Concrete Calculator (Australia)
A 3 m × 2 m slab 100 mm thick needs about 0.66 m³ of concrete with 10% extra, or 72 bags of 20 kg.
Updated · Australian units: metres, millimetres, cubic metres · Built and checked by AI, which can make mistakes
Work out how much concrete you need
Result
This is an estimate. Real quantities depend on uneven ground, settling, compaction and the exact product, and bag sizes and weights vary by brand. Check the bag or ask your supplier before you buy.
Built and checked by AI. AI wrote this calculator and its explanations, researched the rates and ran the tests. AI can make mistakes, and the checks may not catch them all, so confirm anything important with an official source or a qualified professional. How we use AI.
Quick answer
A 3 m × 2 m slab 100 mm thick needs about 0.66 m³ of concrete with 10% extra, or 72 bags of 20 kg.
Working in feet and inches? Use our US concrete calculator.
How to use the concrete calculator
- Choose the shape of the area: a rectangle, a circle, or enter the area if you already know it.
- Enter the measurements in metres and the thickness in millimetres.
- Pick the bag size you plan to buy, and the extra to allow for spillage.
- Optionally open “Compare costs” to see whether bags or bulk is cheaper.
- Press Calculate to see how much you need, with the steps.
How it’s calculated
Area = length × width (m²), or π × (diameter ÷ 2)² for a circle
Volume (m³) = area (m²) × depth (mm) ÷ 1,000
Litres = m³ × 1,000
Concrete to order = volume × (1 + extra % ÷ 100)
Bags = m³ × 108, rounded up (20 kg bags)
Worked example
Concrete for a 3 m × 2 m slab at 100 mm
- Area: 3 m × 2 m = 6 m².
- Volume: 6 m² × 100 mm ÷ 1,000 = 0.6 m³.
- Add 10% for spillage and uneven ground: 0.66 m³.
- In litres: 0.66 m³ × 1,000 = 660 L.
- Bags: 0.66 m³ × 108 bags per m³ = 71.3, rounded up to 72 bags of 20 kg.
Bags or ready-mix?
Bunnings states that 108 bags of Maxpro 20 kg concrete mix make one cubic metre, so each bag makes about 9.3 litres. Other brands may differ slightly, so check the bag.
For larger slabs, ready-mixed concrete delivered by truck is usually easier than mixing dozens of bags. Suppliers sell it by the cubic metre and many have minimum orders, so ask before you book. Concrete work can be dangerous and may need permits. Follow your plans, the product instructions and local building rules.
When you’d actually use a concrete calculator
- Pouring a small slab or path. Work out how many bags to buy, with a little extra.
- Setting fence or deck posts. Use the circle option for each post hole and multiply by the number of holes.
- Choosing bags or ready-mix. Compare the cost once you know the volume.
What this calculator doesn’t include
- Irregular shapes. Split the area into rectangles and circles and add the results.
- Reinforcement, footings that change depth, and steps. Calculate each part separately.
- Delivery fees, minimum orders and local prices, which vary by supplier.
How we calculate
Area comes from your measurements. Volume is area × thickness ÷ 1,000, in cubic metres. We add 10% extra unless you change it. Bag yields for concrete come from the manufacturer’s figures shown in the bag list. We calculate at full precision and round only for display. Bag counts are rounded up to whole bags.
Sources:
Frequently asked questions
How many bags of concrete do I need?
Multiply the volume in cubic metres by the number of bags per cubic metre on the bag. For Maxpro 20 kg concrete mix, Bunnings states 108 bags make one cubic metre.
How much extra concrete should I allow?
Many people add about 5 to 10% for spillage, uneven ground and forms that bow. The calculator adds 10% by default, and you can change it.
Should I use bags or ready-mix?
Bags suit small jobs such as posts and small pads. For bigger slabs, ready-mixed concrete delivered by the cubic metre is usually easier, but suppliers often have minimum orders.