Concrete Bag Calculator
Find how many bags of concrete you need, and what they will cost.
Measure your project
Your answer
That makes 0.09 m³. We added 5% extra.
If you buy bags
Dricon says its bags make about 0.008 m³ (20 kg), 0.01 m³ (25 kg) and 0.016 m³ (40 kg).
If you order a truck (ready-mix)
Ask for 0.40 m³. Mini-mix trucks take loads from about 0.4 m³, so we price at least that.
This is a guide. For posts, pads or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
But you must mix 9 bags by hand. That is 0.2 tonnes to lift.
NZ prices from October 2026, with 15% GST. 25 kg bag: middle of 6 sellers. Ready-mix: one merchant only. 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 0.9 × 0.9 m pad, 100 mm thick
- 9 bags of 25 kg, counting 5% extra. At $18.46 each, the bags cost about $166.
- Bags in 1 m³
- 100 bags of 25 kg. For 40 kg bags it is 62.5, so buy 63. Those are Dricon's figures.
- One fence post
- 1 bag of 20 kg Cemix Fastcrete, for a post 0.9 m above ground and 0.3 m below.
- When a truck is cheaper
- Past about 12 bags of 25 kg, if Allied's $115 small-load fee applies. With no fee, past about 6 bags.
- Price
- A 25 kg bag costs about $18.46 with GST, the middle price of 6 NZ sellers.
How to work it out
- Find the m³: multiply length, width and thickness, with each in metres.
- Make it 5% bigger to cover spills.
- Divide by what one bag makes: 0.01 m³ for 25 kg, 0.016 m³ for 40 kg.
- Round up: shops sell whole bags only.
- To get the cost, multiply the bag count by the price per bag.
0.9 × 0.9 × 0.1 = 0.081 m³
0.081 + 5% = 0.085 m³
0.085 ÷ 0.01 = 8.5 → 9 bags
9 × $18.46 = $166
What each bag of concrete makes
Premixed concrete is sold dry in a bag, ready for water. Dricon sells its HandiCrete in 25 kg and 40 kg bags.
| Bag | Makes | Bags per m³ | Covers at 100 mm | Price with GST |
|---|---|---|---|---|
| High Strength RapidSet 20 kg | 0.008 m³ (8 L) | 125 | 0.08 m² | Not found |
| HandiCrete 25 kg | 0.01 m³ (10 L) | 100 | 0.1 m² | $18.46 (6 sellers) |
| RapidSet 25 kg | 0.01 m³ (10 L) | 100 | 0.1 m² | $24.78 (8 sellers) |
| HandiCrete 40 kg | 0.016 m³ (16 L) | 62.5 (buy 63) | 0.16 m² | $34.42 (4 sellers) |
| Drymix 20 kg (Mitre 10) | Not given | – | – | $11.29 (1 store) |
What each bag makes is Dricon's figure. Prices are middle prices from October 2026. Only one store gave a Drymix price. Where a store did not say, we took the price as including GST.
Mitre 10's own Drymix 20 kg bag is 20 MPa, for poles, posts and paths. But it does not say how much concrete it makes.
At our middle prices, 1 m³ costs about $1,846 in 25 kg bags and $2,168 in 40 kg bags. But 40 kg bags ran from $23.85 to $44 at 4 sellers. So check your own store.
Bags for everyday NZ jobs
| Job | m³ (+5%) | 25 kg bags | 40 kg bags | Cost (25 kg) |
|---|---|---|---|---|
| Letterbox post: 1 hole, 250 mm wide, 600 mm deep | 0.031 | 4 | 2 | $74 |
| Clothesline: 2 holes, 300 mm wide, 500 mm deep | 0.074 | 8 | 5 | $148 |
| Mowing strip 6 m long, 150 mm wide, 75 mm thick | 0.071 | 8 | 5 | $148 |
| Path 2 m long, 0.6 m wide, 75 mm thick | 0.095 | 10 | 6 | $185 |
| Pad 0.9 × 0.9 m, 100 mm thick | 0.085 | 9 | 6 | $166 |
Sizes are examples. 25 kg bags at $18.46 each with GST. Post holes are counted full, with no post in them.
Your job has more than one piece? In the calculator, use "Add another part" and it adds them up.
Concrete for fence posts
Cemix's Fastcrete data sheet says to allow for 1/3 of the post to be underground. Its one table row is for a post 0.9 m above ground and 0.3 m below. That row takes 1 bag of 20 kg, or 0.8 of a 25 kg bag.
For other holes, use "Round holes" above. A post takes up room, so take it off. For example, a 200 mm hole, 600 mm deep, with a 100 × 100 mm post holds 0.013 m³. With 5% extra, that is 2 bags of 25 kg.
Setting a post with post mix
- Stand the post in the hole and hold it straight.
- Pour in water first: 1.9–3 litres for each 20 kg bag of Fastcrete, says Cemix.
- Pour in the dry mix.
- Fill the rest of the hole after 15 minutes. Light work can start after 1 hour.
- Don't hammer on the posts for 3 days.
Dricon's RapidSet is also sold for post holes. Intelligro, a store, says it reaches 20 MPa after 28 days. We could not open Dricon's own data sheet. So we can't say how many of its bags a hole takes.
For a gate, deck or carport post, check your local building rules or ask a builder or engineer.
When bags cost less than a truck
For a tiny job, bags win, because a truck brings at least 0.4 m³ and may add fees. Past a point, the truck is cheaper. Here is where that point falls.
| Truck charge | 25 kg bags | 40 kg bags |
|---|---|---|
| Smallest load (0.4 m³) at $297.28 per m³, no fees | 6 bags | 3 bags |
| With Allied's $115 small-load fee | 12 bags | 6 bags |
| With $115 and Firth's $9.90 per m³ fuel surcharge | 12 bags | 6 bags |
Bags at our middle prices. Ready-mix at one merchant's 20 MPa price, with GST. Allied's fee is with GST. Firth's fuel surcharge is temporary, and its GST is not stated.
With Allied's fee, a truck can cost less once you need about 13 bags of 25 kg. That is a price point only. For jobs over 3 m³, Golden Bay, a cement maker, says to think about ready-mix.
Mini-mix firms bring loads of about 0.4–2.4 m³. We found no mini-mix price. To mix cement and builders mix yourself, see our concrete calculator.
Questions people ask
How many bags of concrete do I need?
It depends on the size of the job. A 0.9 × 0.9 m pad, 100 mm thick, needs 9 bags of 25 kg with 5% extra. Type your own sizes into the calculator above.
How many 20 kg bags of concrete make a cubic metre?
125, if each makes 0.008 m³. That is Dricon's figure for its 20 kg High Strength RapidSet. Mitre 10's Drymix 20 kg bag gives no figure.
How many cubes are in a bag of concrete?
If a "cube" means 1 m³, then 100 bags of 25 kg make one. Each makes 0.01 m³. A 40 kg bag makes 0.016 m³.
How many bags of concrete per fence post?
Cemix says 1 bag of 20 kg Fastcrete, or 0.8 of a 25 kg bag. That is for a post 0.9 m above ground and 0.3 m below.
How much does a 25 kg bag of concrete cover?
About 0.1 m² at 100 mm thick, by Dricon's figure of 0.01 m³ a bag. At 75 mm, the thickness of a path, it covers about 0.13 m².
How much is a bag of concrete in NZ?
HandiCrete costs about $18.46 for 25 kg, with GST: the middle of 6 sellers. The 40 kg bag is about $34.42, from 4 sellers.
How much concrete does a 40 kg bag make?
About 0.016 m³, or 16 litres, says Dricon. That covers about 0.16 m² at 100 mm thick.
How much water goes in a bag of post mix?
Cemix says 1.9–3 litres for a 20 kg bag of Fastcrete. Pour the water into the hole first. We found no water figure for HandiCrete.
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)
- RapidSet 25 kg price: the middle of 8 NZ sellers (Oct 2026)
- Mitre 10 (Rangiora): Drymix 20 kg concrete $11.29
- Dricon RapidSet 25 kg at Intelligro (Christchurch): $33.00
- Cemix Fastcrete post mix: bags and water per post hole
- CCANZ driveway and path guide: mesh by slab thickness
- BRANZ Bulletin 541: NZ house slab rules (NZS 3604)
- Kiwi Timber (Waikato): ready-mix 20 MPa $297.28/m³ incl. GST
- 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
- Allied Concrete FAQ: add a 5% buffer for waste
- Golden Bay Cement: bags of cement and builders mix per m³
Made by BuildersCount. Last checked 4 Oct 2026.
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