Concrete Calculator

How the Concrete Calculator works

The formula. Slab volume = length × width × depth, plus a 10% allowance; post hole volume = π × radius² × depth. For example, a 3 × 3 m slab 100 mm thick is 0.9 m³, or 0.99 m³ with waste. A second example: a 300 mm hole 600 mm deep is π × 0.15² × 0.6 = 0.042 m³ before waste. A cubic metre takes about 112 bags of 20 kg premix, which is how many bags turn the slab into 111.

Concrete slab calculator: length × width × depth, in one unit

Concrete volume is length × width × depth, and every one of those three numbers has to be in the same unit before you multiply. That is where most kitchen-table calculations fall over: the slab is measured in metres and the thickness in millimetres, and a factor of a thousand goes missing. This page takes the units with the number, so you can type the dimensions the way you measured them.

Post hole concrete calculator: the radius is half the diameter

Post holes are the other shape worth its own mode, because they are round. The volume of a cylinder is π × radius² × depth, and the radius is half the diameter — squaring the full diameter by mistake gives you four times too much concrete. Multiply by the number of holes and you have the pour.

Why the waste allowance is not padding

Order the exact calculated volume and you will run short, every time. Ground is uneven, so the base of a slab sits lower than the string line in places; formwork bows outward under the weight of wet concrete; subgrade absorbs water and a little material; some is left in the barrow, the chute and the mixer. Ten per cent is the conventional allowance for a slab on prepared ground and is what this page uses when you leave the field blank. Push it to 15% on rough or sloping ground, on a hand-dug excavation, or where the formwork is improvised. On post holes, allowance matters more than anywhere else: a hand-augered hole is never a clean cylinder, and belling out at the bottom — which is desirable structurally — means real holes routinely swallow 20% more than the geometry predicts.

The failure mode is asymmetric, and that should drive the decision

Too much concrete costs you the price of the excess and a wheelbarrow of spoil to dispose of. Too little means a cold joint: the first pour begins curing before the second arrives, and the two never bond into one slab. That joint is a permanent plane of weakness and a crack waiting to open along it. For a ready-mix delivery, round up to the next supplier increment — usually 0.2 m³ — and accept that you will waste some. For bagged premix, buy at least one spare bag; an unopened bag goes back to the store, an unfinished slab does not.

Bags of premix

The bag counts here use the yield printed on the bag: about 9 litres of wet concrete from a 20kg bag, about 0.45 cubic feet from a 60lb bag. Yields vary a little by brand and by how much water you add, and mixing by hand is slower than people expect — beyond roughly half a cubic metre, a mixer or a delivery is almost always the better call.

Concrete slab table

A 100 mm thick slab with 10% waste.

Concrete volume and 20 kg bags for a 100 mm slab

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