How to Calculate How Many Bags of Concrete You Need
The formula for working out how many bags of concrete you need has three steps. First, calculate the volume of concrete required for your project. Second, add a waste allowance. Third, divide by the yield per bag and round up to the nearest whole bag.
Step 1 - Calculate volume: For a rectangular slab or footing, multiply length (m) x width (m) x depth (m). Depth is usually measured in millimetres on plans, so divide by 1,000 to convert to metres. For a cylindrical post hole, use: volume = pi x radius² x depth, where radius = diameter / 2.
Step 2 - Add waste allowance: Multiply the net volume by (1 + waste fraction). A 10% allowance means multiplying by 1.10. For a 3 m x 3 m x 0.10 m slab: 0.9 m³ x 1.10 = 0.99 m³.
Step 3 - Divide by yield per bag and round up: A standard 20 kg premixed concrete bag yields approximately 0.009 m³. Dividing 0.99 m³ by 0.009 gives 110 bags exactly. Always round up, never down.
Bag Yields and Common Applications
| Bag Size | Approx Yield | Best For | Approx NZ Price |
|---|
| 20 kg | 0.009 m³ | Post holes, small repairs, DIY slabs (easy to handle) | $10 - $14 |
| 25 kg | 0.011 m³ | Medium pours, garden edging, footings (good balance) | $12 - $17 |
| 40 kg | 0.018 m³ | Larger slabs, commercial DIY, where handling is not a constraint | $18 - $26 |
Prices are approximate retail prices at major NZ hardware stores (Bunnings, Mitre 10, PlaceMakers). Bulk pricing and trade accounts will differ. Always check the current price at your supplier.
Typical Slab Depths
The depth of concrete you need depends on the application. Using the wrong depth is one of the most common mistakes in DIY concrete projects.
| Application | Typical Depth | Notes |
|---|
| Garden path (pedestrian only) | 75 - 100 mm | Minimum for foot traffic; use 100 mm for durability |
| Residential driveway | 100 - 125 mm | 100 mm for cars; 125 mm for heavier vehicles or clay subgrades |
| Shed or garage floor | 100 mm | Standard for single-storey light structures |
| House floor slab | 100 - 150 mm | Requires engineering input; typically with reinforcing mesh |
| Post footing (fence or deck) | Hole depth 600 - 900 mm | Below frost line and at least 1/3 of post height in ground |
| Retaining wall footing | 200 - 300 mm | Depends on wall height; engineer design recommended above 1.5 m |
When to Use Premixed Bags vs Bulk Concrete
Premixed concrete bags are ideal for small to medium DIY projects where a truck pour is impractical or uneconomical. Ready-mix concrete from a truck is usually more cost-effective once you exceed approximately 1 cubic metre, because bulk concrete is priced per cubic metre and the labour of mixing bags at scale becomes significant. As a rough guide:
- Under 0.5 m³: bags are almost always more practical
- 0.5 to 1.5 m³: bags versus ready-mix depends on access, timing, and your preference
- Over 1.5 m³: a ready-mix truck is usually cheaper and easier
Ready-mix concrete in New Zealand is typically priced at $200 to $350 per cubic metre (plus delivery and minimum load charges). For a 3 x 3 m slab at 100 mm depth (0.9 m³), approximately 110 x 20 kg bags at $12.50 each would cost around $1,375, whereas ready-mix might cost $600 to $900 delivered if you can get a small truck. The trade-off is control: bags let you pour in stages, ready-mix requires the whole pour to happen at once.
Worked Example
A 3 m x 3 m garden shed base at 100 mm depth, using 20 kg bags with a 10% waste allowance:
- Net volume: 3 x 3 x 0.100 = 0.900 m³
- With 10% waste: 0.900 x 1.10 = 0.990 m³
- Yield per 20 kg bag: 0.009 m³
- Bags needed: 0.990 / 0.009 = 110 bags
- Total weight of bags: 110 x 20 kg = 2,200 kg
- Estimated cost at $12.50/bag: $1,375
This matches the calculator's default output. For the same slab using 40 kg bags, you would need 55 bags, which is the same total weight (2,200 kg) but fewer trips from the car.
Related Calculators
Sources and method: Bag yield figures are based on typical NZ product specifications for premixed concrete (cement, fine aggregate, and coarse aggregate). Common brands include Cement Australia Postix, Firth Surepost, and Golden Bay Portland Cement mixes. Yields can vary slightly between products and mixing technique; always check the specific bag your supplier stocks. Waste allowance guidance follows standard building trade practice.
This calculator provides an estimate only. Actual bag requirements depend on the specific product yield, mixing water content, subgrade conditions, and pour technique. Always purchase a small buffer above the calculated amount to avoid running short mid-pour. For structural concrete work (house slabs, retaining wall footings, and load-bearing posts), seek advice from a licensed building practitioner or structural engineer.
