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 explains the quantity calculations in metric. The volume maths is universal; dimensions, mix specification and construction requirements still come from the design and applicable local rules.
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.
- Apply an example 10% allowance for this sum: 2.4 × 1.10 = 2.64 m³.
- Round up to order. If your supplier sells in 0.1 m³ increments, round 2.64 up to 2.7 m³. Confirm their order increments and the allowance for your pour.
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. For this example only, assume the selected bag yields 0.009 m³ (9 litres) of mixed concrete. Use the actual product’s declared mixed yield.
For that 2.4 m³ slab:
2.4 m³ ÷ 0.009 m³ per bag ≈ 267 bags.
That is 267 bags before allowance, or 294 bags for the 2.64 m³ allowance-adjusted quantity. Pallet counts and the cost crossover with ready-mix vary by supplier. Compare delivery, small-load charges, access and labour.
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 ratio means one part cement, two sand and three aggregate. That describes proportions, not guaranteed strength or suitability. Take the mix specification from the approved design and supplier; do not choose a structural mix from a generic ratio.
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 ≈ 0.0424115 m³.
- Post volume. 0.090 × 0.090 × 0.600 = 0.00486 m³.
- Concrete needed. 0.0424115 − 0.00486 ≈ 0.03755 m³ per hole.
Ten holes need about 0.37552 m³ before allowance. At the example 9-litre bag yield, that is 42 bags when batching the combined quantity, or 46 bags with a 10% allowance. One hole alone needs 5 whole bags before allowance. Keep intermediate values unrounded and check the actual yield.
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.)