The Definitive Guide to Calculating Concrete Volumes
Pouring concrete for a shed slab, a path, or post hole footings requires precision. In Australia, commercial concrete is ordered and sold by the cubic metre (m³). Calculating the exact volume keeps you from running short mid-pour, which risks a visible cold joint in the finished surface.
Choosing the Right Depth
- Pedestrian Footpaths & Patios: 75–100mm deep, typically with SL72 reinforcement mesh.
- Shed Slabs & Residential Driveways: 100–125mm deep over a compacted road base layer.
- Structural Footings & Posts: Often 300–600mm deep cylindrical holes, depending on soil movement class.
Bags or Ready-Mix?
A 20kg bag of pre-mix concrete yields approximately 0.009m³ — roughly 110 bags per cubic metre, confirmed against manufacturer technical data. Bagged concrete typically costs several times more per cubic metre than a ready-mix truck delivery, but ready-mix suppliers charge a minimum delivery fee that makes very small pours impractical. As a general guide, once a project passes roughly 0.5m³ of concrete, a ready-mix delivery is usually the cheaper and more consistent option, even after accounting for the delivery fee.
Frequently Asked Questions
Select "Square / Rectangular Slab" above and enter your shed's length and width. Most residential shed slabs use 100–125mm depth over a compacted road base layer — the calculator will convert that straight into cubic metres, bag counts, and a ready-mix cost estimate.
Select "Cylindrical Pier / Post Hole Footings" above, then enter the hole diameter, depth, and how many holes you're filling. A common fence post footing is around 300mm diameter and 600mm deep — the calculator totals the volume across all your holes at once and converts it to a bag count.
Ready-mix suppliers charge a minimum delivery fee regardless of how little concrete you order, since a truck trip costs the same whether it delivers 0.2m³ or 2m³. For very small pours like a couple of post holes, that flat fee can outweigh the per-cubic-metre savings.
Yes — 0.009m³ per 20kg bag (around 110 bags per cubic metre) is confirmed by manufacturer technical datasheets. Some specific product lines are rated slightly differently (around 100 bags per m³), so check your chosen brand's packaging if you want an exact match.
For most pedestrian paths and patios, standard SL72 mesh is common practice to control cracking. Driveways and structural slabs often need heavier mesh or bar reinforcement — check your local building requirements or ask your supplier for the correct grade for your application.
Concrete weighs roughly 2.4 tonnes per cubic metre once cured. This matters for structural load calculations, and for practical site access — a concrete pump or agitator truck needs to know the total load it's dealing with.