Concrete Slab Calculator
Find how much concrete a slab or pad needs, in cubic metres or bags, and what goes under it.
Measure your project
Your answer
We added 5% extra.
If you buy bags
Dricon says each 25 kg bag of HandiCrete makes about 0.01 m³.
If you order a truck (ready-mix)
Ask for 0.80 m³. We round up to the next 0.2 m³.
This is a guide. For a house slab, drive or anything that holds weight, check your local building rules. Or ask a builder or engineer.
Which costs less?
You also skip mixing 76 bags by hand. Have the edges ready before it comes.
NZ prices from October 2026, with 15% GST. 25 kg bag: middle of 6 sellers. Ready-mix: one merchant's 20 MPa 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 × 2.4 m shed pad, 100 mm thick
- 0.72 m³. With 5% extra, 0.76 m³. That is 76 bags of 25 kg, or 0.8 m³ of ready-mix.
- Garden shed pad
- At least 80 mm in the middle and 100 mm at the edges, says Duratuf, a NZ shed maker.
- Vibrate it?
- Yes for house slabs and drives, say CCANZ and the placers' code. A small pad can be tamped by hand, says Golden Bay.
How to work it out
- Find the area: length times width, in metres. The answer is in m².
- Choose how thick the slab will be. In metres, 100 mm is 0.1 m.
- Multiply the area by the thickness. Now you have m³.
- Allow 5% more for spills and low spots.
- Order that m³ as ready-mix, or divide it by 0.01 for 25 kg bags.
3 × 2.4 = 7.2 m²
7.2 × 0.1 = 0.72 m³
0.72 + 5% = 0.756 m³
0.756 ÷ 0.01 = 75.6 → 76 bags
How to calculate concrete volume for a slab or pad
Area × thickness gives the volume. Here are some common jobs.
| Job | m³ (+5%) | Ready-mix to order | Truck cost | 25 kg bags |
|---|---|---|---|---|
| Shed pad 2.4 × 1.8 m, 100 mm | 0.45 | 0.6 m³ | $293 | 46 |
| Path 10 × 0.9 m, 75 mm | 0.71 | 0.8 m³ | $353 | 71 |
| Pad 3 × 3 m, 100 mm | 0.95 | 1 m³ | $412 | 95 |
| Driveway 15 × 3 m, 100 mm | 4.73 | 4.8 m³ | $1,542 | 473 |
Truck cost at one merchant's 20 MPa price of $297.28 per m³ with GST, plus Allied's $115 small-load fee. We add that fee to every load.
Garden shed pads: thickness and consent
The Building Act does not say a shed needs a concrete base. Duratuf, a NZ shed maker, says a timber floor kit or a concrete pad both work, on level ground.
- Garden shed: Duratuf says at least 80 mm thick in the middle, and 100 mm at the edges. Make it 15 mm narrower and 15 mm shorter than the shed, so the cladding runs past the floor.
- Bigger shed: for one bigger Lifestyle shed, Duratuf asks for a 100 mm slab with 200 mm thick edges. It sits on at least 75 mm of fill, with 20 MPa concrete and 665 or SE62 mesh.
Building consent (Schedule 1)
- Up to 10 m²: no consent for a detached, single-storey shed. Its floor can be up to 1 m off the ground, and it can be up to 3.5 m tall. It can't have a toilet, stored water or a bed.
- 10–30 m²: no consent if it is lightweight and built to B1/AS1, a set of approved building methods. Or if a licensed building practitioner (LBP) builds it. Either way, it must be at least 1 m from any boundary or house.
Our 3 × 2.4 m example is 7.2 m², under the 10 m² line. Check with your council too.
House slab rules (NZS 3604)
BRANZ and CCANZ explain NZS 3604 in two guides from Dec 2011. They are summaries of the standard. We could not check the government's site for later changes.
| Layer | What the guides say |
|---|---|
| Hardfill (gravel fill) | 75–600 mm, packed down in layers of up to 150 mm |
| Sand blinding | 25 mm, so the DPM does not tear |
| DPM (damp-proof sheet) | Usually 0.25 mm polythene, joins taped |
| Mesh | Grade 500E, at least 2.27 kg/m² (e.g. SE62), 30 mm below the top |
| Concrete | 100 mm. 17.5 MPa, or 20 MPa if exposed to the weather in zone C, 25 MPa in zone D |
| Under inside load-bearing walls | Thicken to 200 mm, unless the wall carries only the roof of a one-storey house |
From BRANZ Bulletin 541 and the CCANZ slab guide. MPa is the strength. Zones B, C and D are NZS 3604 exposure zones, set by where you build.
500E mesh is ductile: it bends a long way before it snaps. Grade 500N 665 and 668 mesh do not meet the rule for house slabs. CCANZ keeps slab bays to 6 m or less, and 5 m is better.
For a house slab, check your local building rules or ask a builder or engineer.
Mesh in a slab, drive or path
| Job | Concrete | Mesh | Joints |
|---|---|---|---|
| House slab | 100 mm | 500E, 2.27 kg/m² or more | Bays up to 6 m |
| Driveway | 100 mm or more | 665 or 668 | Saw cuts every 3 m |
| Path | 75 mm | none or 668 | Every 3 m or less |
| Thicker paving | 150 mm | 663 | – |
Sources: BRANZ and CCANZ, and the placers' driveway code (NZMCPA).
Mesh holds cracks closed. CCANZ says it does not add load capacity or let you pour thinner. CCANZ does not advise a driveway with no mesh.
How many sheets of SE62 mesh?
United Steel's SE62 sheet is 4.45 × 2.02 m. With 200 mm laps, each sheet covers 7.56 m². So a 10 × 12 m slab takes 16 sheets. Ask your merchant to check.
Concrete driveway: base, slope and joints
The NZ concrete placers' driveway code says:
- At least 100 mm thick. Trucks or heavy vehicles need a designed slab.
- 20 MPa concrete or stronger.
- A base of 100 mm of GAP 20, a quarry gravel, packed down.
- A slope (fall) of at least 1:100. That is 10 mm per metre, so 150 mm along a 15 m drive.
- Saw-cut joints every 3 m, 1/4 of the slab deep, 24–48 hours after the pour.
- Keep it wet for at least 4 days.
For a drive, check your local building rules or ask a builder or engineer.
Vibrating the concrete
The placers' code says all driveway concrete must be vibrated. CCANZ says house-slab concrete must be too. Allied Concrete says to vibrate or rod it.
For a small DIY pad, Golden Bay says hand tamping works, though vibration is best. Tamp with a thick timber on edge, or use a vibrating beam.
No vibrator? CCANZ's path guide says to order a wetter mix: a 120–130 mm slump. Ask the plant for it, rather than adding water.
Questions people ask
Does a garden shed need a concrete base?
The Building Act does not ask for one. A 10–30 m² lightweight shed must be built to B1/AS1, which covers foundations too. Duratuf, a shed maker, says a timber floor or a concrete pad both work.
Do I need mesh in my concrete slab?
Yes for a house slab: Grade 500E mesh of at least 2.27 kg/m² (NZS 3604, as summed up in 2011). Yes for a drive: 665 or 668 mesh. A 75 mm path can have none or 668 mesh if joints are 3 m apart or less.
Do you need to vibrate a concrete slab?
Yes for a house slab, says CCANZ. The placers' code says all driveway concrete must be vibrated. For a small DIY pad, Golden Bay says hand tamping works, but vibrating is best.
Do I need building consent for a shed?
Not for a detached, single-storey shed up to 10 m² that meets set rules. From 10 to 30 m², none if it meets set rules. One rule: at least 1 m from boundaries and houses.
How much hardfill goes under a concrete slab?
For a house slab, 75–600 mm, packed down in layers of up to 150 mm. The driveway code asks for 100 mm of GAP 20.
How many sheets of mesh do I need?
Divide the slab area by 7.56 m² for United Steel's 4.45 × 2.02 m SE62 sheet, then round up. A 6 × 4 m slab takes 4 sheets.
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)
- 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
- Allied Concrete FAQ: add a 5% buffer for waste
- BRANZ Bulletin 541: NZ house slab rules (NZS 3604)
- CCANZ leaflet: house slab mesh, cover and vibration
- United Steel SE62 mesh: 2.29 kg/m², 7.56 m² per sheet
- NZ concrete placers' driveway code: 100 mm, 20 MPa, 665 mesh
- CCANZ driveway and path guide: mesh by slab thickness
- CCANZ driveway guide: 100 mm sub-base, 20 MPa, slump and vibration
- Allied Concrete: vibrate or rod concrete when placing
- Golden Bay Cement: bags of cement and builders mix per m³
- Building Act: no consent for sheds up to 10 or 30 m²
- Duratuf: shed pad 80 mm centre, 100 mm edges
- Duratuf Lifestyle shed slab: 100 mm, 20 MPa, 665/SE62 mesh
Made by BuildersCount. Last checked 4 Oct 2026.
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