# Concrete Calculations: The Complete On-Site Guide How much concrete do I need? Work out slab volume, bags per cubic metre, mix ratios and post holes by hand — the on-site concrete numbers at a glance, in metric. - Canonical HTML: https://www.chippy.tools/building-codes/concrete/concrete-calculations-guide/ - Publisher: Chippy Tools - Updated: 2026-09-06 - Market: global 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](https://www.chippy.tools/building-codes/concrete/concrete-volume-slab/) | | Bags vs ready-mix | volume ÷ bag yield | [Bags of concrete per m³](https://www.chippy.tools/building-codes/concrete/bags-of-concrete-per-m3/) | | Post & fence holes | hole volume − post volume | [Concrete for post holes](https://www.chippy.tools/building-codes/concrete/concrete-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**. 1. Put the thickness in metres. 100 mm = **0.100 m**. 2. Multiply the three. 6.0 × 4.0 × 0.100 = **2.4 m³**. 3. Apply an example 10% allowance for this sum: 2.4 × 1.10 = **2.64 m³**. 4. 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. **Diagram labels:** W = 4.0 m; L = 6.0 m; D = 0.100 m; V = L × W × D = 2.4 m³ Slab volume — length × width × depth. 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: 1. Hole volume (a cylinder: π × radius² × depth). Radius = 150 mm = 0.150 m. π × 0.150² × 0.600 ≈ 0.0424115 m³. 2. Post volume. 0.090 × 0.090 × 0.600 = 0.00486 m³. 3. 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.)* This page is general information for tradespeople and is not compliance, engineering or legal advice. Requirements are subject to state and territory variations and adoption timing. Always confirm requirements with your building certifier and relevant state authority. ## Frequently asked questions ### How much concrete do I need for a slab? Multiply length × width × thickness in metres. A 6 m × 4 m slab at 0.1 m thick is 2.4 m³. An example 10% allowance gives 2.64 m³. Agree the allowance and order increments with the supplier before rounding the order. ### How many bags of concrete are in a cubic metre? Use the manufacturer's mixed yield, not bag weight alone. If a bag yields 0.009 m³ (9 litres), 1 ÷ 0.009 = 111.11, so round up to 112 bags before any allowance. Compare delivered ready-mix and bag prices for your actual quantity. ### How do I calculate concrete for a post hole? Use cylinder volume minus buried post volume. A 300 mm wide, 600 mm deep hole with a 90 × 90 mm post needs about 0.03755 m³. At an assumed 9-litre mixed yield, round up to 5 bags for one hole, before allowance. Check the actual bag yield. ### What is the best concrete mix ratio? Use the specified mix for the job. Nominal cement:sand:aggregate ratios explain proportions but do not establish concrete strength or suitability. Follow the approved design and supplier instructions. ### How much extra concrete should I order for wastage? Agree an allowance with the supplier based on the pour, ground and formwork. The 5–10% figures in these examples are assumptions, not a universal rule. Check delivery increments and plan the pour with the responsible contractor.