Concrete is just sums until the truck turns up. On the day, every question comes down to the same few sums: how much volume, how many bags, what mix, and how deep the post holes. This page is the on-site concrete calculator, all in one place — each method worked out by hand, in metric. No rules to look up: a cubic metre is a cubic metre in every country.
What you can work out
Four jobs cover almost everything you’ll pour on a house block. Each one has its own step-by-step guide:
| Job | The sum | Deep dive |
|---|---|---|
| Slab / footing volume | length × width × depth | Concrete volume for a slab |
| Bags vs ready-mix | volume ÷ bag yield | Bags of concrete per m³ |
| Post & fence holes | hole volume − post volume | Concrete for post holes |
| Mix ratio | parts cement : sand : aggregate | covered below |
Concrete volume: length × width × depth
Everything starts here. Concrete is sold and mixed by volume, so you need cubic metres. Work in metres and the answer comes out in m³ — no converting needed.
Say you’re pouring a shed slab, 6.0 m long × 4.0 m wide × 100 mm thick.
- Put the thickness in metres. 100 mm = 0.100 m.
- Multiply the three. 6.0 × 4.0 × 0.100 = 2.4 m³.
- Add wastage. The ground is never flat and the barrow spills some. Add 10%: 2.4 × 1.10 = 2.64 m³.
- Round up to order. Ready-mix comes in 0.1 m³ steps, so order 2.7 m³.
The one thing that trips people up is mixing units. Leave the thickness in millimetres and multiply it by metres, and your answer comes out a thousand times too big. Get everything into metres first, every time.
Bags or ready-mix?
Bagged pre-mix is handy for small jobs, but the bag count adds up fast. A 20 kg bag makes about 0.009 m³ — check this number on the bag, because it changes by product.
For that 2.4 m³ slab:
2.4 m³ ÷ 0.009 m³ per bag ≈ 267 bags
That’s most of a pallet, mixed by hand. Somewhere around 0.5 m³ it flips: a truck of ready-mix becomes cheaper, faster and more even than bags. Use bags for post holes and small pads. Order a truck for slabs.
Mix ratios in one line
If you’re mixing your own, the mix is written as parts of cement : sand : aggregate, by volume. A 1:2:3 mix — one part cement, two sand, three aggregate — is a good all-rounder for most jobs. A thinner 1:2:4 mix is fine for fill that doesn’t carry load. A richer mix is used where you need more strength. Measure the parts by volume, not weight. The right one depends on the job, so treat 1:2:3 as a starting point, not a rule — and follow the product guidance or your engineer’s spec.
Post holes: hole minus post
A post hole isn’t a full cylinder of concrete — the post sits in the middle. So work out the hole, then take away the post.
A 300 mm wide hole, 600 mm deep, for a 90 × 90 mm timber post:
- Hole volume (a cylinder: π × radius² × depth). Radius = 150 mm = 0.150 m. π × 0.150² × 0.600 = π × 0.0135 = 0.042 m³.
- Post volume. 0.090 × 0.090 × 0.600 = 0.005 m³.
- Concrete needed. 0.042 − 0.005 = 0.037 m³ per hole.
Ten of those is 0.37 m³ — that’s bag work (about four 20 kg bags a hole). For a run of fence posts, count the holes, multiply, and add a bit for the odd deep one.
Do it in 5 seconds
Sources
Working out concrete is just shapes and sums, not rules — there’s no code behind a cubic metre, so nothing here changes by state or country. Two numbers do change by product: check the amount each bag makes on the actual bag you’re buying, and take the mix ratio and any strength you need from the maker’s guidance or your engineer. (The bag-yield and mix-ratio figures above are typical examples, vary by product — always check the current bag.)