How the calculator works
Every tab does the same three things: it converts your measurements to metres, multiplies them to get cubic metres, adds the waste percentage and rounds the bag count up to a whole bag. Thickness is entered in centimetres by default, but every field also accepts millimetres, metres, inches or feet.
| Project | Formula (metres) |
|---|---|
| Slab, footing, wall | length × width (or height) × thickness |
| Round slab, column | π × (diameter ÷ 2)² × thickness (or height) |
| Post holes | (π × (hole diameter ÷ 2)² − post width²) × depth × number of holes |
| Steps | rise × going × width × n(n + 1) ÷ 2, plus the top landing if there is one |
Two details most calculators skip: the post-hole formula subtracts the post itself, because a 90 mm post in a 250 mm hole takes up about 17% of it, and the steps formula treats a stair as a stack of blocks, which is how a solid-poured stair is formed.
volume (m³) = length (m) × width (m) × thickness (cm) ÷ 100
Three worked examples
4 × 3 m patio, 120 mm thick. 4 × 3 × 0.12 = 1.44 m³. Add 10%: 1.58 m³. In 20 kg bags (9.5 L each): 170 bags, more than three tonnes to carry. As ready-mix: order 1.6 m³ and expect a minimum-load charge of A$60–A$150 under about 3 m³.
Eight fence post holes, 250 mm wide, 600 mm deep, 90 × 90 mm posts. One hole: π × 0.125² × 0.6 = 0.0295 m³; the post takes 0.09 × 0.09 × 0.6 = 0.0049 m³, leaving 0.0246 m³. Eight holes: 0.197 m³, plus 10% = 0.216 m³, which is 22 bags of Rapid Set at about 10 L each.
Four steps, 170 mm rise, 280 mm going, 1 m wide, 1 m landing. Steps: 0.17 × 0.28 × 1 × (4 × 5 ÷ 2) = 0.476 m³. Landing: 1 × 1 × 0.17 × 4 = 0.68 m³. Total 1.156 m³, plus 10% = 1.27 m³. That is ready-mix territory, or nine to ten loads of a 140 L mixer by hand.
Bag yields by product
Yields come from the manufacturers' data sheets. Premix is sold by weight, but you need volume, and the ratio is nearly the same across brands: about 2,150 kg of dry mix per cubic metre of placed concrete, so a 20 kg bag gives roughly 9.5 litres and a 25 kg bag about 11.5 litres.
| Product | Bag | Yield | Bags per m³ |
|---|---|---|---|
| Boral Concrete Mix / Cement Australia Concrete Mix | 20 kg | ≈ 9.5–10 L | 105–107 |
| Dingo Concrete Mix | 20 kg | ≈ 9.5 L | 107 |
| Cement Australia Rapid Set (fence posts, no mixing) | 20 kg | ≈ 10 L | 100 |
| Cement alone, GP cement (mix 1:2:3) | 20 kg | 350 kg per m³ | 18 |
Source: Boral, Cement Australia, Dingo product data sheets, checked October 2026. Use the water quantity printed on the bag; a wetter mix does not give you more concrete, only weaker concrete.
Mixing it yourself
For general-purpose concrete at 350 kg of cement per m³ (slabs, footings, posts) the 1 : 2 : 3 rule is the simplest: one part cement, two parts sharp sand, three parts 20 mm gravel, and about half a part of water. For 1 m³: 350 kg cement, 400 L sand (about 680 kg), 800 L gravel (about 1,175 kg), 175 L water. A 140 L mixer gives about 90 L of placed concrete per load: one 25 kg bag of cement, four buckets of sand, six buckets of gravel and half a bucket of water, adjusted for how wet the sand is.
How thick, and how strong
A patio or shed base is poured at 100 mm. A driveway for cars needs 120 to 150 mm with mesh; for a van, caravan or boat trailer, 150 mm. Below 100 mm a slab cracks at the first frost or the first delivery van.
| Use | Thickness | Concrete grade (AS 3600 and AS 2870) |
|---|---|---|
| Garden path, shed base | 100 mm | N20 for paths and shed slabs |
| Patio, garage floor | 100–120 mm | see grades above; air-entrained outdoors where it freezes |
| Driveway | 120–150 mm | driveway grade, with mesh |
| Footings, foundations | per plan | sized from the soil and the frost or soil-movement depth |
Recommended grades: N20 for paths and shed slabs, N25 for driveways and garage floors, N32 where a truck or boat trailer sits. Bagged mix is formulated for about 25 to 30 MPa at 28 days, so strength is rarely the problem with bags; thickness, sub-base and curing are. For footings, Footing depth is set by the site's soil class under AS 2870, from 300 mm on stable sand to 600 mm or more on reactive clay. AS 2870 (residential slabs and footings) classifies the site soil; a footing that stops short of that depth moves with the ground.
Reinforcement and joints
Mesh does not stop concrete from cracking; it holds the cracks tight so the slab stays flat. Use SL72 mesh (7 mm bars, 200 mm grid) for patios and shed slabs, SL82 for driveways, set on chairs in the middle of the slab. Mesh that lies on the hardcore and gets walked into the bottom of the pour does nothing.
Control joints give the slab a place to crack on purpose: every 4 to 5 m in both directions, panels no larger than 20 to 25 m², cut to a quarter of the depth within 24 hours of finishing. Keep the panels roughly square; a long narrow strip cracks in the middle.
What concrete costs in 2026
Prices move with cement and diesel, so treat these as planning numbers and get two local quotes. These are the ranges we saw from suppliers and retailers in October 2026; cities run higher, rural plants lower.
| Item | Typical price | Notes |
|---|---|---|
| Ready-mix, driveway grade, delivered | A$260–A$360 per m³ | 2026 quotes; add 5–10% for air-entrained or fibre mix |
| Small-load charge | see note | minimum-load charge of A$60–A$150 under about 3 m³ |
| Truck waiting time | charged per minute | after the first 30–45 minutes on site |
| Concrete pump | half-day hire | only if the truck cannot reach the forms |
| Bagged concrete | 20 kg Boral or Cement Australia bag A$9–A$14 | DIY retail; post mix: Cement Australia Rapid Set 20 kg, about 10 L per bag |
| Finished slab by a contractor | A$100–A$160 per m² | formwork, hardcore, pour, finish |
Break-even: one cubic metre in bags is about 107 bags of 20 kg at A$11, so about A$1,180 of material, 2.2 tonnes to carry and a day at the mixer. The same cubic metre delivered is A$260–A$360 per m³ plus the small-load charge. Below about half a cubic metre, bags win; above one cubic metre, the truck wins on both price and quality, because every batch is identical.
Mixing, placing and curing
Water first into the mixer, then gravel, cement and finally sand, and let it turn for three minutes. The mix is right when a slice cut with a shovel stands up but slumps slowly. The most common DIY mistake is too much water: an extra litre per bag can cost a quarter of the strength and leaves a dusty surface.
Place within an hour of mixing, screed level, let the bleed water disappear, then float and finish. After finishing the slab must stay damp for 7 days: polythene sheet, wet hessian or a spray-on curing compound. Keep foot traffic off for 24 to 48 hours, cars off for 7 days, and expect design strength at 28 days. Above 32 °C pour at dawn, dampen the sub-base and keep the slab shaded; most Australian slab failures are hot-weather plastic cracking, not frost.
Weight and density
| Concrete type | Density | Weight per m³ |
|---|---|---|
| Normal-weight (bags and ready-mix) | 2,300–2,400 kg/m³ | 2.3–2.4 t |
| Lightweight structural | 1,400–1,800 kg/m³ | 1.4–1.8 t |
| Dry premix, as bagged | 2,150 kg per m³ of concrete produced | 2.15 t of bags per m³ |
The calculator uses 2,400 kg/m³ for the weight line. That matters for two things: whether your trailer can carry the bags (one cubic metre of bags is 2.2 tonnes, far beyond a 750 kg unbraked trailer), and whether a balcony or decked terrace can carry a slab on top.
Frequently asked questions
How many 20 kg bags of concrete make 1 m³?
About 107 bags of 20 kg, each giving 9.5 litres of placed concrete. The exact yield is printed on the bag.
How much concrete for a 20 m² slab, 120 mm thick?
20 × 0.12 = 2.4 m³, and 2.64 m³ with 10% extra. That is a ready-mix job; in bags it would be 282 bags of 20 kg, nearly six tonnes.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes for normal concrete, or 2,300 to 2,400 kg/m³.
What mix ratio for a patio slab?
350 kg of cement per m³: one part cement, two parts sharp sand, three parts 20 mm gravel, half a part of water. In a 140 L mixer: one 25 kg bag of cement, four buckets of sand, six buckets of gravel, half a bucket of water.
How much concrete for a fence post?
A 250 mm wide, 600 mm deep hole with a 90 × 90 mm post needs 0.025 m³, which is three bags of Rapid Set at 10 L each. Dig the hole three times the post width and to the depth your soil class needs.
Does the calculator subtract the post from the hole?
Yes. Enter the post width in the optional field and its volume is removed from the hole. Leave it empty if the hole is filled solid, for example for a ground sleeve or a bollard.
How much extra should I allow?
5 to 10%. Wheelbarrow spillage, an uneven sub-base and formwork that bows all eat concrete. For a hand-dug footing on rough ground use 15%; running out mid-pour creates a cold joint you cannot fix.
How thick should a shed base be?
100 mm for a garden shed or workshop, 120 mm with SL72 mesh if a car, ride-on mower or heavy machine will sit on it.
Can I pour a slab in two batches on different days?
You can, but the join is a cold joint and will crack. Plan the pour so each panel between control joints is finished in one go, and if you must stop, stop at a joint.
How long before I can walk or drive on new concrete?
Walk after 24 to 48 hours, park a car after 7 days, and treat 28 days as full strength. Keep it damp for the first 7 days; concrete that dries out early never reaches its rated strength.
How we built this calculator
The formulas are the plain geometric ones, shown on the result board for every calculation. Bag yields are taken from the manufacturers' data sheets and the dry-mix ratio of 2,150 kg per m³ (133 lb per ft³) is what those sheets imply; density is 2,400 kg/m³ (150 lb/ft³), the Portland Cement Association's figure for normal-weight concrete. Prices are ranges we collected from supplier and retailer price lists in the month shown and are not quotes.
References
Estimates for planning purposes. Always check the yield printed on your bags and local building rules.