Concrete Calculator

To calculate the concrete volume needed for a slab, multiply the length by the width by the thickness — all measured in metres — to get the result in cubic metres (m3). For example, a slab 4 metres long, 5 metres wide, and 0.1 metres (100mm) thick requires 4 x 5 x 0.1 = 2.0m3 of concrete. Always add at least 10% atu to account for uneven subgrade, spillage, and over-pour at edges, so order 2.2m3 in this example. For irregular shapes, break the area into rectangles and sum the volumes. Also account for any thickened edges or rebates. For reinforced slabs, the concrete volume does not change but you will also need to calculate rebar or mesh quantities separately. Source: Firth Industries firth.co.nz; Cement and Concrete Association NZ.

Ready-mix concrete in New Zealand typically costs $200-350 per cubic metre for a standard 25 MPa mix, depending on your region and supplier. Most ready-mix suppliers have a minimum load of 3-4 cubic metres, so small jobs incur a short-load surcharge that can add $100-200 to the order. Delivery is usually included within a certain radius, with extra charges for longer distances. If the truck cannot reach the pour location, concrete pump hire adds approximately $500-1,000 per day. Specialty mixes (high-strength, coloured, exposed aggregate) cost more. Always confirm pricing with local suppliers such as Firth, Holcim, or Allied Concrete, as prices vary by region. Source: Firth Industries firth.co.nz; Cement and Concrete Association NZ.

Approximately 108 x 20kg bags of premix concrete are required to make one cubic metre (1m3) of mixed concrete, based on the typical yield of around 9.3 litres per 20kg bag. This makes premix bags practical for small jobs such as setting posts, filling holes, or repairing footings, but very labour intensive and expensive for volumes over 0.5m3. Premix concrete bags in New Zealand typically cost $10-15 per 20kg bag at hardware stores, so 1m3 of premix costs approximately $1,080-1,620 in materials alone — significantly more than ready-mix ($200-350/m3). For jobs over about 0.5m3, it is usually more economical to order ready-mix concrete. Source: Firth Industries firth.co.nz; Cement and Concrete Association NZ.

Concrete strength requirements in New Zealand are specified in megapascals (MPa) and depend on the application. Residential floor slabs and driveways typically require 25-30 MPa concrete. Footings for residential buildings are generally 20-25 MPa. Structural elements such as columns and beams are specified by a structural engineer, typically 30-40 MPa or higher. NZS 3109 (Concrete Construction) sets out the New Zealand standard for concrete construction, while NZS 3101 covers reinforced concrete design. Ready-mix suppliers offer standard mixes at 17.5, 20, 25, 30, and 40 MPa. For earthquake-prone NZ, higher-strength concrete and ductile reinforcing are often required in structural elements per NZS 3101. Source: NZS 3109 Concrete Construction; Firth Industries firth.co.nz.