Concrete Calculator NZ: Cubic Metres for Slabs, Footings & Post Holes
Work out how many cubic metres of concrete you need for a slab, path, driveway section, rectangular footing or post holes. Add a project allowance and see the result in both m³ and litres.
Reviewed 10 August 2026 · Metric units for New Zealand projects
Direct answer
How much concrete do I need?
For a rectangular pour, concrete volume is length × width × depth. A 4 m × 3 m slab that is 100 mm thick is 1.2 m³ before any allowance. Use the calculator below for slabs, footings and circular post holes.
Your concrete dimensions
Enter valid positive dimensions and a quantity of at least 1.
Your concrete estimate
Concrete volume formulas
| Project | Formula | Notes |
|---|---|---|
| Slab, path or footing | length × width × depth | Convert depth from millimetres to metres first. |
| Round post hole | π × radius² × depth | Multiply by the number of holes. |
| Round post in round hole | (hole circle area − post circle area) × depth | The post displaces some concrete. |
| Square post in round hole | (hole circle area − post width²) × depth | Assumes the post runs through the full entered hole depth. |
Concrete calculator examples
Example 1: 4 m × 3 m slab at 100 mm thick
Convert 100 mm to 0.1 m, then calculate 4 × 3 × 0.1 = 1.2 m³. With a 5% allowance, the planning total is 1.26 m³.
Example 2: ten 300 mm diameter post holes, 600 mm deep
A 300 mm diameter is a 0.15 m radius. One hole is approximately π × 0.15² × 0.6 = 0.0424 m³. Ten holes are about 0.424 m³ before an allowance and before subtracting any post volume.
How much extra concrete should I allow?
The exact allowance depends on how accurately the area is formed or excavated. Uneven ground, over-dig, spillage and measurement differences can increase the real volume. This calculator starts at 5% because that is a common guide used by major NZ ready-mix calculators, but the field is editable.
NZ supplier references: Firth Concrete Volume Calculator and Allied Concrete Volume Calculator. Both describe their results as guides; confirm your actual order with the supplier.
Concrete m³ vs 20kg bags
This page targets concrete volume for pours and ready-mix planning. It deliberately does not assume a fixed 20 kg bag yield, because premix products can produce different volumes. If you are doing a smaller bagged-concrete job, use the separate Concrete Bag Calculator and select the yield that matches the product packaging.
Before ordering concrete
Concrete Calculator NZ FAQs
How do I convert millimetres to metres for concrete?
Divide millimetres by 1,000. For example, 100 mm = 0.1 m and 150 mm = 0.15 m.
How many litres are in one cubic metre of concrete?
One cubic metre is 1,000 litres. A 0.75 m³ concrete volume is therefore 750 litres.
Can I calculate a driveway or path?
Yes. Use the slab/rectangular option and enter the length, width and average concrete thickness. Complex shapes can be divided into rectangles and calculated separately.
Can this calculate concrete for fence posts?
Yes. Choose a post-hole option. You can calculate the full circular hole or subtract a round or square post volume.
Should I order exactly the amount shown?
Not automatically. The calculator gives a planning volume. Confirm the job dimensions and the supplier's ordering guidance, minimum quantities and allowance before purchase.