Concrete Calculator
Find how many cubic metres of concrete you need, what goes in the mix, and what a truck costs.
Measure your project
Your answer
We added 5% extra.
If you buy bags
Dricon says a 25 kg bag of HandiCrete makes about 0.01 m³, and a 40 kg bag about 0.016 m³.
If you order a truck (ready-mix)
Ask for 1.00 m³. We round up to the next 0.2 m³.
This answer is a guide. For slabs, footings, drives or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 95 bags by hand. Have the edges ready before it comes.
NZ prices from October 2026, with 15% GST. Bags: middle of 6 sellers. Ready-mix: one merchant's price. We add Allied's $115 small-load fee to every truck. One merchant lists it, and the load size it applies to isn't stated. Set it to $0 if your order is big enough.
Quick answers
- A 3 × 3 m slab, 100 mm thick
- 0.9 m³. With 5% extra, 0.95 m³. Order 1 m³ of ready-mix.
- Ready-mix price
- About $297.28 per m³ for 20 MPa, with GST. Only one merchant lists a price. A cost guide says $280–$360 delivered, GST not stated.
- Builders mix for 1 m³
- About 1.2–1.25 m³ of builders mix and 5–8 bags of 40 kg cement, says Golden Bay.
- Smallest truck load
- Mini-mix firms bring loads from about 0.4 m³. A plant may add a small-load fee. Allied's is $115, listed by one merchant.
How to work it out
- Measure how long and how wide the area is, in metres.
- Measure how thick the concrete will be, in mm. Divide by 1,000 to change it to metres.
- Multiply the three sizes together. That is the volume in m³.
- Add 5% for spills and uneven ground.
- To count 25 kg bags, divide by 0.01 and round up.
3 × 3 × 0.1 = 0.9 m³
0.9 + 5% = 0.945 m³
0.945 ÷ 0.01 = 94.5 → 95 bags
Measure up: the concrete volume formula
Multiply length by width by thickness, with each size in metres. The answer is the volume in cubic metres (m³).
A cubic metre is a cube 1 m on each side. It holds 1,000 litres.
| Thickness | m³ per m² | 25 kg bags per m² |
|---|---|---|
| 75 mm | 0.075 | 7.5 |
| 100 mm | 0.1 | 10 |
| 150 mm | 0.15 | 15 |
These have no extra in them. Put the 5% on top.
Odd shapes
- L shape: treat it as two rectangles, each as its own part.
- Triangle with a square corner: work out the full rectangle, then halve it.
- Piles and post holes: choose "Round holes", then type the width and depth.
- Strip under a wall: use the "Footing" part.
Cubic yards?
Some calculators use cubic yards. NZ plants sell by the cubic metre, a metric unit. One cubic yard is about 0.76 m³.
Ready-mix prices and fees in NZ
In 25 kg bags, 1 m³ costs about $1,846. That is about 6 times the ready-mix price.
Our ready-mix price is weak. No NZ plant we checked lists its prices, so ask for a quote. Only one merchant lists ready-mix prices:
| Mix | Per m³, with GST |
|---|---|
| 20 MPa | $297.28 |
| 20 MPa line-pump mix | $313.72 |
| 25 MPa line-pump mix | $323.21 |
Kiwi Timber (Waikato), Bowers ready-mix, special order, October 2026. The listing does not say if delivery is in.
NZ Tradie Tools, a cost guide, gives $280–$360 per m³ delivered. GST is not stated.
Small loads and fees
- Mini-mix firms bring small loads of about 0.4–2.4 m³.
- Allied Concrete's small-load charge is $115 a load with GST. One merchant lists it, and the size it applies to isn't stated. Our calculator adds it to every truck.
- Firth adds a temporary fuel surcharge of $9.90 per m³ from 5 Oct 2026. GST is not stated.
- Golden Bay, a cement maker, suggests ready-mix for jobs over 3 m³.
Change any fee under "Change prices". How many bags before a truck wins? See our concrete bag calculator.
Concrete mix: builders mix and cement for 1 m³
Builders mix is sand and gravel (the coarse aggregate) already mixed. Golden Bay calls it "all-in". You add cement and water.
| Job | Cement : builders mix | 40 kg cement bags | Builders mix | Cost per m³ |
|---|---|---|---|---|
| Paths and mowing strips | 1 : 6 | 6 | 1.25 m³ | $293 |
| Driveways | 1 : 5 | 7.5 | 1.2 m³ | $331 |
| Fence posts and bedding | 1 : 8 | 5 | 1.25 m³ | $264 |
| Foundations and walls | 1 : 5 | 8 | 1.2 m³ | $345 |
Builders mix is the loose amount to buy. Cost uses 20 kg cement at $14.29 (only 2 sellers) and builders mix at $97 per m³ (only 3 sellers). With GST, before delivery.
Using 20 kg cement bags? Buy 2 times as many. By these prices, mixing your own costs about $264–$345 per m³. Ready-mix is about $297 before fees, and you skip the mixing.
Cement for each strength (NZS 3104)
| Strength | Use | Cement per m³ | 40 kg bags | 20 kg bags |
|---|---|---|---|---|
| 10 MPa | bedding, fence posts | 200 kg | 5 | 10 |
| 15 MPa | paths, implement sheds | 240 kg | 6 | 12 |
| 17.5 or 20 MPa | NZS 3604 work | 300 kg | 7.5 | 15 |
| 25 MPa | exposed concrete in sea-spray areas | 340 kg | 8.5 | 17 |
NZS 3104 mixes as Golden Bay gives them, made with damp builders mix.
Golden Bay warns that too much water affects the quality. The water per m³ is in the questions below.
A house slab has its own rules. Our concrete slab calculator covers NZS 3604.
Before you order ready-mix
- Get the price per m³, and check that it includes GST.
- Ask about small-load and fuel charges.
- Ask how long the truck can wait. Allied may charge for time over 10 minutes per m³ on site. For a 2 m³ load, that is 20 minutes.
- Check the truck can reach. One Christchurch mini-mix truck is 6 m long, 2.4 m wide and 3.2 m high. Its chute reaches 3 m behind.
- Need a pump? MaxPump in Auckland charges at least $430 a job. GST is not stated.
Questions people ask
How do I work out how much concrete I need?
Find the volume: length × width × thickness, with all three in metres. Then add 5% more, as Allied Concrete suggests.
How much builders mix for 1 cubic metre of concrete?
About 1.2–1.25 m³ of builders mix, says Golden Bay. Add 5–8 bags of 40 kg cement, by the job.
How many bags of cement make 1 m³ of concrete?
With builders mix, 5–8 bags of 40 kg, says Golden Bay. NZS 3104 uses 300 kg for 17.5 or 20 MPa. That is 7.5 bags of 40 kg, or 15 of 20 kg.
How much is a cubic metre of concrete in NZ?
One merchant lists 20 MPa ready-mix at $297.28 per m³ with GST. A cost guide says $280–$360 delivered, GST not stated. Fees can be extra, so get a quote.
How much water is in 1 cubic metre of concrete?
For the 15–25 MPa mixes, about 150 litres with damp builders mix (NZS 3104, via Golden Bay). Use 100 litres if it is wet, or 200 if it is SSD (soaked inside, dry on the surface).
How many litres are in a cubic metre of concrete?
1,000 litres. So a 25 kg bag, which makes 0.01 m³, gives about 10 litres.
How many wheelbarrows are in 1 m³ of concrete?
We found no NZ figure. Heidelberg Materials, a UK maker, says about 35 barrow loads. That is about 29 litres a load.
What MPa concrete should I order?
MPa is the strength. Golden Bay's NZS 3104 table uses 10 MPa for bedding and fence posts, and 15 MPa for paths. It uses 17.5 or 20 MPa for NZS 3604 work, and 25 MPa for exposed concrete in sea-spray areas.
Where our numbers come from
- Concrete per bag: Dricon (Firth) HandiCrete and RapidSet
- HandiCrete 25 kg price: the middle of 6 NZ sellers (Oct 2026)
- HandiCrete 40 kg price: the middle of 4 NZ sellers (Oct 2026)
- CCANZ driveway and path guide: mesh by slab thickness
- BRANZ Bulletin 541: NZ house slab rules (NZS 3604)
- NZ concrete placers' driveway code: 100 mm, 20 MPa, 665 mesh
- Kiwi Timber (Waikato): ready-mix 20 MPa $297.28/m³ incl. GST
- NZ Tradie Tools cost guide: ready-mix $280–360 per m³
- Small-load charge: Allied Concrete $115 a load (Kiwi Timber listing)
- Fuel surcharge: Firth, $9.90 per m³ of ready-mix (from 5 Oct 2026)
- Minimixers (Wellington): small loads 0.4–2.4 m³
- Coastwide Mini Mix (Kapiti): small loads 0.4–2.2 m³
- Mini Mix Concrete (Christchurch): small truck size and reach
- Allied Concrete terms: waiting time after 10 minutes per m³
- Allied Concrete FAQ: add a 5% buffer for waste
- Golden Bay Cement: bags of cement and builders mix per m³
- Builders mix price: the middle of 3 NZ sellers (per m³, Oct 2026)
- Cement 20 kg price: the middle of 2 NZ sellers (Oct 2026)
- MaxPump (Auckland) pump hire: $430 minimum, $120/h, $12/m³
- Full loads, barrows and mixes: Heidelberg Materials UK merchant guide
- Unit sizes: NIST Handbook 44, Appendix C (1 ft = 0.3048 m; 1 US gallon = 231 cubic inches)
Made by BuildersCount. Last checked 4 Oct 2026.
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