Calculating how much concrete you need comes down to one simple formula: length × width × depth = volume in cubic metres (m³). Get this right before you order and you'll avoid costly over-ordering or frustrating shortfalls. Here's a practical, step-by-step guide tailored for common NZ DIY concrete projects.
The Basic Formula
All concrete volume calculations start with the same principle:
Volume (m³) = Length (m) × Width (m) × Depth (m)
The key is to make sure all measurements are in metres before you multiply. Depth in particular is often measured in millimetres on plans — remember to convert (e.g. 100 mm = 0.1 m).
Quick example: a garden path
Say you're pouring a path that is 10 m long, 1.2 m wide, and 100 mm (0.1 m) deep:
10 × 1.2 × 0.1 = 1.2 m³
Always add 10% for waste, spillage, and uneven subgrade:
1.2 × 1.10 = 1.32 m³ — so you'd order 1.4 m³ to be safe.
Recommended Depths for Common NZ Projects
Using the right depth is just as important as measuring the area accurately. Here are typical minimum depths used in NZ residential work:
| Project | Typical Depth | Notes |
|---|---|---|
| Garden path (foot traffic) | 75–100 mm | 100 mm preferred for longevity |
| Driveway (cars) | 100–125 mm | 125 mm for heavier vehicles |
| House slab (light) | 100 mm | Engineer/council may specify more |
| Garage floor | 100–150 mm | Check with your local council |
| Shed base | 75–100 mm | Depends on shed size and load |
| Footings/foundations | Varies | Always consult engineer or council |
Note: Slab thickness and reinforcing requirements for habitable buildings are governed by the NZ Building Code and standards such as NZS 3604. Always check with your local council whether a building consent is required before pouring a slab — some work is exempt under Schedule 1, but rules vary by project type and size.
Calculating Irregular Shapes
Not every pour is a neat rectangle. Here's how to handle common shapes:
Circular slabs (e.g. a round pad)
Volume = π × radius² × depth
For a 2 m diameter circle (radius = 1 m), 100 mm deep: 3.1416 × 1² × 0.1 = 0.314 m³ (plus 10% = ~0.35 m³)
L-shapes or irregular areas
Break the shape into separate rectangles, calculate each one individually, then add the volumes together.
Sloped ground
If your ground isn't level, measure the average depth across the area. For significant slopes, it's worth getting a concreter to assess the site.
Bagged Concrete vs. Ready-Mix: Which to Use?
Once you know your volume, you need to decide how to source your concrete.
- Bagged concrete mix (available from Bunnings or Mitre 10): Practical for small jobs under about 0.5 m³. A standard 20 kg bag typically yields around 0.009–0.010 m³ when mixed, so you'll need roughly 100 bags per cubic metre — check the bag for the manufacturer's stated yield.
- Ready-mix (truck-delivered): More economical and consistent for anything over 0.5–1 m³. Suppliers generally have a minimum order (often around 0.5–1 m³) and charge a short-load fee for small quantities. Ring your local ready-mix supplier for current minimums and pricing.
- Volumetric mixer (on-site mixing): A good middle-ground option for medium jobs where you want fresh concrete without a full truck. Ask your local concrete supplier if this is available in your area.
Concrete Strength (Grade)
For most residential NZ work, 17.5 MPa or 20 MPa concrete is standard for paths and slabs. Driveways and garage floors often call for 25 MPa. If you're unsure what strength to specify, ask your ready-mix supplier or a licensed concreter — using the wrong grade can cause premature cracking or surface failure.
Tips to Get Your Measurement Right
- Measure twice — small errors in depth add up quickly over a large area.
- Account for formwork thickness — measure the inside dimensions of your forms, not the outside.
- Check for soft spots — if your subgrade is uneven or soft, your actual depth may vary. Compact and level the base before calculating.
- Use a concrete calculator — many ready-mix suppliers and hardware stores offer online calculators as a handy cross-check.
- For projects involving structural elements, retaining walls over 1.5 m, or any habitable building, consult MBIE Building Performance guidance and your local council before proceeding.
When to Call a Professional
While calculating concrete volume is something most homeowners can do themselves, pouring and finishing concrete well is a skilled trade. Consider hiring a qualified concreter for:
- Slabs larger than a small garden shed base
- Driveways, especially with reinforcing mesh or steel
- Any structural footings or foundations
- Sloped or complex sites
- Projects requiring a building consent
A professional concreter will also verify your volume calculations, specify the correct mix design, and ensure the pour meets the NZ Building Code. You can find qualified local concreters on tradedirectory.co.nz.
For guidance on best-practice construction in NZ, BRANZ is an excellent independent resource.
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