Working out how much concrete you need
The sum is length × width × thickness. What catches people is the units: thickness is in millimetres and everything else is in metres, so a 100mm slab is 0.1 in the sum, not 100. Get that wrong and you are out by a factor of a thousand.
The second trap is ordering the neat volume. A slab poured on a scraped subgrade is never exactly the thickness you nominated — the ground dips, the screed rides up over a high spot, and the real average comes out thicker than the drawing. Five per cent is the minimum on flat, well-prepared ground. On a hand-dug trench or soft fill, ten is more honest.
A worked example — a 5m × 3m shed slab
- Neat volume: 5 × 3 × 0.1 = 1.5 m³
- Plus 5% for waste: 1.575 m³
- Ready-mix, rounded up to the 0.2 step: 1.6 m³
- Or in bags: 1.575 ÷ 0.0093 = 170 × 20kg bags — that is 3.4 tonnes of bags to cart, load and mix, which tells you straight away this is a truck job
Common pours, done for you
| Job | Neat volume | Order (ready-mix) | 20kg bags |
|---|---|---|---|
| Path — 10m × 900mm × 100mm | 0.9 m³ | 1.0 m³ | 102 |
| Single garage — 6m × 3.5m × 100mm | 2.1 m³ | 2.4 m³ | 238 |
| Shed slab — 6m × 4m × 100mm | 2.4 m³ | 2.6 m³ | 271 |
| Driveway — 12m × 3m × 125mm | 4.5 m³ | 4.8 m³ | 509 |
| Four post holes — 300mm × 600mm | 0.17 m³ | 0.2 m³ | 20 |
All of these include the 5% waste allowance before rounding.
Post holes and columns
A round hole is π × radius² × depth. People underestimate these badly, because a hole looks small and a cubic metre sounds big. A 300mm hole 600mm deep is 42 litres — nearly five 20kg bags on its own. Multiply that by a fence line of fifteen holes and you are into a tonne and a half of premix.
| Hole | Volume | 20kg bags per hole |
|---|---|---|
| 200mm × 600mm deep | 19 L | 3 |
| 250mm × 600mm deep | 29 L | 4 |
| 300mm × 600mm deep | 42 L | 5 |
| 300mm × 900mm deep | 64 L | 8 |
| 400mm × 900mm deep | 113 L | 13 |
| 450mm × 1200mm deep | 191 L | 22 |
Bag counts include 5% waste and are rounded up per hole. The post itself takes some of that back — a 100 × 100 post in a 600mm hole displaces about 6 litres, so on a big fence line you can knock a few bags off the total. On a handful of holes, leave the allowance where it is.
Bags or a truck?
Roughly, under half a cubic metre, use bags. Over that, ring the plant. Half a cubic metre is already 54 bags — over a tonne to shift by hand, and about two hours of mixing before you have even started finishing.
- Bags win on post holes, small pads, a mower strip, a single step, or anywhere a truck and a barrow run cannot get to.
- Ready-mix wins on anything you want to finish as one slab. Concrete you mix in batches over three hours is not one pour — the early batches set before the late ones arrive and you get a cold joint through the middle of the slab.
- Ask about the short-load fee. Most Australian plants add a charge below about a cubic metre, and some have a minimum load regardless. Two small pours booked on the same truck is usually cheaper than two deliveries.
Order up, never down. Concrete you did not use is a wheelbarrow of waste. Concrete you ran short of is a cold joint, a second delivery fee, and a visible line across the customer's new driveway forever.
What this calculator does not cover
Volume only. It does not size your reinforcement, your slab thickness, your footing depth or your ground preparation — those depend on soil classification, loading and the design, and they come from an engineer, not from a form on a web page. If the job needs council sign-off, the design comes first and the volume follows it.
Also not in the number: mesh and bar, chairs, plastic under the slab, formwork, sand blinding and the cost of getting the truck close enough to chute it. On a quote, those are usually more money than the concrete.
Ordering from the plant
When you ring, they will want the volume, the strength grade, the aggregate size, the slump, and whether you want it pumped. Have all five ready. Say the volume the way this calculator gives it — in whole 0.2 m³ steps — because that is how they batch, and asking for 2.53 m³ just means somebody rounds it for you without telling you which way.
Concrete sorted. Now get paid for it.
Yamate turns the truck, the bags and your hours into a quote the customer accepts on their phone, then an invoice, then a payment — with the GST worked out and the follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberConcrete volume & bags FAQ
How many 20kg bags of concrete in a cubic metre?
About 108. Each 20kg bag of premix makes roughly 0.0093 m³ of set concrete, so a cubic metre is 108 bags — a bit over two tonnes. Check the yield printed on the bag you are buying, because it varies slightly by brand.
How much concrete for a 6m × 4m slab?
At 100mm thick, 2.4 m³ neat. Add 5% for waste and an uneven subgrade and you order 2.6 m³ — or 271 × 20kg bags, which is why nobody bags a slab that size.
How much concrete goes in a post hole?
A 300mm hole 600mm deep takes about 42 litres, which is five 20kg bags with a bit of waste. A 200mm hole at the same depth takes 19 litres, or three bags. Enter your own hole size and count above.
Should I use bags or order ready-mix?
Under about half a cubic metre, bags. Over that, order the truck. Half a cubic metre is 54 bags and over a tonne to carry, and anything you mix in batches across a few hours will not finish as one slab.
Why does the calculator round the order up to 0.2 m³?
Because that is the step most Australian plants batch in, and running short mid-pour is the one mistake you cannot fix. A barrow of leftover concrete costs a few dollars. A cold joint through a new driveway is there for good.
How thick should a concrete slab be?
This calculator will not tell you — thickness depends on the soil classification, what is going on top of it and the design, so it comes from your engineer or the plan. Domestic shed and patio slabs are commonly 100mm, driveways 100 to 125mm, but treat those as context, not a specification.
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. This is a volume estimate to help you order and quote, not a structural design. Slab thickness, footing depth, reinforcement and ground preparation depend on soil classification and loading and must come from your engineer or your plans. Bag yields vary by brand — check the bag. Confirm quantities, grade, slump and delivery minimums with your concrete supplier before you book.