Concrete Bag Calculator NZ
Use this concrete bag calculator to quickly work out how many 20kg or 25kg bags you need for your project in New Zealand.
How to use this concrete calculator
Enter your length and width in metres, and depth in millimetres. Most New Zealand projects use:
75–100mm for paths and patios
100–125mm for driveways
150mm+ for heavy vehicles
300mm+ for fence posts
Select your bag size and click calculate.
The result includes a 10% waste buffer, which is recommended for most jobs to avoid running short.
NZ concrete depth guide
Getting the depth right is one of the most common mistakes on DIY projects.
Use these standard guidelines:
Footpaths / garden paths: 75–100mm
Driveways (cars): 100–125mm
Driveways (heavy vehicles): 150mm+
Concrete slabs: 100mm standard
Fence posts: minimum 300mm deep (or 1/3 of post height)
Deck footings: 400–600mm depending on load
How many concrete bags per cubic metre (NZ)
For standard ready-mix concrete in New Zealand:
20kg bags: ~108 bags per m³
25kg bags: ~86 bags per m³
These figures are based on typical products from suppliers like Firth and Drymix.
Always allow an additional 10% for waste, uneven ground, and spillage.
Bags vs ready-mix truck (what to use)
Choosing the right option can save you time and money:
Under 0.5m³: bags (Mitre 10 / Bunnings)
0.5m³ – 1m³: mini-mix truck (often cheaper than bags)
1m³+: ready-mix truck delivery
Major NZ suppliers include Allied Concrete, Firth, and Holcim.
Related tools:
- Roofing Calculator NZ
Frequently asked questions
How much area will a 20kg bag of concrete cover?
One 20kg bag covers approximately 0.009 cubic metres. For a 100mm deep slab that's roughly 0.09 square metres per bag. For a 75mm deep path it's around 0.12 square metres per bag.
How do I calculate how many concrete bags I need?
Multiply length by width by depth in metres to get cubic metres. Then multiply by 108 for 20kg bags or 86 for 25kg bags. Always add 10% extra for waste.
How many 20kg bags of concrete make 1m3?
Approximately 108 bags of 20kg concrete make one cubic metre. This is based on standard NZ ready-mix products.
How many bags of concrete per cubic metre?
108 bags of 20kg or 86 bags of 25kg per cubic metre of concrete.
What depth should a concrete driveway be in NZ?
Standard NZ residential driveways are 100mm to 125mm deep. For heavy vehicles or trailers increase to 150mm minimum.