Concrete Calculator NZ
This calculator works out how much concrete you need for a slab, path, footing or post holes, and turns that volume into a number of premix bags so you can decide whether to buy bags or order a ready-mix truck. Enter the length and width of the area in metres and the thickness in millimetres, since slab thickness is normally specified that way, then add a wastage allowance as a percentage to cover the concrete that is always lost to spillage, uneven ground and over-filling, with 5 to 10% a sensible default. The calculator multiplies length by width by thickness, converted to metres, to get the base volume, then adds your wastage percentage on top. It returns the total volume needed in cubic metres, the base volume before wastage, and how many 20kg and 25kg premix bags that volume would take, based on typical yields of about 0.009 and 0.0112 cubic metres per bag. Treat the bag counts as a guide only, since yield varies by brand, and check the figure printed on your chosen product. As a rule of thumb, bags suit small jobs up to around half a cubic metre; beyond that, ordering ready-mix concrete delivered by truck usually works out cheaper, faster and gives a more consistent finish. These figures are indicative estimates for planning a pour and do not replace advice from your concrete supplier or a builder for structural work.
Bag counts use typical yields (20kg bag about 0.009 m³, 25kg bag about 0.0112 m³). Check the yield on your chosen product. For pours over about 0.5 m³, ready-mix is usually better value.
How to calculate concrete volume
Concrete volume is simply length times width times thickness, with every measurement in the same unit. Because slab thickness is usually given in millimetres, divide it by 1,000 to convert to metres first. The result is the volume in cubic metres, which is how ready-mix concrete is ordered.
Worked New Zealand example
You are pouring a 4 m by 3 m shed slab at 100 mm thick. The base volume is 4 x 3 x 0.1, which is 1.2 cubic metres. Adding a 10% wastage allowance brings it to about 1.32 cubic metres.
That is well over half a cubic metre, so ready-mix delivered by truck would be the sensible choice rather than mixing roughly 147 bags by hand.
Bags or ready-mix?
- Bags suit small jobs: post holes, small paths, repairs, up to about 0.5 cubic metres.
- Ready-mix suits larger pours: driveways, slabs, and floors. It is cheaper per cubic metre at volume and gives a consistent mix.
- Always add a wastage allowance of 5 to 10% so you do not run short mid-pour.
Frequently asked questions
How many 20kg bags are in a cubic metre?
Roughly 110, because a 20kg bag yields about 0.009 cubic metres. A 25kg bag yields about 0.0112 cubic metres, so about 88 per cubic metre. Confirm the figure on the bag.
What thickness should a slab be?
That depends on the use and must meet the relevant building requirements. A common domestic path or shed slab is 100 mm, but always follow the engineering or council specification for your project.
Who this calculator is for
This calculator is for DIYers and builders estimating the concrete needed for a slab, footing, path or post holes.
What this calculator assumes
- The dimensions you enter.
- Volume is given in cubic metres.
- You should add a wastage allowance, as some concrete is always lost.
- Bag yields vary by product, so check the bag for its coverage.
Related calculators
- Cubic Metre Calculator: volume of any box-shaped space.
- Square Metre Calculator: floor area in square metres.
- Volume Conversion Calculator: convert between volume units.
- Area Conversion Calculator: convert between area units.
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