How much concrete do I need for a 12m × 12m slab?

A 12m × 12m slab covers 144 square metres. At 100 mm thick that is 14.40 cubic metres of concrete, or 15.84 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 21.60 cubic metres, 23.76 with waste. Thickness scales the volume linearly, so half again the depth is half again the concrete.

How many bags of concrete for a 12m × 12m slab?

About 1,440 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: mixing that many by hand is well beyond a single day of work, and ready-mix will be cheaper as well as faster.

How many ready-mix loads for a 12m × 12m slab?

2 loads at 100 mm thick, assuming an eight cubic metre truck. More than one load means the joint between them matters: either the next truck is on site before the first finishes, or the pour stops at a planned construction joint with reinforcement carried through. A gap long enough for the first concrete to stiffen leaves a cold joint, which is a permanent weak plane.

What is a 12m × 12m slab usually for?

Agricultural or commercial floor. 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 long before I can use the slab?

You can walk on it after a day or so, but hold off on real load for a week and full load for four. Concrete reaches roughly two thirds of its strength in the first seven days and the rest over the following three weeks, which is why the specification figure is quoted at twenty-eight days. Keeping it damp during the first week matters more than most people realise — concrete cures by chemical reaction with water, not by drying out.

Any slab exposed to weather should, at about one in a hundred — ten millimetres of fall per metre. A dead-level outdoor slab will pond somewhere, because no screed is perfect over any real distance, and standing water freezes, stains and works into the surface. Interior slabs are laid level except where a drain is involved. Always fall away from buildings, never towards them.

How much waste should I allow?

Ten percent is a sensible default and is what the figures here include. It covers an uneven sub-base swallowing more than the nominal depth, spillage, the last of the load stuck in the chute, and formwork that bows slightly under the weight of wet concrete. The asymmetry is what justifies it: surplus concrete is a modest waste, while running short mid-pour leaves a cold joint through the slab, which is permanent.

What finish should the surface have?

It depends where it is. An interior floor is usually power-floated smooth, which is durable and easy to clean. An outdoor slab should be broomed, because a smooth troweled surface is dangerously slick when wet. Draw the broom across the direction people will walk. For a slab that will be tiled or covered later, a lightly tamped finish gives the adhesive something to grip.

Can I pour concrete in cold or wet weather?

Cold is the more serious problem. Below about five degrees the hydration reaction slows sharply, and concrete that freezes before it has set is permanently damaged rather than merely delayed. Suppliers offer accelerators and warm mixes for cold conditions. Rain is less dangerous but still unhelpful — water landing on a fresh surface dilutes the paste and weakens the top few millimetres, so a slab caught in a downpour before it has stiffened will dust and scale later.

Concrete for a 12m × 12m Slab

A hundred and forty-four square metres is a commercial-scale pour. At this size almost nothing about the job is DIY: the excavation is a machine job, the sub-base needs proper compaction in layers, the concrete needs a pump or good access for several trucks, and the finishing needs a power float rather than a hand trow...

SLAB DIMENSIONS

LENGTH (metres)

WIDTH (metres)

THICKNESS (millimetres)

CONCRETE PRICE ($/m³, optional)

Enter your local ready-mix price for an estimate.

RESULTS

VOLUME (exact)

14.40 m³

WITH 10% WASTE — order this amount

15.84 m³

25 KG BAGS

1440

≈ 0.01 m³ per 25 kg bag

ESTIMATED MATERIAL COST

$2,377

at $150/m³ + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area144.00 m²

Thickness100 mm

Volume exact14.4000 m³

Volume +10%15.8400 m³

25 kg bags1440 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 12 by 12 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 14.40 cubic metres this is past the point where bags make sense on cost or effort.
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 12m × 12m.

WORKED EXAMPLE

A 12m × 12m slab at 100 mm thick. The area is 12 × 12 = 144 m². Multiplying by the 0.1 m thickness gives 14.40 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 15.84 m³. That is about 38.0 tonnes. In 25 kg bags, at roughly 0.01 m³ of finished concrete per bag, that is 1,440 bags. From a truck holding 8 m³ it is 2 loads. At 150 mm instead, the same area needs 21.60 m³, or 23.76 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.