How Much Concrete Do I Need? And Why It Is Heavier Than You Think

The calculation is trivial. The numbers it produces are the part that surprises people.

volume = length × width × depth, all in metres

A slab 4 m by 3 m at 100 mm deep is 4 × 3 × 0.1 = 1.2 cubic metres.

That is about 130 bags of 20 kg pre-mixed concrete, and it weighs just under three tonnes.

The unit mistake that ruins the order

Work entirely in metres and the answer comes out in cubic metres, which is what everything is sold and quoted in.

The overwhelmingly common error is mixing millimetres and metres. A depth of 100 mm is 0.1 m. Entering 100 gives an answer a thousand times too large, and it is easy to do when the length and width are naturally in metres and the depth is naturally in millimetres.

If your answer looks absurd, this is why.

How thick does it need to be

• Footpath or 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, on 150 mm of hardcore
• Heavy vehicles or a garage floor: 150 mm minimum, reinforced, and worth taking advice on
• Strip footing for a garden wall: typically 450 mm wide and 200 mm deep

Foundation depth for anything structural is governed by ground conditions, frost depth and building regulations rather than by rules of thumb. If it holds a building up, that is a conversation with building control, not a blog post.

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 within a couple of winters.

Bags or ready-mix

This is where the arithmetic changes plans.

A cubic metre needs roughly 108 bags of 20 kg. That is not a typo.

• Under 0.3 m³: bags are sensible. Postholes, a small pad, a step, a shed base for a small shed.
• 0.3 to 1 m³: possible with a mixer and a long day, and genuinely hard work. Budget for help, and get a ready-mix quote anyway because minimum charges often make it competitive from around half a cubic metre.
• Over 1 m³: order ready-mix.

The last point is not only about effort. A large slab poured in batches over several hours develops cold joints, where one batch has begun to set before the next arrives. Those are structural weaknesses running through the slab, and no amount of care while mixing prevents them. A single continuous pour avoids the problem entirely.

Weight and access

Concrete weighs about 2,400 kg per cubic metre.

A builder’s barrow holds around 0.06 m³ and, loaded with concrete, weighs roughly 150 kg. So a cubic metre is about 17 barrow loads, each one heavy enough that the route genuinely matters: soft lawn, steps, gates and tight corners turn a two-hour job into a day.

Work this out before the truck arrives. Ready-mix drivers charge waiting time, and the concrete starts going off whether or not you are ready for it.

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

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.

Note that dry materials compact when mixed, so you need roughly 1.5 times the finished volume in dry ingredients. A cubic metre of concrete needs about 1.55 m³ of dry material.

Water is where home mixes usually go wrong. More water makes concrete far easier to place and substantially weaker, and the relationship is steeper than people expect. Aim for a mix that holds a ridge when you draw a shovel through it, not one that flows.

Do not pour in a hard frost or strong sun. Below about 5°C the cure stalls and the concrete may never reach strength. In heat it dries too fast and cracks.

Cover it. Concrete cures by chemical reaction with water, not by drying out. Covering a fresh slab with plastic sheeting for a few days holds moisture in, which is exactly what curing needs. It is free and it makes a real difference to the final strength.

Postholes

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

So a post standing 2.4 m above ground wants about 600 mm below it, in a 300 mm hole. Ten such holes with 100 mm posts come to about 0.36 m³ of concrete once you deduct what the posts themselves displace.

Two details that add years:

Put 100 mm of gravel in the bottom before the concrete. A post sitting in a sealed concrete cup holds water at exactly the point where the timber is most vulnerable.

Slope the top of the concrete away from the post so rain runs off rather than pooling against it.

Add a waste allowance

Between 5% and 10%. Ground is never as flat as it looks, trenches over-dig, some spills, and a small shortfall on a large pour is a much worse outcome than a small surplus.

If you are ordering ready-mix, round up to the next quarter cubic metre. Running out mid-pour is the one failure mode with no good recovery.