Concrete Bags Calculator

This calculator works out how many bags of premix concrete you need for a project in New Zealand, whether you are pouring a small slab, setting a path, bedding posts or filling a footing. Bagged premix is the easy option for jobs too small to justify a concrete truck, but it is surprisingly hard to judge by eye how many bags a hole or slab will swallow, and running out halfway through a pour is a real problem because fresh concrete will not bond properly to concrete that has already started to set. Buying far too many bags wastes money and leaves you lugging them back to the store. This tool removes the guesswork. You enter the length, width and depth of what you are filling, choose a waste allowance to cover spillage and uneven ground, and tell it the yield of the bag you are buying, which is usually printed on the bag at around 0.009 to 0.011 cubic metres for a 20kg bag. The calculator returns the number of bags to buy, the bare volume of concrete in cubic metres, and the volume once waste is added. Knowing the cubic metre figure is also handy if you decide the job is big enough to order ready-mix instead, since concrete is sold by the cubic metre. As a rule of thumb you need roughly 90 to 110 bags of 20kg premix per cubic metre, so once you pass about half a cubic metre, ordering ready-mix usually works out cheaper and easier. This is an estimate to help you buy with confidence; always check the yield on your chosen bag.

A 20kg premix bag yields about 0.009 to 0.011 m³; check your bag. Past roughly 0.5 m³, ready-mix is usually cheaper. An estimate only.