up in the wall or the slab.
Handy conversions for bucket batching:
- A 20kg bag of cement is about 14 litres of loose powder
- One 20kg bag of cement mixed at N25 makes roughly 54 litres of concrete, so a cubic metre is about 19 bags
- A typical hire mixer holds around 60 to 70 litres of mixed material — call it 25 batches to the cubic metre, which is a long day
Aggregate size, and the honest limit of a nominal mix
Use 20mm aggregate for slabs, footings and anything thicker than about 75mm. Drop to 10mm for thin sections, congested reinforcement, or anywhere you need to work the mix into a narrow space. Bigger stone means less cement paste needed for the same strength, so do not go smaller than the job needs.
One thing to be straight about: these ratios are nominal. AS 1379 grades describe supplied concrete that has been batched to a design and tested for strength. Mixing 1:2:3 in a hire mixer gets you into the same neighbourhood and it is what generations of paths and shed slabs are made of — but a nominal ratio is not a graded product, and nobody has tested your batch. For anything structural, anything your engineer has nominated a grade for, or anything a certifier will look at, order it from a plant with a docket.
Mixed the concrete. Now bill the job.
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Try Yamate free → Free to try · no card · sign in with a link, no password to rememberConcrete mix ratio FAQ
What is the mix ratio for concrete in Australia?
The common nominal ratios by volume are 1:2:4 for N20, 1:2:3 for N25, 1:1.5:3 for N32 and 1:1:2 for N40, cement to sand to aggregate. N25 at 1:2:3 covers most domestic slabs and driveways.
How much cement do I need for 1 cubic metre of concrete?
About 370kg — 19 × 20kg bags — for N25 at 1:2:3, along with roughly 0.51 m³ of sand and 0.77 m³ of aggregate. N20 needs 16 bags, N40 needs 28.
Why do I need 1.54 cubic metres of dry material for 1 cubic metre of concrete?
Because loose dry sand and stone are full of air. Wetting and compacting the mix fills those voids with cement paste and the volume closes up by about a third. 1.54 is the standard allowance for it.
What does N25 mean?
It is the AS 1379 grade name for concrete with a 25 megapascal characteristic compressive strength at 28 days. N20 is 20 MPa, N32 is 32, N40 is 40. Higher number, more cement in the mix.
How much water goes in a bag of cement?
About 10 litres per 20kg bag at a 0.5 water-cement ratio. Add it slowly and stop when the mix holds a ridge off the shovel — extra water makes the mix easier to place and the concrete measurably weaker.
Can I mix N32 concrete on site?
You can mix to the 1:1.5:3 ratio, but a nominal mix is not the same thing as supplied N32 concrete, which is batched to a design and strength tested. If an engineer has specified a grade, order it from a plant and keep the docket.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Nominal mix ratios are a materials estimate for site-mixed batches, not a concrete specification. AS 1379 strength grades apply to supplied concrete batched and tested to a design; a hand-mixed batch to the same ratio has not been tested and should not be treated as equivalent. Densities and the 1.54 dry factor are typical published figures and vary with your actual materials. For structural work, follow your engineer and order graded concrete with a delivery docket.