Concrete Calculator for NZ Builders & Tradies
Use this calculator to work out how much concrete you need for slabs, paths, strip footings, or post holes. Enter your dimensions and get the volume in m³ plus the equivalent number of 20kg bags.
Concrete Rules of Thumb
- Slabs & paths: 100mm deep is standard for pedestrian areas; 150mm for vehicle traffic
- Ready-mix vs bagged: Ready-mix is more economical for anything over 0.5 m³ — typically $260–$320/m³ in NZ
- One 20kg bag of dry-mix concrete yields approximately 0.01 m³
- Always add 10% wastage to account for uneven subgrade and spillage
Concrete Strength Grades (NZ)
| Grade | Common Use | Typical Compressive Strength |
|---|---|---|
| 17.5 MPa | Non-structural fills | Low-load paths |
| 25 MPa | Residential slabs, paths | Standard residential |
| 30 MPa | Driveways, structural | Vehicle loads |
| 40 MPa | Commercial, structural | Heavy loads |
Hiring a Concrete Contractor?
Check our driveway cost guide for typical NZ prices, or find a local builder or concreter for quotes.