The right ratio of cement, sand and aggregate for the job — and exactly how much of each to buy for your volume.
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Mix (cement : sand : aggregate)Not sure of the volume? Use the concrete calculator first.
Uses the traditional dry-volume method with a 0.5 water–cement ratio — it runs slightly generous on cement versus manufacturer tables, so you won't come up short. For structural or engineered work, order batched ready-mix to the specified MPa instead.
Ratios are cement : sand : aggregate, measured by volume — use the same bucket for every part. More cement makes a stronger, more workable mix; more aggregate makes a cheaper, leaner one.
Hand-mixing suits small jobs. Once you're past about half a cubic metre — check your volume here — ordering ready-mix is cheaper, faster and far more consistent.
| Use | Ratio | Approx. strength |
|---|---|---|
| Mass fill / blinding | 1 : 3 : 6 | ~10–15 MPa |
| Paths & pads | 1 : 2 : 4 | ~15–20 MPa |
| Slabs & driveways | 1 : 2 : 3 | ~20–25 MPa |
| Posts & heavy duty | 1 : 1.5 : 3 | ~25–30 MPa |
Strengths are indicative — actual MPa depends on water content, compaction and curing. For engineered work, order batched concrete to spec.
The classic general-purpose mix is 1 part cement, 2 parts sand and 4 parts aggregate by volume (1:2:4), with about half a part of water. For stronger structural work use 1:2:3 — the '1-2-3 rule' — go 1:1.5:3 for high strength, and stretch to 1:3:6 for non-structural mass fill. Measure parts with the same bucket or shovel for consistency.
For a 1:2:3 mix, allow roughly 320–370 kg of cement per cubic metre — about 16–19 × 20kg bags. Manufacturer tables like Cement Australia's sit at the lower end; the traditional dry-volume method our calculator uses sits at the upper end, so you won't run short. You'll also need around 0.5 m³ of sand and 0.8 m³ of coarse aggregate. Leaner mixes need less cement; richer mixes need more.
Aim for a water-to-cement ratio of about 0.5 by weight — roughly 10 litres per 20kg bag of cement — when you're batching your own mix from cement, sand and aggregate. That figure is for straight cement only: pre-mixed concrete bags need far less, typically 1.75–2.5 litres per bag, so always follow the bag. Add most of the water, mix, then add the rest gradually — extra water makes concrete easier to work but permanently weakens it.
In Australia, 20 MPa suits paths and non-structural work and is the minimum for internal house slabs under AS 2870, 25 MPa is what's most commonly ordered for house slabs and driveways, and 32–40 MPa covers heavy-duty or engineered work. Hand-mixed ratios only approximate these strengths — where an engineer specifies an MPa, order batched ready-mix concrete instead.
Cement is the grey powder that binds everything; concrete is the finished product — cement mixed with sand, aggregate and water. Mortar (for bricklaying) is cement and sand only, with no coarse aggregate.
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