Why concrete jobs get under-ordered
Concrete is one of those materials where the maths looks simple and the ordering still goes wrong. Length by width by depth gives you a volume, and volume is what a supplier sells. The mistake almost never happens in the multiplication. It happens in the measuring, in forgetting that a slab isn't flat ground and a hole isn't a perfect cylinder, and in rounding a number down because it feels close enough. A truck that shows up short on a Saturday, with the rest of the pour already open and drying, is a much more expensive problem than ordering an extra quarter of a cubic metre would have been.
This calculator handles the volume side properly, in cubic metres, for a slab, a footing or strip footing, or a run of post holes. What it can't do is measure your job for you, and a fair bit of the value in a concrete order comes from measuring it properly in the first place.
Measuring a slab properly
A slab looks like a single rectangle from above, and for the plan area, that's all it is: length times width. The part people get wrong is depth. A slab isn't a fixed thickness by default. It's a design decision, made by whoever is engineering the job, based on what the slab has to carry. A garden shed slab and a slab under a house extension are not the same thickness, and neither is a slab for a carport compared to one that'll have a car parked on it every day. If you've got engineered drawings, the thickness is on them. If you don't, that's a conversation with a builder or a structural engineer before you order anything, not a number to guess at because it sounds about right.
Measure length and width at the actual formwork line, not the fence line or a rough pace across the yard. A tape measure at both ends and the middle catches a site that isn't quite square, which is most of them. If the two ends come out different, use the larger figure. Running short on a pour because the site turned out to be a few centimetres wider than the first measurement costs a lot more than a slightly generous order.
Footings and strip footings
A footing is a long, narrow version of the same maths, run in millimetres for the width and depth because that's how a footing is specified. Measure the total run, not just one wall, if you're doing a full perimeter. It's easy to measure one side, work out a volume, and forget that a strip footing under a boundary fence or a shed base usually goes all the way around. Add up every run before you put it into the calculator, or run each side through separately and add the results.
Footing width and depth, like slab thickness, come from engineering, not habit. Soil type matters here more than it does for a slab. Reactive clay, which a lot of Victoria sits on, needs a different footing to sand or rock, and a soil test is what tells an engineer which one you've got. If a job involves a structural footing under a wall that'll carry real load, that's past the point where a homeowner should be sizing it themselves, and it's the kind of work that usually needs a permit anyway.
Post holes
Post holes are the fiddly one, because a hole is round, a post inside it usually isn't, and the concrete only fills the gap between the two. This calculator treats the hole as a plain cylinder and works out the full volume of it, which is the safer number to order against. If you're backfilling around a timber or steel post rather than pouring a solid pier, the post itself displaces some of that volume, so the true concrete usage will be a bit less than the raw hole volume. That's a reasonable margin to have in hand rather than a shortfall to chase later, especially once you add a wastage allowance on top.
Auger the hole to a consistent diameter and depth if you can. A hand-dug hole that flares out near the top, or narrows partway down where the auger hit a root or a rock, throws the volume out more than most people expect, because a hole's volume grows with the square of its radius. A hole that's 10% wider than planned isn't 10% more concrete. It's closer to 20%.
Why you order more than the raw number
The wastage allowance in this calculator exists because the raw volume from a tape measure is never quite what ends up in the ground. Formwork isn't perfectly rigid, ground isn't perfectly flat, and some concrete is always lost off a barrow, a shovel or the edge of a form. Ten percent is a reasonable starting point for a straightforward slab or footing poured on prepared ground. A hand-mixed job, an irregular site, or a first-time DIY pour is worth pushing higher, because the places concrete gets wasted, spillage, over-digging, an uneven base, all get worse when the person doing the pour is doing it for the first time.
Running short mid-pour is the outcome you're actually trying to avoid, and it's worse than it sounds. Concrete starts setting from the moment it's mixed, so a slab poured in two batches with a gap between them, because the first batch ran out and the second one took an hour to arrive, ends up with a visible cold joint and a weaker bond at exactly the point where the two batches meet. A bit of extra volume ordered up front is cheap insurance against a defect that's expensive to fix afterwards.
Bags versus premix
The bag yield field is there because bag yields genuinely differ by brand and mix, and a figure typed into a calculator is never as reliable as the number printed on the actual bag you're buying. Twenty-kilogram bags typically yield somewhere around a hundredth of a cubic metre once mixed, but read your own bag before you rely on it, because a general-purpose mix and a rapid-set mix aren't identical, and manufacturers do change formulations.
The threshold this calculator flags, around half a cubic metre, is a rough but useful line. Below it, mixing bags by hand or in a small hire mixer is realistic for one or two people over a day. Above it, the sheer number of bags, the mixing time and the physical effort usually make a premix truck the better call, even for a job you were planning to do yourself. A concrete supplier can tell you their minimum load and delivery charges, and it's worth ringing one even if you end up mixing bags, just to know what the alternative would have cost and how it compares on time.
Irregular shapes and site quirks
Not every slab is a clean rectangle. An L-shaped patio, a slab with a curved edge, or a footing that steps down a slope all need to be broken into simpler pieces first. Split an L-shape into two rectangles, work out the volume of each separately, and add them together. A stepped footing on a sloping block is the same idea: treat each level section as its own run with its own depth, rather than trying to average a single depth across the whole thing, because averaging tends to under-order the deep end and over-order the shallow end in a way that doesn't cancel out cleanly.
A sloping site more broadly is where a lot of DIY concrete jobs stop being a DIY job. Getting formwork level and square on a slope, keeping a consistent depth when the natural ground line moves under your feet, and knowing when the site needs cut, fill or retaining before a slab even makes sense, all sit closer to a builder's or a concreter's daily work than a homeowner's. If that describes your block, it's worth getting a quote before you order any concrete at all.
What to ask a supplier
Once you've got a volume figure from this calculator, a concrete supplier still needs a bit more from you to quote properly: the mix strength you need (which usually comes from an engineer or the job type), whether the site has truck access or needs a pump, and what day and time you want delivery. Ask about their minimum order, because small pours often get charged at the minimum load regardless of the exact volume, and ask what happens if you order slightly short or slightly long on the day, since both are common enough that most suppliers have a standard answer.
When it stops being a DIY job
A garden edge, a small paver base or a handful of post holes for a fence are jobs plenty of people handle themselves. A structural slab under a building, anything near a boundary or a service, a footing that's carrying real load, or a pour big enough to need a truck and a pump, is a different scale of job, and usually a different level of risk if it's done wrong. Councils in Victoria generally require a permit for structural work and sometimes for slabs over a certain size, so it's worth checking before you dig rather than after the concrete's already in the ground. When in doubt, a quick call to a local concreter or your council's building department is a lot cheaper than getting it wrong.
If you're the one running a concreting business rather than pouring your own slab, our page on websites for concreters covers how a site that actually converts enquiries is built, and our other calculators cover a range of the other numbers a trade business runs on day to day.