How concrete volume is calculated

Every formula these calculators use, why each one is the way it is, and how the arithmetic is checked.

The formulas, in short

Rectangular pour: length × width × depth
Round hole or pier: π × radius² × depth
Waste allowance: volume × (1 + allowance ÷ 100)
Order quantity: the allowed volume rounded up to the next 0.2 m³
Bags: volume ÷ the yield of one bag, rounded up

Everything below is the detail behind those five lines. All of it runs in your browser — no measurement you type is ever sent to us.

Units come first

Every measurement is converted to metres before any arithmetic happens. Mixing units inside a calculation is one of the easiest ways to be out by a factor of ten and not notice.

1 mm = 0.001 m
1 cm = 0.01 m
1 inch = 0.0254 m
1 foot = 0.3048 m

That is why you can enter a length in metres and a depth in millimetres on the same form and still get the right answer. If a unit is ever unrecognised the calculator raises an error rather than carrying on — a wrong number presented confidently is worse than no number.

What counts as a number

Input is parsed strictly. 5, 5.5, 1,250 and values with stray spaces are accepted. 5m, 5..5 and anything else are rejected outright rather than being half-read as 5. Silently discarding the part of your input it did not understand would be the worst possible behaviour.

Rectangular pours

Slabs, footings, driveways, paths and shed bases are all the same calculation:

Volume (m³) = length (m) × width (m) × depth (m)

A 5 m × 4 m slab at 100 mm: 5 × 4 × 0.1 = 2.00 m³.

Slabs with a thickened edge

Most Australian slabs on ground drop into a deeper beam around the perimeter. That beam only adds concrete where it goes below the underside of the slab, because the slab pour already fills the top of the trench.

Extra volume = beam footprint × (edge depth − slab thickness)

The footprint is the part that catches people out. It is calculated as the whole slab area minus the untouched middle:

footprint = (L × W) − ((L − 2b) × (W − 2b)), where b is the beam width.

The obvious alternative — perimeter × beam width — counts each corner twice and over-orders. On an 8 m × 6 m slab with a 300 mm beam the difference is about 0.1 m³; the error grows as the beam gets wider. If the beam is wide enough to meet in the middle, the calculator treats it as a slab at the full edge depth rather than adding more concrete than the shape can hold.

Round holes and piers

Volume = π × (diameter ÷ 2)² × depth, multiplied by the number of holes.

Where a post sits in the hole, the volume the post occupies is subtracted — its width squared, times the depth — on the assumption the post reaches the bottom. If the post is entered as wider than the hole, the calculator says so instead of returning a negative volume.

Waste allowance

Recommended volume = volume × (1 + allowance ÷ 100)

At the default 10%, 2.00 m³ becomes 2.20 m³.

The allowance is not padding for its own sake. Ground is never perfectly flat, sub-base compacts unevenly, timber formwork bows outward under the weight of wet concrete, and some concrete is always left in the barrow or on the ground. Running short mid-pour is considerably more expensive than ordering slightly over — a cold joint where the first load has begun to set is a structural problem, not an inconvenience.

Order quantity

Most Australian suppliers sell ready-mix in fixed increments, commonly 0.2 m³, and apply a minimum load. So the figure you actually order is the allowed volume rounded up to the next increment:

2.31 m³ → order 2.4 m³. An exact multiple is never pushed to the next increment: 2.2 m³ stays 2.2 m³.

The cost calculator prices the order quantity rather than the raw volume, because that is what appears on the invoice. Pricing the raw volume consistently under-estimates the bill.

Bags

Bags = volume ÷ yield per bag, always rounded up, because you cannot buy part of a bag.

The important word is yield. Two 20 kg bags from different manufacturers do not necessarily make the same volume of concrete, so a bag count worked out from weight alone is a guess. These calculators use the yield in litres, and you can enter the figure printed on the product you are actually buying. The defaults are typical Australian values only — roughly 9 litres for a 20 kg bag, which is about 110 bags per cubic metre.

Bag counts are calculated from the volume after the waste allowance, not before.

Rounding, and why it is harder than it looks

Results are shown to two decimal places, because that is how concrete is ordered and discussed. Getting that rounding right takes more care than it appears to.

Computers store decimals in binary, and most decimal fractions have no exact binary representation. A 0.3 m × 5 m path at 150 mm is exactly 0.225 m³ in decimal arithmetic. The nearest value a computer can store is 0.22499999999999998 — very slightly under. Round that the obvious way and you get 0.22, when the answer is 0.23.

It is not a rare curiosity. Across the range of dimensions people realistically enter, there are more than 1,500 combinations where naive rounding reports the wrong figure. These calculators correct for that drift, by an amount proportional to the value, so numbers that were already correct are never moved.

One deliberate exception: a single small post hole can come to less than 0.005 m³. Rounded to two decimals that displays as "0.00 m³", which reads like a broken calculator. Volumes that small are shown to three decimals instead.

How this is checked

Arithmetic that looks right is not the same as arithmetic that is right. Every formula on this page is covered by an automated test suite that runs against the same code your browser loads — not a copy of it, and not a description of it.

The suite currently runs 505 checks. They cover:

The worked examples above, including 5 m × 4 m × 100 mm = 2.00 m³, and 2.20 m³ with a 10% allowance.
The floating-point case described above, asserting the naive result is wrong and ours is right.
Equivalent measurements in different units producing identical volumes.
Zero, negative, blank and malformed input producing a clear message rather than a broken or confident-looking wrong answer.
Bag counts rounding up rather than down, and using the post-allowance volume.
Order quantities landing on supplier increments, including that exact multiples are left alone.
The edge-beam footprint, the strip-versus-pad footing distinction, and the driveway apron.

The number above is not typed by hand and left to rot. A test reads it from this page and fails if it does not match the real count, so this section cannot quietly become untrue.

A second suite checks the live site rather than the source: it loads each published page, runs the real calculator code against it, and confirms the numbers that appear on screen are the numbers expected. That exists because the two are not the same thing — code can be correct while the page a visitor receives is not.

What this does not tell you

All of the above answers "how much". None of it answers "is this adequate". These calculators do not determine slab thickness, reinforcement, concrete strength class, footing size, or whether any of it suits your soil and loads. Those depend on your site and require someone who can assess it and take professional responsibility for the answer.

Quantities are estimates. Confirm them with your supplier before ordering, and see the terms of use for the full position.

If you think any formula here is wrong, tell us — include the numbers you entered and the result you expected. That is the most useful message we receive.