2m × 2m slab, 100mm deep
Volume: 2 × 2 × 0.1 = 0.4m³, or 400L.
10L bags: 40 before waste.
With 10%: 44 bags.
Concrete Bag Calculator NZ: How Many Bags Do I Need?
Calculate bagged concrete for slabs, paths, post holes, footings, trenches and block fill. Enter the project dimensions, select a 20kg, 25kg or 40kg product yield, add waste and optionally estimate the total cost.
Suitable for homeowners, DIY renovators, landscapers, builders and tradies preparing an initial concrete quantity estimate.
Last reviewed by NZ Calculator: 5 July 2026 · Estimated completion time: under two minutes
- A standard 20kg premixed concrete bag often yields about 0.010m³, or 10 litres, but some rapid-set post mixes yield only 0.008m³.
- At a 10-litre yield, one cubic metre requires 100 bags before waste. At an 8-litre yield, it requires 125 bags.
- Bag weight alone does not determine mixed volume. Use the yield printed on the exact product bag or technical data sheet.
- The calculator rounds up to whole bags and then applies your selected waste allowance.
How much concrete is in a 20kg bag?
Many standard New Zealand 20kg premixed concrete products make about 0.010m³, or 10 litres. Some 20kg rapid-set post-hole products make about 0.008m³, or 8 litres, so there is no single yield for every 20kg bag.
Project volume:Length × width × depth = concrete volume in cubic metres
Bags required:Project litres ÷ litres produced per bag, rounded up
Including waste:Project litres × (1 + waste percentage) ÷ bag yield, rounded up
Example: a 2m × 2m slab at 100mm depth is 0.4m³, or 400 litres. At 10 litres per bag it needs 40 bags before waste, or 44 bags with a 10% allowance.
Your concrete project
Use the specified design depth. This calculator does not determine structural slab thickness or reinforcement.
Block fill varies by block size, core geometry and whether every cell is filled. Replace the example value with the block supplier's published fill volume.
Check the project dimensions. All required entries must be greater than zero and use the units shown.
The product label or current technical sheet overrides every preset.
Common NZ yield quick picks
Your concrete bag estimate
Slab estimateCalculating your estimate…
Comparison of project volume, selected buffer and the mixed volume provided by the rounded whole-bag quantity.
Bags to buy
0 bags
Project volume
0.000m³
| Project concrete volume | 0.000m³ |
| Project volume in litres | 0L |
| Selected bag yield | 0L |
| Bags before waste allowance | 0 |
| Volume including 10% allowance | 0L |
| Whole bags to buy | 0 |
| Mixed volume supplied by those bags | 0L |
| Total dry product weight | 0kg |
| Coverage per bag at entered depth | — |
| Estimated bag cost | Enter a price |
How this Concrete Bag Calculator NZ works
The calculator first converts the selected shape into cubic metres. It then converts that volume into litres, divides by the mixed yield of one bag and rounds up. The waste allowance is applied to the required concrete volume before the final whole-bag quantity is calculated.
This yield-based method is more accurate than assuming every 20kg or 25kg bag makes the same amount. Product density, aggregate size, strength class and rapid-set formulation can change the volume made by each bag.
- Slab or path: length × width × depth.
- Post hole or round pier: π × radius² × depth × quantity, less the optional post volume.
- Footing or trench: total length × width × depth × quantity.
- Block fill: number of blocks × the supplier's litres-per-block figure.
How many concrete bags per cubic metre? NZ product yields
Published New Zealand product yields differ even when bags have the same weight. Use the following as examples only and confirm the current bag or data sheet before buying.
| Product example | Bag weight | Published or planning yield | Bags per 1m³ before waste |
|---|---|---|---|
| Cemix Multicrete or Maxcrete | 20kg | 0.010m³ / 10L | 100 |
| Drymix Easy To Mix or Hi-Strength guide | 20kg | about 9.5–10L | about 100–106 |
| Firth Dricon High Strength RapidSet | 20kg | 0.008m³ / 8L | 125 |
| Firth Dricon HandiCrete | 25kg | 0.010m³ / 10L | 100 |
| Firth Dricon HandiCrete | 40kg | 0.016m³ / 16L | 63 |
Sources: Cemix Multicrete, Cemix Maxcrete, Drymix concrete calculator, Firth Dricon High Strength RapidSet and Firth Dricon HandiCrete.
How much does a 20kg bag of concrete cover?
Coverage depends on the bag's mixed yield and the required concrete depth. The table below uses a 10-litre yield, which is 0.010m³ per bag.
| Concrete depth | Area covered by one 10L bag | Bags per m² before waste |
|---|---|---|
| 50mm | 0.20m² | 5 |
| 75mm | 0.133m² | 7.5 |
| 100mm | 0.10m² | 10 |
| 125mm | 0.08m² | 12.5 |
| 150mm | 0.067m² | 15 |
A 20kg rapid-set product yielding 8 litres covers 20% less area than the 10-litre examples above. Use the calculator's editable yield field for the product you are actually buying.
Worked concrete bag examples
2m × 3m slab, 100mm deep
Volume: 2 × 3 × 0.1 = 0.6m³, or 600L.
10L bags: 60 before waste.
With 10%: 66 bags.
250mm round hole, 600mm deep
Hole volume: π × 0.125² × 0.6 = 29.5L.
8L rapid-set: 3.7 bags before rounding.
Buy: 5 bags with a 10% allowance, before any post deduction.
How much is a 20kg bag of concrete in New Zealand?
Prices vary by brand, strength, setting time, retailer and store. When this page was reviewed, Bunnings listed Cemix Multicrete 20kg at $10.89 and Mitre 10 listed Drymix Easy To Mix Concrete 20kg at $11.65. Rapid-set and higher-strength products were generally more expensive.
These are dated retail examples, not guaranteed current prices. Enter the current shelf, click-and-collect or trade price in the calculator to estimate your actual bag cost.
Price sources reviewed 5 July 2026: Bunnings New Zealand concrete range and Mitre 10 concrete range.
Concrete bags for slabs, post holes, footings and block fill
Slabs, paths, patios and small pads
Use the finished concrete dimensions, not the excavation dimensions if compacted basecourse will sit below the slab. Structural slabs require an appropriate design, reinforcement, subgrade preparation and Building Code compliance.
Post holes, round tubes and piers
Measure the finished hole diameter and depth. The optional post deduction can improve the volume estimate, but irregular holes, loose soil and bell-shaped bases usually justify a contingency allowance.
Footings, kerbs and trenches
Use the total run length and the actual width and depth. Changes in excavation width can materially increase the amount of concrete required.
Concrete block fill
Do not assume a universal volume per block. Core dimensions and fill requirements vary. Obtain the litres-per-block or cubic-metres-per-square-metre figure from the block supplier or project specification.
Bagged concrete or ready-mix?
Bagged concrete is convenient for small jobs, difficult access and staged work. As the volume increases, the labour of transporting, opening and mixing many bags becomes significant. Compare a local ready-mix or mini-mix quote when the estimate reaches dozens of bags, especially for time-sensitive or structural pours.
- Compare the delivered concrete volume, minimum-load charge, access, barrow distance and disposal costs.
- Plan enough labour and mixing capacity to place the concrete continuously where the project requires it.
- Confirm the specified concrete strength, slump, reinforcement, curing and joint requirements before ordering.
Common concrete estimating mistakes
- Using bag weight instead of bag yield. Two 20kg products can produce different mixed volumes.
- Entering millimetres as metres. Convert slab depth, footing width and hole diameter correctly.
- Rounding before adding waste. Apply the allowance to the required volume, then round up to whole bags.
- Assuming excavations are perfectly shaped. Post holes and trenches are often wider or less regular than planned.
- Using a post-setting product for the wrong application. Check strength, intended use and manufacturer limitations.
- Ignoring site access and mixing time. A mathematically correct bag quantity may still be impractical for a large pour.
- Confusing cement with premixed concrete. A bag of cement alone still requires correctly proportioned sand and aggregate.
NZ building, structural and safety considerations
This calculator estimates quantity only. It does not select concrete strength, reinforcement, footing dimensions, slab design, curing method or whether building consent is required. Building Performance states that all building work must comply with the Building Code, even where an exemption from building consent applies.
For structural slabs, foundations, retaining walls, load-bearing posts and primary structural work, use the project drawings and advice from an appropriate designer, engineer, Licensed Building Practitioner or council.
Safety: dry cement and wet concrete can irritate or burn skin and eyes, and dry mixing can create dust. Follow the product safety data sheet, use suitable protective equipment, avoid breathing dust and wash contamination promptly.
Official guidance: Building Performance: check if you need consents and the safety instructions supplied with the selected concrete product.
How to use the Concrete Bag Calculator NZ
- Select the project shape. Choose slab, post holes, footing or block fill.
- Enter finished dimensions. Use metres for horizontal slab dimensions and millimetres where the field specifies them.
- Select the exact bag yield. Use a quick pick or enter the mixed litres stated on the product bag.
- Add a realistic allowance. Adjust the default 10% for excavation accuracy, spills and site conditions.
- Review the whole-bag quantity. Enter the current price per bag if you also need a material-cost estimate.
Concrete Bag Calculator NZ Frequently Asked Questions
A standard 20kg premixed concrete product commonly makes about 0.010m³, or 10 litres, but some 20kg rapid-set post mixes make about 0.008m³, or 8 litres. Check the exact product yield rather than relying on bag weight alone.
For many standard products, a 20kg bag makes about 10 litres of mixed concrete. Drymix publishes around 9.5 to 10 litres for its 20kg mixes, while Firth Dricon High Strength RapidSet publishes 8 litres per 20kg bag.
At a 10-litre yield, 100 bags make one cubic metre before waste. At a 9.5-litre yield, about 106 bags are needed. At an 8-litre yield, 125 bags are needed. Add the selected contingency allowance after confirming the product yield.
A 20kg bag yielding 10 litres covers about 0.20m² at 50mm depth, 0.10m² at 100mm depth or 0.067m² at 150mm depth. A lower-yield product covers less area.
The slab volume is 0.6m³ or 600 litres. At 10 litres per bag, it needs 60 bags before waste and 66 bags with a 10% allowance.
Calculate the cylindrical hole volume using π × radius squared × depth, subtract the post volume if appropriate, then divide by the product yield. Hole shape and loose soil can increase the real quantity, so include a contingency allowance.
Multiply slab length by width by depth, with every measurement converted to metres. Multiply cubic metres by 1,000 to obtain litres, then divide by the mixed litres produced by one bag.
Price depends on product, strength, retailer and location. At the 5 July 2026 review, standard 20kg examples at major retailers were around $10.89 to $11.65, while rapid-set and high-strength mixes cost more. Check the live local price before ordering.
Compare ready-mix or mini-mix once the bag count becomes large enough that transport, mixing labour and continuous placement are difficult. The break-even point varies by location, access, delivery minimums and product price.
Only when the selected product and construction method meet the project's specified strength and other design requirements. Some rapid-set products have use limitations. Follow the plans, product literature and professional or council requirements.
Ten percent is a common early estimating allowance for small projects, but the appropriate amount depends on excavation accuracy, subgrade, spills, uneven surfaces and the consequences of running short.
No. It estimates volume, bag quantity, dry weight and optional bag cost. It does not determine slab thickness, reinforcement, concrete strength, foundations, retaining-wall design, curing, consent or structural compliance.
This calculator provides a preliminary material estimate only. Actual bag yield, excavation volume, concrete specification, reinforcement, strength, curing, price, delivery requirements and Building Code obligations vary by project and product. Confirm quantities and suitability with the product supplier and an appropriately qualified professional before structural work.