Work out a whole concrete job, with every step shown. Add slabs, footings, columns, piers, beams and walls, and get the total volume, cement bags, sand, stone, water, ready-mix order and formwork area. Results also compare the published bag method with the common 1.54 dry volume method.
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| Element | Size | Volume | Formwork |
|---|
| Method | Cement | Sand | Stone |
|---|
Includes your waste allowance. The docket uses the published method.
Wet Concrete Needed
0m³
This concrete computation calculator handles a whole job in one go, and it shows the working so you can check every line.
For a single simple pour, our Concrete Volume Calculator is quicker. For a batch recipe in buckets, use the Concrete Mix Ratio Calculator. If your measurements came from the site in mixed units, start with the Concrete Measurement Calculator.
Concrete computation is the work of turning a set of plans into quantities. Builders and estimators call it a take-off. It answers four questions. How much concrete do I need? What goes into it? What does it weigh? And what else does the pour need, such as forms?
Each element is computed on its own, then added up. That way, you can check any line, and change one item without starting again. A good computation sheet is also easy for someone else to follow, which is why this calculator lists each step in plain words.
The maths itself is simple. The care goes into getting the sizes right, using the right units, and not counting anything twice.
Concrete Basics also says ready-mix is sold in 0.2 m³ steps, so the calculator rounds your order up to the next one.
All sizes below are in metres. Take care with units, as plans mix metres and millimetres.
| Element | Volume | Example | Answer |
|---|---|---|---|
| Slab Or Path | Length × width × thickness | 6 × 4 × 0.1 | 2.4 m³ |
| Strip Footing | Length × width × depth | 20 × 0.3 × 0.3 | 1.8 m³ |
| Square Column | Width × depth × height | 0.3 × 0.3 × 3 | 0.27 m³ |
| Round Column Or Pier | 3.14 × diameter² ÷ 4 × height | 3.14 × 0.45² ÷ 4 × 0.9 | 0.143 m³ |
| Beam Below Slab | Length × width × depth below slab | 6 × 0.3 × 0.4 | 0.72 m³ |
| Wall | Length × height × thickness | 8 × 2.4 × 0.2 | 3.84 m³ |
Stairs, tapered footings and odd shapes need their own methods. Our Concrete Steps Calculator handles stairs. Bunnings also suggests splitting irregular areas into sections, and adding them together.
If one element needs more detail, use the matching tool. Try the Concrete Slab Calculator, Concrete Footing Calculator, Concrete Post Hole Calculator or Concrete Foundation Wall Calculator.
Where elements meet, they share the same concrete. If you count it in both, your total runs high. A structural take-off guide suggests this order: slab, then column, then beam.
That is why the calculator asks for Depth Below Slab on a beam. A 500 mm deep beam under a 100 mm slab has a depth below slab of 400 mm.
Large voids should come off. Small ones often do not matter. The same guide says that some measurement codes do not deduct small openings, using 1 m² under the UK NRM2 as an example, so check which rules your job uses. For a house job, simply work out the volume of large openings or pits, and enter it under Take Off Openings. Bunnings says to subtract anything that will displace concrete, such as posts or pipes.
Say a shed needs a slab, a strip footing and four piers, all in a 1 : 2 : 3 mix.
The sizes above are only an example, not a design. At this size, ready-mix is the obvious choice, as 84 bags of cement plus almost 9 tonnes of sand and stone is a lot to handle. Our Concrete Order Calculator can take it from there.
The docket uses the same method as our Mix Ratio Calculator, so the two pages agree. It starts with published bag counts from the Cement Australia General Purpose Cement sheet.
| Use | Mix By Volume | 20 kg Bags Per m³ | Sand Per m³ | Stone Per m³ |
|---|---|---|---|---|
| Paths And Driveways | 1 : 2 : 3 | 16 | 0.44 m³ | 0.67 m³ |
| Foundations And Footings | 1 : 3 : 3 | 13 | 0.54 m³ | 0.54 m³ |
| Higher Strength Or Water Tightness | 1 : 1.5 : 3 | 17 | 0.35 m³ | 0.71 m³ |
The bag counts are published. The sand and stone figures are our working from those bags and the ratio, using loose densities of 1,440 kg/m³ for cement, 1,600 for sand and 1,500 for stone. These are starting values, as materials and moisture vary. For a custom ratio, the calculator spreads 1.3 m³ of loose material across the parts. We fitted that figure to the published table.
Search for concrete computation and you will soon meet the number 1.54. Many guides, such as this one, multiply the wet volume by 1.54 to get the dry volume, then share it across the mix parts.
Dry materials take up more room than the finished concrete, as the gaps between stones get filled by sand, and the gaps in the sand get filled by paste. Hence the factor. Here is how the two methods compare for 1 m³, with no waste.
| Mix | Published Cement | 1.54 Cement | Published Sand | 1.54 Sand | Published Stone | 1.54 Stone |
|---|---|---|---|---|---|---|
| 1 : 2 : 3 | 16 bags | 18.5 bags | 0.44 m³ | 0.51 m³ | 0.67 m³ | 0.77 m³ |
| 1 : 3 : 3 | 13 bags | 15.8 bags | 0.54 m³ | 0.66 m³ | 0.54 m³ | 0.66 m³ |
| 1 : 1.5 : 3 | 17 bags | 20.2 bags | 0.35 m³ | 0.42 m³ | 0.71 m³ | 0.84 m³ |
So the 1.54 method gives about 15% to 20% more of everything. It is not exactly wrong. It simply assumes a larger loose volume. The real figure depends on your sand, stone and moisture. We lead with published Australian bag counts, as they come from a local cement maker. The results show both, so you can see the difference.
If you are mixing by hand and want a margin, the 1.54 figure gives you a built-in buffer. But do not add it on top of a waste allowance without thinking, or you could over-order by a third.
Water is computed from the cement weight, not the volume of concrete.
Concrete Basics gives 20 litres of water with 40 kg of cement as an example of a ratio of 0.5, and notes that 1 litre weighs 1 kg. So a 1 m³ mix with 320 kg of cement needs about 160 litres.
Treat this as a guide. Sand and stone hold moisture, especially after rain, so you will often add less. Too much water weakens concrete. Add it slowly, and only until the mix is workable. The lower the ratio, the stronger the concrete, as long as it can still be placed and compacted.
The docket uses 2,400 kg/m³, a common planning figure, on the measured volume. So the shed base above weighs about 4.77 × 2.4 = 11.5 tonnes.
Weight matters for loads, lifts, access and truck planning. For a different type of concrete, or to work out unit weight in kN/m³, use our Concrete Density Calculator.
Add up the weight of your materials, and compare it with 2,400 kg per m³. It is a quick concrete computation check that few guides mention. Here is 1 m³ of a 1 : 2 : 3 mix, by both methods.
| Material | Published Method | 1.54 Method |
|---|---|---|
| Cement | 320 kg | 370 kg |
| Sand | 711 kg | 821 kg |
| Stone | 1,000 kg | 1,155 kg |
| Water | 160 kg | 185 kg |
| Total | 2,191 kg | 2,531 kg |
The planning figure of 2,400 kg sits between the two. So on these loose densities, the published method runs a little lean on sand and stone, and the 1.54 method runs a little rich. The real amounts depend on your materials and moisture, so buy a little extra sand and stone when you mix on site. Running short halfway through a pour is the mistake to avoid. The cement bag count stays as published.
Formwork is measured in square metres of contact with the concrete. The Deakin University measurement guide measures column forms as height × perimeter, and beam forms as length × girth, the exposed perimeter of the beam.
| Element | Formwork Area | Example | Answer |
|---|---|---|---|
| Slab Edge | Perimeter × thickness | 20 × 0.1 | 2 m² |
| Footing | 2 × (length + width) × depth | 2 × (20 + 0.3) × 0.3 | 12.18 m² |
| Square Column | Perimeter × height | 1.2 × 3 | 3.6 m² |
| Round Column | 3.14 × diameter × height | 3.14 × 0.45 × 0.9 | 1.27 m² |
| Beam Below Slab | Length × (width + 2 × depth) | 6 × (0.3 + 0.8) | 6.6 m² |
| Wall | 2 × length × height | 2 × 8 × 2.4 | 38.4 m² |
The results use these. They treat a slab as sitting on the ground, so only its edge is counted. A suspended slab also needs its underside formed, and a footing poured straight into a clean trench may need no forms at all. The take-off guide says that plywood typically lasts about 6 to 8 pours, so forms are not a one-time cost on big jobs.
Wet concrete pushes hard on forms. Concrete Basics says forms should be properly braced, so they do not flex or move.
This calculator does not compute steel, as it depends on your structural design. But the basic sum is easy. A common formula gives the weight of a bar per metre.
| Bar Diameter | Sum | Weight Per Metre |
|---|---|---|
| 10 mm | 100 ÷ 162 | 0.62 kg |
| 12 mm | 144 ÷ 162 | 0.89 kg |
| 16 mm | 256 ÷ 162 | 1.58 kg |
| 20 mm | 400 ÷ 162 | 2.47 kg |
Add laps as your engineer sets them. The same guide uses about 40 bar diameters in compression and 50 to 60 in tension, and adds about 3% to 5% for wastage. Those are general rules, so follow your plans. For slabs on the ground, mesh is often used instead. Our Concrete Material Cost Calculator works out mesh sheets and more.
Your computation tells you which way to go. The results show all three for the same job.
| Option | Best For | Watch Out For |
|---|---|---|
| Premix Bags | Very small jobs | Hundreds of bags on bigger jobs |
| Site Mix | Small jobs with a mixer | Consistency, and a lot of handling |
| Ready-Mix | Most jobs over about 1 m³ | Truck access, small-load fees |
Premix bags use 0.0093 m³ per 20 kg bag, from the Dingo MaxPRO listing. That is about 108 bags per m³, so the shed base above would take 565 bags. No one wants that. The 1 m³ rule is our own rule of thumb. For costs, see our Concrete Cost Calculator and the Concrete Cost Per Square Metre page. Our Concrete Truck Calculator covers delivery.
A small frame has 4 columns, each 300 mm square and 3 m high, and 3 beams, each 6 m long, 300 mm wide and 400 mm below the slab. These sizes are an example only.
Structural work like this should use an engineer's design and a graded ready-mix, not a site mix. The example simply shows how the sums work.
A 1.2 m × 1.2 m pad at 100 mm is 0.144 m³, or 0.158 m³ with 10% waste. At 1 : 2 : 3, that is 2.53 bags, so 3 bags of cement, with about 70 litres of sand, 106 litres of stone and 25 litres of water. Or you could use 18 bags of 20 kg premix. For a job this small, premix is often simpler, as you buy just one product. Our Concrete Bag Calculator checks the count, and the Concrete Pad Cost Calculator covers the cost.
A tidy sheet saves mistakes, and it lets someone else check your work. If you do your own concrete computation on paper or in a spreadsheet, use columns like these. The take-off sheet in the results follows the same idea.
| Ref | Element | Number | Length | Width | Depth | Volume |
|---|---|---|---|---|---|---|
| S1 | Shed Slab | 1 | 6.000 | 4.000 | 0.100 | 2.400 |
| F1 | Strip Footing | 1 | 20.000 | 0.300 | 0.300 | 1.800 |
| P1 | Pier, 450 mm Round | 4 | Round | 0.450 | 0.900 | 0.573 |
| Total | 4.773 |
Holcim says to base your final order on site measurements rather than drawings. So treat a plan take-off as your budget figure, then check it on site before you order.
Keep three decimals through your concrete computation, so rounding errors do not build up. Round only at the end, and always round up. Bags come whole, and ready-mix comes in steps of 0.2 m³, so 5.25 m³ becomes 5.4 m³. So a part bag always becomes a whole bag, and sand and stone are best bought with a little to spare.
If you do mix on site, your concrete computation does not stop at the total. You also need to know what goes into each mixer load. The simplest way is to build each batch around one 20 kg bag of cement.
| Mix | Cement | Sand | Stone | Water At 0.5 | Batch Makes |
|---|---|---|---|---|---|
| 1 : 2 : 3 | 13.9 L | 27.8 L | 41.7 L | 10 L | 63 L |
| 1 : 3 : 3 | 13.9 L | 41.7 L | 41.7 L | 10 L | 77 L |
| 1 : 1.5 : 3 | 13.9 L | 20.8 L | 41.7 L | 10 L | 59 L |
The cement volume comes from 20 kg ÷ 1,440 kg/m³. The yield comes from dividing 20 kg by the cement in each cubic metre. So one bag of 1 : 2 : 3 makes about 63 litres of concrete, and the shed base above needs 84 mixes. That is a very long day, which is why ready-mix wins on all but small jobs. Our Concrete Mix Ratio Calculator turns these into a bucket recipe, and Concrete Basics warns never to use a shovel as a measure, as volumes can vary widely.
Do not worry if a supplier or concreter comes back with a slightly different figure. There are a few common reasons. They may have measured the site rather than the plans, or used a different waste allowance. They may add thickened edges or beams that you left out. Ready-mix suppliers also work to their own mix designs by strength grade, not to a simple volume ratio, so the idea of cement bags does not apply to a truck order.
A large gap is worth a question, though. If your concrete computation and a quote differ by more than your waste allowance, go through the elements one at a time. Most errors turn out to be a missed item, a repeat that was not multiplied, or a unit slip. Your take-off sheet makes this quick to find.
Many searches for concrete computation come from overseas, so you may see figures that do not match Australian practice. The main differences are bag size and mix.
In the Philippines, guides often use a factor method with 40 kg bags. One guide lists these amounts per cubic metre of concrete.
| Class | Mix | Cement Bags | Sand | Gravel |
|---|---|---|---|---|
| AA | 1 : 1.5 : 3 | 12 | 0.50 m³ | 1.00 m³ |
| A | 1 : 2 : 4 | 9 | 0.50 m³ | 1.00 m³ |
| B | 1 : 2.5 : 5 | 7.5 | 0.50 m³ | 1.00 m³ |
| C | 1 : 3 : 6 | 6 | 0.50 m³ | 1.00 m³ |
Pinoy Builders labels Class A as 1 : 1.5 : 3 rather than 1 : 2 : 4, so even those letters differ from source to source. They are not Australian classes. In Australia, cement commonly comes in 20 kg bags at hardware stores, and ready-mix is ordered by strength grade. See Concrete Grades Explained.
To use any of these here, enter the ratio under Custom Ratio. Bag counts will be in 20 kg bags. If your bags differ, use the cement weight instead of the count.
Wet concrete and cement can burn skin, and dry cement dust irritates the eyes and lungs. Wear gloves, boots, eye protection and a mask when you handle bags. Cutting cured concrete creates dust that contains silica. See Safe Work Australia for more.
Multiply length, width and thickness, all in metres. A 6 m × 4 m slab at 100 mm is 6 × 4 × 0.1 = 2.4 m³. For a job with several parts, work out each one and add them together.
Find the wet volume, add waste, then use your mix. For a 1 : 2 : 3 mix, Cement Australia lists 16 bags of 20 kg cement per m³. Sand is twice the cement by volume, and stone is three times the cement by volume.
It is a common textbook factor that turns wet volume into dry volume. You multiply the wet volume by 1.54, then share it across the mix parts. It gives more material than published Australian bag counts, as the results show side by side.
On Cement Australia figures, a 1 : 2 : 3 mix takes 16 bags of 20 kg per m³, a 1 : 3 : 3 mix takes 13 bags, and a 1 : 1.5 : 3 mix takes 17 bags. The 1.54 method gives about 18.5 bags for 1 : 2 : 3.
At a water to cement ratio of 0.5, a 1 : 2 : 3 mix with 320 kg of cement needs about 160 litres. Damp sand and stone already hold water, so you may need less. Keep the mix only as wet as it needs to be.
Multiply the width, depth and height, in metres. A 300 mm square column 3 m high is 0.3 × 0.3 × 3 = 0.27 m³. For a round column, use 3.14 × diameter² ÷ 4 × height.
Use the length × width × depth below the slab. Do not count the part inside the slab depth, as the slab already includes it. A 6 m beam, 300 mm wide and 400 mm below the slab, is 0.72 m³.
Multiply the total length by the width and depth. A 20 m strip that is 300 mm wide and 300 mm deep is 20 × 0.3 × 0.3 = 1.8 m³.
Concrete Basics says to order about 10% extra. Bunnings suggests 5% to 10%. The calculator starts at 10%, and you can change it under Mix And Allowances.
A common formula is bar diameter squared ÷ 162, which gives kg per metre. So a 12 mm bar is 144 ÷ 162 = 0.89 kg per metre. Your engineer sets the bars, laps and spacing.
No. It works out quantities from sizes you enter. It does not say if a size or mix is strong enough. Your plans and engineer decide that.
No. Your numbers stay in your browser and nothing is uploaded. Use Download Sheet to keep a copy.
Figures reviewed November 2026. Sources: Cement Australia, Concrete Basics, Bunnings, Holcim, Deakin University, Civil At Work, BIM Planit, Aedo Construction, Pinoy Builders and Safe Work Australia. Read our Disclaimer and Calculator Methodology.