How it’s worked out
Slab or footing: length × width × thickness (converted to metres). Post holes: π × radius² × depth for each hole, minus the volume of the post. Then we add your waste allowance, because ground is never perfectly level and holes are never perfectly round.
Bags: volume ÷ the bag’s yield. We default to 0.0095 m³ per 20 kg bag, slightly under the 0.01 m³ many bags quote, so you don’t run short. Rapid-set post mixes often yield a little less; check the bag.
Before you pour
- Check whether the structure needs council approval, such as sheds or decks.
- Call Before You Dig Australia (1100) before digging post holes or footings to locate underground services.
- Wear gloves and eye protection: wet cement is caustic and can burn skin.
- For paths and driveways, you’ll also need road base under the slab. See the soil, mulch and gravel calculator.
Questions
How many bags of concrete do I need per cubic metre?
A 20 kg bag of concrete mix typically makes about 0.01 m³, so roughly 100–110 bags per cubic metre. Check the yield printed on the bag, as it varies by product.
How many bags of concrete for a fence post?
It depends on the hole. A 300 mm wide, 600 mm deep hole holds about 0.042 m³, or about 0.036 m³ once a 100 × 100 mm post is in it, which is roughly 4 × 20 kg bags. Smaller holes for light posts may need only 2.
When should I order premixed concrete instead of bags?
Once you need more than about 1 m³ (100+ bags), a premix truck is usually cheaper and far less work. Many suppliers have a minimum order and charge extra for small loads.
How thick should a concrete slab be?
Garden shed and path slabs are commonly 75–100 mm, and driveways around 100–125 mm with reinforcing mesh. Anything structural, or a slab for a building that needs approval, must follow the engineer’s or building approval’s specification.
Estimates only. Check the product label and your measurements before you buy. More free tools: see them all.