New Zealand calculator
Firth Concrete Calculator NZ
Enter your slab, path or post-hole dimensions and get the concrete volume in cubic metres, plus the number of premix bags required based on the bag yield you are using.
Last reviewed by NZ Calculator:
How many bags of concrete do I need?
Work out the volume in cubic metres (length × width × depth), convert to litres by multiplying by 1,000, then divide by the yield of one bag in litres. A Firth Dricon HandiCrete 25kg bag yields roughly 10 litres and a 40kg bag roughly 16 litres, so a 0.5 m³ pour needs about 50 of the 25kg bags. Always add a waste allowance.
Your pour dimensions
Your concrete estimate
Bag yields vary by product and how wet you mix. Always check the yield printed on the bag you are buying before ordering.
Bags or ready-mix?
Bagged premix suits small jobs — post holes, a garden path, a shed pad. Once you get past roughly one cubic metre the bag count, mixing time and cost usually make a ready-mix delivery the better option. Use the table below as a rough guide.
| Volume | 25kg bags (10L) | 40kg bags (16L) | Usual approach |
|---|---|---|---|
| 0.1 m³ | 10 | 7 | Bags, mix by hand |
| 0.5 m³ | 50 | 32 | Bags with a mixer |
| 1.0 m³ | 100 | 63 | Borderline — consider ready-mix |
| 2.0 m³ + | 200 | 125 | Ready-mix delivery |
Getting the depth right
Depth drives the volume more than anything else, and a base that dips by 20mm across a slab adds real cost. A garden path is commonly 75–100mm, a domestic driveway 100–125mm with reinforcing, and post holes are calculated as hole volume minus the post itself. Check the specification for your job before ordering.
Why the waste allowance matters
An over-dug base, spillage, a bit lost in the barrow and uneven ground all eat into your order. Ten percent is a common allowance for small pours. Running short mid-pour and creating a cold joint costs far more than one spare bag.
Independent tool: NZ Calculator is not affiliated with, endorsed by or connected to Firth Industries. Product names and yields are referenced for planning only — confirm the current yield on the product bag or with your supplier.
NZ source: Building Performance — B1 Structure.
Frequently asked questions
How many bags of concrete do I need per cubic metre?
It depends on the bag yield. At a 10 litre yield you need about 100 bags per cubic metre, and at a 16 litre yield about 63 bags. Always add a waste allowance on top.
What is the yield of a Firth Dricon HandiCrete bag?
As a planning figure, a 25kg bag yields around 10 litres of concrete and a 40kg bag around 16 litres. Check the current yield printed on the bag, as it varies with product and mix consistency.
How do I calculate concrete volume in cubic metres?
Multiply length in metres by width in metres by depth in metres. A depth entered in millimetres is divided by 1,000 first, so 100mm becomes 0.1m.
How thick should a concrete slab be in New Zealand?
A garden path is commonly 75–100mm and a domestic driveway 100–125mm with reinforcing. Structural slabs must meet the Building Code and any engineer's specification, so check before you pour.
Should I use bags or ready-mix concrete?
Bags suit pours under about one cubic metre. Beyond that, the number of bags, mixing time and cost usually favour a ready-mix delivery.
How much waste should I allow?
Around 10% is a common allowance for small pours to cover spillage, an uneven base and over-dig. Increase it if the ground is rough or the excavation is imprecise.
How do I calculate concrete for post holes?
Calculate the volume of the hole, then subtract the volume of the post that sits below ground. Set the number of areas to the number of holes.
Is this an official Firth calculator?
No. NZ Calculator is independent and not affiliated with Firth Industries. Confirm product yields and specifications with your supplier before ordering.