Work out how much concrete you need for slabs, footings and post holes — in cubic metres, 20kg bags and dollars. Built by ConConnect for Aussie concreters and DIYers.
Driveways, shed floors, paths and patios.
Estimates only. Confirm quantities with your batch plant and allow for site conditions, mix design and access.
Concrete is measured in cubic metres (m³). Multiply your dimensions in metres, then add a wastage allowance. These are the formulas the calculator uses.
Length × width × thickness. Thickness is usually quoted in mm, so divide it by 1,000 first.
6m × 4m × 0.1m = 2.4 m³
Total trench length × width × depth. Measure every run of trench and add the lengths together.
12m × 0.3m × 0.45m = 1.62 m³
π × radius² × depth, multiplied by the number of holes. Radius is half the hole diameter.
π × 0.15² × 0.9m × 4 = 0.25 m³
A 6 m × 4 m shed slab at 100 mm thick measures 6 × 4 × 0.1 = 2.4 m³. Add 10% wastage for uneven sub-base and spillage = 2.64 m³, then round up to the next 0.2 m³ truck increment and order 2.8 m³. In 20kg pre-mix bags that would be about 294 bags — which is exactly why a job this size gets ready-mix, not a mixer.
Thickness drives your volume more than anything else — 25 mm extra over a big slab adds up fast. These are the typical thicknesses used in Australia on a properly compacted base.
These figures follow Australian guidance: AS 3727.1 allows 75 mm for foot-traffic-only residential paths, AS 2870 governs residential slab design, and some councils set their own minimums — Brisbane, for example, requires 125 mm driveways. If an engineer has specified your slab, footing or pier sizes, always use those figures instead.
| Application | Typical thickness |
|---|---|
| Paths & patios | 75–100 mm |
| House slabs & shed floors | 100 mm |
| Driveways (cars) | 100–125 mm |
| Heavy vehicles & machinery | 150–200 mm |
If the truck cannot place the concrete directly, use the concrete pump hire cost guide and quote calculator to check minimum hours, travel, volume fees and GST.
Multiply the length by the width by the thickness of the pour, all in metres, to get cubic metres (m³). For a slab 6 m long and 4 m wide at 100 mm thick: 6 × 4 × 0.1 = 2.4 m³. Add 5–10% for wastage, uneven ground and spillage.
One 20kg bag of pre-mixed concrete yields roughly 0.009 m³, so you need around 110 bags per cubic metre. For anything over about 0.5 m³, ready-mix delivered by truck is usually cheaper and far less work than mixing bags.
As a rough guide, ready-mix concrete costs around $250–$400 per cubic metre supplied regionally, and up to $450 delivered in metro areas, depending on the mix (MPa strength) and delivery distance. Special mixes, short loads under about 4 m³ and weekend pours cost more. Laid and finished concrete typically runs $60–$85 per m² for a plain finish and $100–$180 per m² for decorative finishes like exposed aggregate or stamped concrete.
In Australia, foot-traffic-only paths and patios can be as thin as 75 mm, while 100 mm is the standard for house slabs, shed floors and anywhere a vehicle might travel. Driveways are typically 100–125 mm — some councils, like Brisbane, require 125 mm — and slabs carrying heavy vehicles or machinery are 150–200 mm with appropriate reinforcement. Always follow your engineer's specification where one applies.
Yes. Order 5–10% more than your measured volume to cover uneven sub-base, spillage and pump line loss. Running short mid-pour means a cold joint and a second delivery fee, which costs far more than the extra concrete.
You need a pump when the truck can't get close enough to chute directly into the forms — think backyards, tight access, pours above ground level or large slabs where you'd otherwise be barrowing all day. A line pump suits most residential jobs; a boom pump suits high or large pours.
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