Concrete, Gravel and Sand Calculator

Guide

How the amount is worked out

Slab volume = length × width × thickness, in metres. Add waste, then divide by how much one bag makes.

Example. A 3 × 3 m slab 100 mm thick is 3 × 3 × 0.1 = 0.9 m³. A 20 kg bag of premix makes about 0.01 m³, so that is 90 bags with no waste allowance, and 99 bags with 10% extra.

Bag yields

The Easy Mix Super Strength data sheet lists about 99 bags of 20 kg per cubic metre, and a Bunnings-hosted Easy Mix sheet gave about 0.015 m³ for a 30 kg bag when we checked it through a search (check your bag). The tool rounds the 20 kg figure to 0.01 m³. Other brands differ, so enter the yield from your bag or data sheet.

Ready-mix or bags?

For more than about a cubic metre it is usually easier to order ready-mixed concrete than to mix bags by hand. Suppliers have minimum loads and short-load fees, so call and ask.

Gravel, sand and topsoil

Suppliers sell these by the cubic metre or the tonne. To go from volume to weight multiply by a density. The densities here are editorial starting points, not standards: they change with moisture and grading, so ask your supplier.

Safety and rules

Before you dig, request plans of underground services from Before You Dig Australia, and check council rules and any approvals that apply to your property. This is not building or structural advice. Footings and slabs that carry loads need design and approval under the building rules in your state.

Common questions