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NZ Building Code · MaterialsConcrete grades & mixes, plain English
A builder’s rundown of NZ concrete grades by MPa, what slump and water/cement ratio actually mean on site, and why the grade you pick matters most near the coast.
Concrete grade is the strength of the mix, measured in MPa (megapascals, or N/mm²). Higher MPa means stronger and denser concrete – and picking the right grade matters, because the wrong call in a coastal zone can cost you a slab in under a decade.
What the grade number means
The MPa figure tells you how strong the cured concrete is. Mix design gets you there through the cement type, the water/cement ratio, the aggregate and any admixtures. As a rule, the tighter the water/cement ratio, the stronger the mix.
- 17.5 MPa – standard residential: pile footings, slab on ground, drives (inland). Slump 80mm, W/C 0.55, Type GP (general purpose) cement. Suits NZS 3604 inland Zone A/B.
- 20 MPa – garage slab, slightly exposed slabs, Zone C inland. Slump 80mm, W/C 0.55, Type GP.
- 25 MPa – coastal Zone D slabs and footings, suspended slabs, NZS 4229 masonry. Slump 80–100mm, W/C 0.50, Type GP or HE (high-early). Mandatory for coastal per NZS 3604.
- 30 MPa – high-load slabs, light commercial, heavily-reinforced beams. Slump 80–100mm, W/C 0.45, Type GP.
- 40 MPa – engineered foundations and suspended slabs, commercial. Slump 100mm, W/C 0.40, Type HE. Always engineer-designed.
Slump – how wet the mix is
Slump is how much fresh concrete drops when the cone is removed in the slump test (NZS 3104). It’s your quick read on workability on site.
- 80mm – stiff: footings and slabs you walk on quickly.
- 100mm – workable: most slabs.
- 120mm+ – wet: pumped through hoses, or congested reinforcing.
Watch the trade-off: higher slump means more water, which means lower strength unless it’s balanced with admixtures.
Bags vs ready-mix
Bagged concrete is for the small stuff – fence posts, small pads. But the yield off a bag is deceptively small, so it adds up fast.
- A 20 kg bag yields ~0.01 m³.
- A 25 kg bag yields ~0.012 m³.
- A 30 kg bag yields ~0.015 m³.
To pour 1 m³ you’re looking at roughly 70 × 20 kg bags – and using 25 kg bags it’s about 85 bags, over 2 tonnes. A ready-mix truck is almost always cheaper above 0.5 m³. Hand-mix only for very small jobs like a post hole or a repair patch.
Coastal concrete (Zone D)
NZ Zone D mandates 25 MPa minimum, and it’s not a corner worth cutting.
- Lower-grade concrete in coastal Zone D lets chloride attack the reinforcing early, which can spall within 5–10 years.
- The extra $30–50/m³ for the right grade is worth it against a slab that fails inside a decade.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
What does the MPa number on concrete mean?
MPa (megapascals, or N/mm²) is the strength of the cured concrete. Higher MPa means stronger and denser. The grade is achieved through the mix design – cement type, water/cement ratio, aggregate and admixtures.
What concrete grade do I need for a coastal (Zone D) slab?
NZ Zone D mandates 25 MPa minimum. Going lower in coastal Zone D lets chloride attack the reinforcing early, which can cause spalling within 5–10 years. The extra $30–50/m³ is worth it.
What does slump tell me about the mix?
Slump (measured by the NZS 3104 slump test) is how much fresh concrete drops when the cone is removed. 80mm is stiff (footings), 100mm is workable (most slabs), and 120mm+ is wet (pumped or congested reinforcing). Higher slump means more water and lower strength unless balanced with admixtures.
How many bags of concrete make up 1 m³?
About 70 × 20 kg bags, or roughly 85 × 25 kg bags (over 2 tonnes). A 20 kg bag yields ~0.01 m³, 25 kg ~0.012 m³, and 30 kg ~0.015 m³. Ready-mix is almost always cheaper above 0.5 m³ – hand-mix only for very small jobs.
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