How much concrete do I need for a 3m × 3m slab?

A 3m × 3m slab covers 9 square metres. At 100 mm thick that is 0.90 cubic metres of concrete, or 0.99 once you add the usual ten percent for waste. At 150 mm — the thickness you want if a vehicle will use it — it rises to 1.35 cubic metres, 1.48 with waste. Thickness scales the volume linearly, so half again the depth is half again the concrete.

How many bags of concrete for a 3m × 3m slab?

About 90 bags of 25 kg at 100 mm thick, on the basis that a 25 kg bag yields roughly 0.01 cubic metres of finished concrete. That figure alone answers the bags-or-truck question: that is a long but feasible day of hand-mixing, and worth pricing against a delivery before deciding.

How many ready-mix loads for a 3m × 3m slab?

1 load at 100 mm thick, assuming an eight cubic metre truck. One load covers it, which is the ideal case — a slab poured continuously from a single delivery is monolithic, with no joint anywhere through it.

What is a 3m × 3m slab usually for?

Garden shed or bin store base. That matters for the specification rather than the quantity: the area gives you the concrete, but what sits on the slab decides the thickness, whether it needs reinforcement, and how good the sub-base has to be. Change the thickness on the calculator above to see the volume move — it is the input that costs the most and gets the least thought.

How thick should a concrete slab be?

A hundred millimetres for a pedestrian floor such as a shed base or garden room, a hundred and fifty where a vehicle will use it, and two hundred or more for lorries, heavy machinery or turning areas. Thickness resists bending, and a slab bends when the ground beneath it is not uniformly supportive — which is why a well-compacted sub-base under a slightly thin slab beats a thick slab on soft ground.

How far apart should control joints be?

No more than about twenty-four times the slab thickness, and with panels kept near square. On a 100 mm slab that gives 2.4 metres; at 150 mm it gives 3.6. The shape rule matters as much as the spacing: a panel much longer than one and a half times its width behaves like a beam and cracks across its middle, so a long slab should be jointed on a grid rather than in strips.

Do I need a damp-proof membrane?

Under anything with a floor people occupy, yes. A concrete slab is not waterproof — moisture wicks up through it by capillary action and appears as damp flooring, lifting adhesive, or condensation on whatever is stored on it. A polythene membrane under the slab stops that. For an outdoor slab such as a patio or yard it is unnecessary, since the surface is exposed anyway and trapped moisture has nowhere useful to go.

How much sub-base does a slab need?

A hundred millimetres of compacted granular material under a domestic slab, a hundred and fifty under anything carrying vehicles, and more on soft or made-up ground. Order it by the loose volume rather than the finished depth, because crushed stone settles by roughly a quarter as it compacts. Compact it in layers of no more than a hundred millimetres — a single deep dump never consolidates properly and the slab settles into the soft spots later.

Does a slab need reinforcement?

It depends on what the slab carries and what it sits on. A small shed base on firm ground genuinely does not need it. Anything carrying vehicles does, and so does any slab on ground that may move — recent excavation, made-up ground, near trees. Reinforcement does not prevent cracking; it holds the pieces together across a crack so it stays tight rather than opening into a trip hazard or a water path.

Concrete for a 3m × 3m Slab

A three by three metre slab is the smallest pour most people ever take on themselves, and it is the size that makes hiring a truck look absurd — nine square metres at a hundred millimetres is under a cubic metre of concrete. That puts it firmly in bag territory, and it is one of the few slabs where mixing by hand genui...

SLAB DIMENSIONS

LENGTH (metres)

WIDTH (metres)

THICKNESS (millimetres)

CONCRETE PRICE ($/m³, optional)

Enter your local ready-mix price for an estimate.

RESULTS

VOLUME (exact)

0.90 m³

WITH 10% WASTE — order this amount

0.99 m³

25 KG BAGS

≈ 0.01 m³ per 25 kg bag

ESTIMATED MATERIAL COST

$149

at $150/m³ + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area9.00 m²

Thickness100 mm

Volume exact0.9000 m³

Volume +10%0.9900 m³

25 kg bags90 bags

THICKNESS GUIDE

75mm

Paths, light paving

Foot traffic

100mm

Patios, residential slabs

Standard residential

150mm

Driveways, garage floors

Vehicles

200mm

Commercial floors

Heavy loads

UNIT CONVERSIONS

1 cubic yard = 27 cubic feet

1 cubic yard ≈ 0.765 m³

80 lb bag ≈ 0.6 cubic feet

60 lb bag ≈ 0.45 cubic feet

25 kg bag ≈ 0.01 m³

Always order 10% extra for waste

Created with❤️byeaglecalculator.com

HOW TO USE

1
The calculator opens with 3 by 3 metres already entered. Adjust the thickness to match what the slab will carry — 100 mm for a pedestrian floor, 150 mm where a vehicle will use it.
2
Read the volume with and without the ten percent waste allowance. Order the figure with waste included; the exact figure is there so you can see how much of the order is margin.
3
Compare the bag count against the ready-mix load count. At 0.90 cubic metres this is small enough that bags are a genuine option.
4
Switch to imperial with the unit toggle if you are working in feet and inches, or change the dimensions if your slab is not exactly 3m × 3m.

WORKED EXAMPLE

A 3m × 3m slab at 100 mm thick. The area is 3 × 3 = 9 m². Multiplying by the 0.1 m thickness gives 0.90 m³ of concrete. Adding ten percent for waste — for an uneven sub-base, spillage, and the last of the load left in the chute — brings the order quantity to 0.99 m³. That is about 2.4 tonnes. In 25 kg bags, at roughly 0.01 m³ of finished concrete per bag, that is 90 bags. From a truck holding 8 m³ it is 1 load. At 150 mm instead, the same area needs 1.35 m³, or 1.48 m³ with waste — half again the concrete for half again the depth.

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Last updated: July 29, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.