concrete.work
Australia

Concrete Slab Calculator

Enter the size and thickness of your slab in metres and millimetres. You get the cubic metres, the bags of 20kg premix, how much readymix to order and, if you want, the SL72 or SL82 mesh sheets.

What are you pouring?
Patio, driveway, shed base, path
Length Width Thickness

Tip: metres and millimetres, or feet and inches if your plans use them, like 10' 6": we convert them.

Reinforcement for slabs (optional)
Gravel base under slabs (optional)

Gravel weight changes from quarry to quarry: ask your supplier for the compacted weight and we turn the volume into tonnes.

Use your own prices (optional)

You need

– m³

–

Poured, it weighs about – kg.

Premixed bags

Bag Bags To carry Est. cost

Mixing it yourself

Cement Australia, General Purpose Cement product data sheet (PDS C01, 2016): mix by volume cement : sand : aggregate and 20kg bags per m³ – high strength 1 : 1.5 : 3 (17 bags), paths and driveways 1 : 2 : 3 (16), foundations and footings 1 : 3 : 3 (13). The sheet gives no water: we use half the cement weight (water/cement 0.5). Kilos of sand and aggregate from 2,400 kg per m³ of fresh concrete, split by the volume parts.

Ready-mix truck

–

How we price

Link to this calculation Download estimate (PDF)

Your materials at a glance

How to calculate concrete

  1. Measure length and width. In metres, inside the formwork. Split an L-shaped slab into two rectangles.
  2. Pick the thickness. In millimetres: 100 mm is common for sheds, garages and driveways.
  3. Multiply. Length × width × (thickness ÷ 1,000) gives cubic metres.
  4. Add 5–10% extra. Ground is never perfectly level and formwork flexes.
  5. Bags or readymix. Divide by 0.0094 for 20kg bags. Above 2–3 m³, a readymix truck saves a lot of mixing.

Worked example: a 6 × 6 m garage slab, 100 mm thick

  1. Volume: 6 × 6 × 0.1 = 3.60 m³. Plus 5%: 3.78 m³.
  2. That is 404 bags of 20kg premix, over 7 tonnes to mix by hand.
  3. Readymix: order 3.8 m³, above the usual 3 m³ minimum.

Concrete for common slab sizes

Slab Thickness m³ 20kg bags
2 × 2 m 75 mm 0.32 34
2 × 2 m 100 mm 0.42 45
3 × 3 m 75 mm 0.71 76
3 × 3 m 100 mm 0.95 101
4 × 3 m 75 mm 0.95 101
4 × 3 m 100 mm 1.26 135
6 × 3 m 75 mm 1.42 152
6 × 3 m 100 mm 1.89 202
6 × 6 m 75 mm 2.84 303
6 × 6 m 100 mm 3.78 404
9 × 6 m 75 mm 4.25 455
9 × 6 m 100 mm 5.67 606

Includes 5% extra for waste. 20kg bags at about 0.0094 m³ each (Australian Builders states 0.01 m³).

Concrete formulas by shape

All measurements in metres; divide millimetres by 1,000 first. Results are in cubic metres.

Slab
length × width × thickness
Round slab
π × (diameter ÷ 2)² × thickness
Monolithic slab
length × width × thickness + perimeter band × (edge depth − thickness), where perimeter band = length × width − (length − 2 × edge width) × (width − 2 × edge width)

1 m³ = 1,000 litres.

Frequently asked questions

How many bags of concrete for a 3m x 3m slab?

At 100 mm thick it is 0.9 m³, or 0.95 m³ with 5% extra: 101 bags of 20kg premix.

How many square metres does 1m³ of concrete cover?

10 m² at 100 mm thick, 13.3 m² at 75 mm and 8 m² at 125 mm.

How many mesh sheets do I need?

Open "Reinforcement for slabs" in the calculator: it counts 6.0 × 2.4 m sheets with a 225 mm overlap for your exact slab. Check the overlap and the mesh type on your plans.

Should a slab have a road base under it?

Most do: 75–100 mm of compacted road base or crushed rock under the slab. Turn on "Gravel base" in the calculator to get the volume and, with your quarry's weight, the tonnes.

About the prices

Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.