🧱 Concrete Calculator
← Life & LivingThat little slab, footing or fence-post hole needs a surprising amount of concrete. Enter your dimensions and see how much to order, how many bags that means, and what a little extra thickness can do to the total.
Here's how much concrete you'll need
A slab you can actually picture
Proportions are schematic — thin shapes are exaggerated so the diagram stays readable.
Where that concrete goes
A centimetre can make a difference
Explore a different thickness without changing your measurements until you apply it.
This changes quantity only. It says nothing about what thickness or depth is structurally suitable.
How many bags are we talking?
The measurements behind the answer
| Area | Dimensions | Volume |
|---|
A few centimetres can change the whole order
Concrete is one of those jobs where a small measuring mistake becomes a very physical problem. A slab looks flat, so it's tempting to think only about the length and width. But thickness is where an extra centimetre can turn into another stack of bags.
Take a five-by-three-metre slab. At 100 mm thick, it holds 1.5 cubic metres. Make it just 10 mm thicker and you've added another 150 litres of space to fill. That's about fifteen more 20 kg bags before you even think about an allowance for uneven ground.
This calculator turns the shape into a volume, then turns that volume into something you can actually order. You can switch from a slab to a footing, calculate a row of fence-post holes, check different bag yields, and see what the job might cost if you've got prices from a supplier.
The number is a planning estimate, not a building specification. It tells you how much space you're filling, not whether the design, mix, depth or reinforcement is right for the job.
Concrete has a way of making small numbers huge
One extra centimetre can mean fifteen extra bags.
A 5 m × 3 m slab that's just 10 mm thicker needs 150 extra litres of concrete. At an example yield of 10 litres per 20 kg premix bag, that's 15 bags before you add any waste allowance. Your tape measure can have a bigger effect on the shopping list than you'd think.
Twice the hole width means four times the concrete.
If two round post holes are the same depth, doubling the diameter makes the concrete volume four times as large. The radius gets squared in the formula, which is the mathematical equivalent of a tiny-looking change becoming a much bigger job.
A 20 kg bag doesn't make 20 litres of concrete.
This one's a trap. A typical Australian 20 kg premixed-concrete product can yield around 10 litres of finished concrete, not 20. That means just one cubic metre can take about 100 bags. Bag weight and final filled volume are two different things.
Even a fence post takes up a meaningful chunk of the hole.
Put a 100 mm square post 600 mm deep into a 300 mm diameter hole. The post displaces 6 litres of space. That's more than half of the approximate finished yield of a 20 kg premix bag. For a whole fence line, those little subtractions can add up.
One square metre can hide ten bags.
Cover just 1 m² to 100 mm deep and you've made a space for 100 litres of concrete: roughly ten 20 kg premix bags at 10 litres each. Multiply the area by ten and the small job can suddenly look like a pallet problem.
How does the concrete calculator work?
The calculator finds the volume of the empty space you want to fill. Everything is converted into the same unit first, so metres, millimetres, feet and inches can be used together without quietly changing the answer. Once the space has a volume, the optional allowance and bag yield turn it into an order estimate.
Slab, pad or rectangular footing
Concrete volume = length × width × thickness (or depth) × number of matching areas
All lengths in metres give cubic metres. For 5 m × 3 m × 100 mm:
Thickness: 100 mm = 0.100 m Volume: 5 × 3 × 0.100 = 1.500 m³ Extra 5%: 1.500 × 0.05 = 0.075 m³ Order allowance included: 1.500 × 1.05 = 1.575 m³
A trench uses exactly the same box-shaped formula even though its purpose is different. Four identical rectangular pads multiply the per-pad volume by four.
Round post holes
Hole volume = π × (diameter ÷ 2)² × depth × number of holes
If posts occupy part of that space:
Concrete actually needed = hole volume − embedded post volume
- Square post:
postWidth × postWidth × embeddedDepth. - Round post:
π × (postDiameter ÷ 2)² × embeddedDepth. - Multiply post displacement by the number of holes and posts.
- Add the optional extra allowance only after displacement is deducted.
One circular hole, no post displacement:
Diameter = 300 mm = 0.3 m
Depth = 600 mm = 0.6 m
Volume = π × (0.15)² × 0.6
≈ 0.0424115 m³ (42.41 L)
With 5% allowance ≈ 0.0445321 m³ (44.53 L)
20 kg bags at 0.010 m³ each = ceil(4.45321) = 5 bags
With a 100 mm × 100 mm square post embedded 600 mm:
Displacement = 0.1 × 0.1 × 0.6 = 0.006 m³ Net hole volume ≈ 0.0364115 m³ With 5% allowance ≈ 0.0382321 m³ 20 kg bags ≈ ceil(3.82321) = 4 bags
If the shape is invalid, the calculator shows an error instead of letting negative concrete volume into the results.
Adding more than one section
Total base volume = sum of each independently calculated non-overlapping section's net volume
Apply the allowance once to the combined total. For a mix of slabs and holes, the recap keeps every section's own volume so you can check for double counting.
Allowance for waste
Volume to allow for = base volume × (1 + extra allowance % ÷ 100)
Extra allowance volume = volume to allow for − base volume
A 5% allowance is not a structural thickness adjustment, and it does not cover every delivery or placement contingency. Both volumes are shown so you can see what is in the shape and what is in the order.
Bags — why you must round up
Whole bags required = ceil(volume to allow for ÷ manufacturer's yield per bag)
For the default example:
1.575 m³ ÷ 0.010 m³ per 20 kg bag = 157.5 bags Order estimate = 158 whole bags
The calculator uses the manufacturer's declared finished-concrete yield, not kilograms as though they were litres. Even the same bag weight may have a different stated yield for a different product. A custom yield appears in the result wherever it changes the bag count.
This bag calculation is for bagged concrete premix, not cement-only bags. No mix-design ratios, aggregate recipes, water dosage or promised strength are inferred from this estimator.
Cost estimates
For bagged concrete:
Estimated premix materials cost = whole bags × entered price per bag
For ready-mix:
Estimated quoted cost = ordered volume × entered rate per volume + entered delivery and surcharge fees
If a supplier increment is set, the allowance-adjusted volume is rounded up to the nearest allowed increment before the priced order volume is calculated. Supplier minima, delivery sizes and fees vary, so only your entered prices are used.
Unit conversions
- 1,000 L = 1 m³.
- 1 ft = 0.3048 m; 1 in = 25.4 mm.
- 1 yd³ ≈ 0.76455 m³.
- 1 m³ ≈ 1.308 yd³.
Unit switches affect only how the dimensions and results are expressed, not the underlying physical project.
Assumptions and limits
- Geometric shapes are assumed uniform and correctly measured.
- Ground may be uneven; measurements may not capture voids, slopes or excavation variation.
- A buffer is an editable planning allowance, not a guaranteed supplier or professional standard.
- Bag yields are example manufacturer figures; check the actual product's published wet-concrete yield.
- Bag product is premixed concrete, not dry cement alone.
- Structural adequacy, site conditions, load capacity, reinforcement, joints, local approvals, material suitability, curing and safe handling are not designed or assessed here.
- Price estimates omit unentered costs, often including labour, delivery, tools, excavation, sub-base and reinforcement.
- The calculator is for quantities and budgeting, not engineering or construction advice.
Example yields referenced from manufacturer documentation: Easy Mix technical data sheet, Bastion 20 kg premix listing, CEMIX Maxcrete data sheet and the QUIKRETE Concrete Mix data sheet. These are product examples, not universal guarantees; availability varies. Cement-only batching is a different calculation — see Cement Australia's batching table.
Concrete Calculator FAQs
How do I calculate how much concrete I need for a slab?
Measure the length, width and thickness, convert them into the same unit, and multiply them together. A slab 5 m long, 3 m wide and 100 mm thick holds 1.5 cubic metres. You can then add your chosen allowance for small measuring differences or uneven ground.
How many 20 kg concrete bags make one cubic metre?
A number of Australian ready-to-use 20 kg concrete mixes list a yield around 0.010 cubic metres, or 10 litres, per bag. At that yield, one cubic metre takes 100 bags before any allowance. Always check the yield stated for the actual product you're buying: bag weight alone doesn't determine the finished volume.
What's the difference between cubic metres and cubic yards?
They're both units of volume. One cubic metre is about 1.308 cubic yards, and one cubic yard is about 0.765 cubic metres. Ready-mix quantities are commonly quoted in cubic metres in Australia and cubic yards in the United States. The calculator keeps the underlying volume the same when you switch units.
Should I include an extra allowance for concrete?
An extra allowance can help you plan for uneven surfaces, small measurement errors or material loss, but no single percentage suits every job. The default 5% is an illustrative starting point that you can adjust. For an important or complex pour, verify the quantity with the concrete supplier or an appropriately qualified professional.
How do I estimate concrete for a fence-post hole?
For a round hole, calculate π × radius² × depth, then multiply by the number of holes. If a post takes up part of that space, you can subtract the volume of its embedded section. This calculates the material space only; it doesn't determine a safe hole depth or suitable post design.
Is a bag of concrete the same as a bag of cement?
No. A bag of premixed concrete includes cement and aggregate and typically needs the specified water added. Cement alone is a binder used with other materials to make concrete. This calculator's bag counts are for products sold as premixed concrete with a published finished-volume yield.
Is bagged concrete cheaper than ready-mix delivery?
That depends on the quantity, local prices, delivery and any minimum-load charges, as well as the time and equipment needed for mixing. Enter your own prices to compare the material estimates. A lower material total doesn't automatically mean a lower total project cost.
Can this calculator tell me how thick my slab or footing should be?
No. Thickness, depth, strength, reinforcement and joint details depend on loads, ground conditions, the intended use and local construction rules. This calculator estimates quantity after you've chosen or obtained those dimensions; it isn't a structural design tool.
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