How many bags of cement do I need per m³ of concrete?

It depends on your mix ratio and bag size, but as a worked reference: for a 1:2:4 mix with the standard 1.54 shrinkage factor, 5% waste, 1440 kg/m³ cement density and 25 kg bags, roughly 1 m³ of placed concrete needs about 13-14 bags of cement. This calculator gives you the exact figure for your specific volume, ratio, waste allowance and bag size rather than a rough per-m³ rule of thumb.

Why is the dry volume of cement, sand and gravel more than the wet (mixed) volume?

Because loose, dry cement, sand and aggregate particles do not pack together perfectly — there are air voids between them. Once water is added and the mix is compacted, those voids collapse and the material settles into a smaller, denser volume. The standard correction factor is 1.54 (accepted range 1.52-1.57): you need roughly 1.54 m³ of dry materials to produce 1 m³ of finished, compacted concrete.

What is the difference between a concrete mix and a mortar mix?

A concrete mix includes cement, sand and coarse aggregate (gravel or crushed stone) in a ratio such as 1:2:4, and is used for structural work — slabs, footings, columns. A mortar mix is cement and sand only, with no coarse aggregate, in a ratio such as 1:4, and is used for blockwork, brickwork, rendering and plastering, where a finer, more workable paste is needed rather than a load-bearing structural material.

How much does a bag of cement cost, and how do I estimate total material cost?

Cement bag prices vary significantly by country, bag size and brand. This calculator multiplies the number of bags needed by a per-bag price you can adjust to your local supplier's quote, giving a material cost estimate for the cement alone — sand, aggregate and labour are typically additional costs on top.

What mix ratio should I use for a concrete slab or footing?

A 1:2:4 cement:sand:aggregate ratio (roughly 15 MPa) is the standard general-purpose choice for slabs, paths and low-rise reinforced concrete. Footings and lightly-loaded foundations can often use a leaner 1:3:6 or 1:4:8 mix, while heavily loaded columns typically need a richer 1:1.5:3 or 1:1:2 mix. Always check your local building code or an engineer's specification for anything structural or load-bearing.

What size cement bag is standard in Australia?

The standard retail cement bag in Australia is 20 kg, widely sold at Bunnings and other hardware chains, with 40 kg bags also common for larger jobs. Always check the bag weight printed on the packaging, since both sizes are stocked side by side.

How much does a bag of cement cost in Australia?

Prices vary by brand and retailer, but a typical Australian price is roughly AUD 10-14 per 20 kg bag of general-purpose cement. This calculator lets you enter your own local quote to get an accurate total material cost for your specific job.

Cement Calculator — How Much Cement Do I Need? | Australia

Cement Calculator: How Many Bags of Cement Do I Need?

If you already know the volume of concrete or mortar you need to place — a footing, a screed, a batch of blockwork mortar — the real question is how many bags of cement to buy, and how much sand and aggregate to order alongside it. This cement calculator takes that wet (placed) volume, applies the dry-volume shrinkage factor that every self-mixer eventually learns the hard way, splits the result by your chosen mix ratio, and gives you a bag count plus a real material cost estimate.

It is built for the specific step that trips up most first-time self-mixers: dry, loose cement, sand and aggregate take up noticeably more volume than the finished, compacted concrete or mortar they produce, because the dry particles settle around air voids that disappear once water is added and the mix is compacted. Skip that correction and you will run short mid-job — this calculator applies it automatically, and shows the working so you can see exactly why your bag count is higher than "volume needed" alone would suggest.

It is different from a general concrete-slab or footing calculator, which starts from shape dimensions (length, width, thickness) and works out volume for you. Here, you already have the wet volume — from a slab calculator, a footing spec, or a plan — and you want the cement/sand/aggregate/bag breakdown on top of it, for either a structural concrete mix (cement:sand:aggregate, e.g. 1:2:4) or a sand-only mortar mix (cement:sand, e.g. 1:4) for blockwork, rendering or plastering.

This is one of the highest-demand construction calculators worldwide, especially in markets where nominal mix ratios (rather than pre-bagged ready-mix) are the normal way to plan a pour — self-builders and small contractors sizing bag orders before a concrete or mortar job, anywhere from a garden wall to a house foundation.

How to Use the Cement Calculator

Choose your mix type: "Concrete" for a cement:sand:aggregate structural mix, or "Mortar" for a cement:sand-only mix (blockwork, rendering, plastering).
Enter the wet (placed) volume you need — the actual volume of concrete or mortar that will end up in place once mixed, poured and compacted — and choose its unit (m³, ft³, or another volume unit).
Check the dry-to-wet shrinkage factor — it defaults to 1.54, the standard mid-point value, but can be adjusted between roughly 1.52 and 1.57 depending on your aggregate and compaction.
Set the waste allowance percentage — 5% is the default, rising to 10% for hand-mixing or a less controlled site.
Select a mix ratio preset (e.g. 1:2:4 for general concrete, or 1:4 for mortar) or enter your own cement:sand:aggregate proportions.
Check or adjust the cement density (defaults to 1440 kg/m³, the standard figure for ordinary Portland cement) and the bag size in kg to match what is actually sold locally.
Read off the dry volume, the cement/sand/aggregate volume split, the cement weight, the number of bags to buy, and the estimated material cost.

The Math Behind the Calculation

The starting point is your wet (placed) volume — the volume the finished, compacted concrete or mortar will occupy once poured. The first correction is the dry-volume shrinkage factor: Dry volume = Wet volume x shrinkage factor (default 1.54). This factor exists because dry, loose cement, sand and aggregate particles do not pack perfectly — there are air voids between them — so a larger dry volume of raw materials is needed to produce a smaller volume of dense, compacted concrete once water is added and the mix consolidates. The accepted range is roughly 1.52-1.57 depending on aggregate grading and compaction method, with 1.54 as the standard default.

A waste allowance is then applied on top of the dry volume: Total dry volume = Dry volume x (1 + waste% / 100). This covers spillage, over-ordering safety margin and minor site losses, typically 5-10%.

That total dry volume is then split between cement, sand and aggregate (or just cement and sand, for mortar) in proportion to your chosen mix ratio. For a ratio of a:b:c, the total parts are a+b+c, and each component gets (its part / total parts) of the total dry volume. For example, in a 1:2:4 mix, total parts = 7, so cement gets 1/7 of the volume, sand 2/7, and aggregate 4/7.

The cement volume is then converted to weight using cement density: Cement weight (kg) = Cement volume (m³) x density (kg/m³), with 1440 kg/m³ as the standard default for ordinary Portland cement. Finally, the number of bags is the cement weight divided by the bag size, always rounded up: Bags = ceiling(Cement weight / Bag size), since a fraction of a bag cannot be bought.

Worked example: you need 2 m³ of placed concrete at a 1:2:4 mix ratio, using the default 1.54 shrinkage factor, a 5% waste allowance, 1440 kg/m³ cement density and 25 kg bags. Dry volume = 2 x 1.54 = 3.08 m³. With 5% waste: 3.08 x 1.05 = 3.234 m³ total dry volume. Split 1:2:4 (7 total parts): cement = 3.234 / 7 = 0.462 m³, sand = 0.924 m³, aggregate = 1.848 m³. Cement weight = 0.462 x 1440 = 665.3 kg. Bags needed = 665.3 / 25 = 26.6, rounded up to 27 bags of 25 kg cement.

Common Mix Ratios (Cement : Sand : Aggregate, by Volume)

1:5:10 — mass concrete, roughly 5 MPa, used for non-structural bulk fill
1:4:8 — footings and other lightly-loaded foundations, roughly 7.5 MPa
1:3:6 — general non-structural concrete, roughly 10 MPa
1:2:4 — general-purpose structural concrete (often labelled "M15"), roughly 15 MPa, the most common default for slabs, paths and low-rise reinforced concrete
1:1.5:3 — higher-load slabs, driveways and low-rise house structural work ("M20"), roughly 20 MPa
1:1:2 — heavily loaded columns and beams, roughly 25 MPa
Mortar-only mixes (cement:sand, no coarse aggregate): 1:3 for high-strength pointing/bedding, 1:4 for general blockwork, 1:5 for rendering/plastering.
These nominal mixes and their approximate strength classes follow the same volumetric convention as IS 456-style nominal mixes used widely across self-build and small-contractor practice; always confirm against your local building code or an engineer's specification for anything structural.
Dry-to-wet shrinkage factor convention: 1.54 is the standard default (accepted range 1.52-1.57), reflecting the voids-between-particles effect that disappears once the mix is wetted and compacted.

Australian Cement Bag Sizes

The standard retail cement bag in Australia is 20 kg, sold at all major hardware and building-supply chains (Bunnings, hardware trade suppliers).
Larger 40 kg bags of general-purpose (GP) cement are also common for bigger DIY and small-contractor jobs, alongside bulk bags for larger site deliveries.
Source: standard Australian retail packaging convention for general-purpose Portland cement, consistent across major suppliers (Bunnings, Boral, Cement Australia).

Pro Tips and Common Mistakes

At a typical Australian price of around AUD 12 per 20 kg bag of cement, the worked example in this guide scales to roughly 33 bags of 20 kg cement for a 2 m³ pour at a 1:2:4 mix (665.3 kg / 20 kg, rounded up) — about AUD 396 in cement alone, before sand, aggregate and delivery.

Always round your bag count up, never down — a partial bag cannot be bought, and running short mid-mix is far worse than having a small surplus.
The 1.54 dry-to-wet shrinkage factor is a mid-point default, not a fixed law: looser, more angular aggregate and less thorough compaction push it toward 1.57, while well-graded, well-compacted mixes can sit closer to 1.52. If you have measured your own site's ratio from past pours, use that instead of the default.
A common mistake is applying the shrinkage factor to the wrong number — it converts wet (placed, compacted) volume to dry (loose, pre-mix) volume, not the other way round. If you already have a dry-materials quantity from a supplier, do not multiply it by 1.54 again.
Bag size varies a lot by country and even by brand within a country — common sizes include 25 kg, 40 kg and 50 kg bags, and 94 lb (roughly 42.6 kg) sacks in the US. Always check or edit the bag size to match what is actually sold near you before ordering.
For mortar (blockwork, rendering, plastering), leave the aggregate ratio at zero and use a cement:sand-only ratio — mortar has no coarse aggregate, unlike structural concrete.
Use the higher end of the waste-allowance range (closer to 10%) for hand-mixing, uneven site conditions, or your first time mixing a particular ratio — spillage and inconsistent batching add up fast at that stage.
For any structural, load-bearing pour (foundations, columns, suspended slabs), treat the mix-ratio table as a starting point only and confirm the specification against your local building code or a structural engineer — nominal mixes are a practical guide, not a substitute for an engineered design where one is required.