Concrete Volume Calculator

The calculation

slab or footing:  length × width × depth       (all in metres)
column or hole:   π × radius² × height

Working in metres throughout means the answer comes out in cubic metres, which is the unit everything is sold and quoted in. The single most common error is mixing millimetres and metres: a depth of 100 mm is 0.1 m, and entering 100 gives an answer a thousand times too large.

How thick does it need to be

• Footpath, garden path: 75 mm on 75 mm of compacted hardcore.
• Patio, shed base, greenhouse: 100 mm on 100 mm of hardcore.
• Driveway for cars: 100 to 150 mm with steel mesh reinforcement, on 150 mm of hardcore.
• Heavy vehicles or a garage floor: 150 mm minimum, reinforced, and worth taking advice on.
• Strip footings for a wall: typically 450 mm wide and 200 mm deep for a garden wall, but foundation depth is governed by ground conditions and frost, and for anything structural it is set by building regulations rather than by rules of thumb.

The sub-base matters as much as the slab. Concrete is strong in compression and weak in tension, so it cracks when the ground beneath it moves. A well-compacted hardcore layer is not optional, and skipping it is the usual reason a slab fails.

Bags or ready-mix

A cubic metre of concrete needs roughly 108 bags of 20 kg pre-mixed product. That is not a typo, and it is the number that changes people's plans.

Rough guidance:

• Under 0.3 m³: bags are sensible. Postholes, a small pad, a step.
• 0.3 to 1 m³: possible with a mixer and a long day, and genuinely hard work. Budget for help.
• Over 1 m³: order ready-mix. It is usually cheaper per cubic metre than bags, the strength is consistent and certified, and it arrives in one pour.

The last point is the one people underestimate. A large slab poured in batches over several hours forms cold joints where one batch has started to set before the next arrives, and those are structural weaknesses. A single continuous pour avoids them entirely.

Weight and access

Concrete weighs about 2,400 kg per cubic metre. A modest 1.2 m³ slab is nearly three tonnes.

A builder's barrow holds around 0.06 m³ and, loaded with concrete, weighs about 150 kg. A cubic metre is therefore roughly 17 barrow loads, and each one is heavy enough that the route matters: soft lawn, steps, gates and tight corners all turn a delivery into a much longer day.

If a mixer truck cannot reach the pour, ask about a concrete pump. It costs more and it is frequently cheaper than a day of barrowing plus the help you would need.

Mixing your own

The conventional general-purpose mix is 1 part cement, 2 parts sharp sand, 4 parts aggregate, by volume. For foundations a wetter 1:2.5:5 is common; for a stronger slab, 1:1.5:3.

Water is where most home mixes go wrong. More water makes concrete much easier to place and substantially weaker, and the relationship is steep. Aim for a mix that holds its shape when you form a ridge in it rather than one that flows off the shovel.

Do not pour in a hard frost or in strong sun. Below about 5°C the cure stalls and the concrete may never reach strength; in heat it dries too fast and cracks. Covering a fresh slab with plastic sheeting for a few days holds moisture in, which is what curing actually needs, and it is free.

Postholes

A rule of thumb for fence posts is to bury a quarter to a third of the total post length, in a hole roughly three times the post width.

So a 2.4 m post above ground needs about 600 mm below it, in a 300 mm hole. Put 100 mm of gravel in the bottom for drainage before the concrete, because a post sitting in a concrete cup that holds water rots at exactly the point where it is weakest.

Slope the top of the concrete away from the post so rain runs off rather than pooling against the timber. It takes a moment and it adds years.