How much concrete do I need for a 4m × 4m slab?

A 4m × 4m slab covers 16 square metres. At 100 mm thick that is 1.60 cubic metres of concrete, or 1.76 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 2.40 cubic metres, 2.64 with waste. Thickness scales the volume linearly, so half again the depth is half again the concrete.

How many bags of concrete for a 4m × 4m slab?

About 160 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 4m × 4m 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 4m × 4m slab usually for?

Workshop, summerhouse or garden room. 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.

Under about half a cubic metre, bags are simpler and there is no minimum charge. Above roughly a cubic metre, ready-mix wins on cost as well as effort once you count the delivery and short-load charges honestly. There is also a quality argument that matters more than the money: a slab poured continuously from one load is monolithic, while one built up from batches mixed over several hours has cold joints running through it that you cannot see and cannot repair.

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.

Concrete for a 4m × 4m Slab

Sixteen square metres is the size at which a garden building stops being a shed and starts being a room. A four by four metre slab typically carries a summerhouse, a garden office, a workshop, or a substantial storage building — things with a floor people stand on for hours, insulation, and often power run out to them....

SLAB DIMENSIONS

LENGTH (metres)

WIDTH (metres)

THICKNESS (millimetres)

CONCRETE PRICE ($/m³, optional)

Enter your local ready-mix price for an estimate.

RESULTS

VOLUME (exact)

1.60 m³

WITH 10% WASTE — order this amount

1.76 m³

25 KG BAGS

160

≈ 0.01 m³ per 25 kg bag

ESTIMATED MATERIAL COST

$265

at $150/m³ + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area16.00 m²

Thickness100 mm

Volume exact1.6000 m³

Volume +10%1.7600 m³

25 kg bags160 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 4 by 4 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 1.60 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 4m × 4m.

WORKED EXAMPLE

A 4m × 4m slab at 100 mm thick. The area is 4 × 4 = 16 m². Multiplying by the 0.1 m thickness gives 1.60 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 1.76 m³. That is about 4.2 tonnes. In 25 kg bags, at roughly 0.01 m³ of finished concrete per bag, that is 160 bags. From a truck holding 8 m³ it is 1 load. At 150 mm instead, the same area needs 2.40 m³, or 2.64 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.