How much concrete do I need for a 20m × 15m slab?

A 20m × 15m slab covers 300 square metres. At 100 mm thick that is 30.00 cubic metres of concrete, or 33.00 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 45.00 cubic metres, 49.50 with waste. Thickness scales the volume linearly, so half again the depth is half again the concrete.

How many bags of concrete for a 20m × 15m slab?

About 3,000 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 20m × 15m slab?

5 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 20m × 15m slab usually for?

Industrial slab or large yard. 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.

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.

How accurate does the excavation need to be?

More accurate than most people expect, because over-excavation is paid for in concrete. Digging fifty millimetres deeper than intended across a hundred square metres adds five cubic metres of concrete — a whole extra load, filling a hole nobody wanted. It is far cheaper to dig slightly shallow and bring the level up with compacted sub-base than to dig deep and fill the difference with the most expensive material on site.

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.

Concrete for a 20m × 15m Slab

Three hundred square metres is the largest size on this list and the one where the calculator is doing the least of the work. At this scale the concrete quantity is straightforward arithmetic; everything difficult is in the ground beneath it, the joint layout across it, and the logistics of getting several hundred tonn...

SLAB DIMENSIONS

LENGTH (metres)

WIDTH (metres)

THICKNESS (millimetres)

CONCRETE PRICE ($/m³, optional)

Enter your local ready-mix price for an estimate.

RESULTS

VOLUME (exact)

30.00 m³

WITH 10% WASTE — order this amount

33.00 m³

25 KG BAGS

3000

≈ 0.01 m³ per 25 kg bag

ESTIMATED MATERIAL COST

$4,950

at $150/m³ + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area300.00 m²

Thickness100 mm

Volume exact30.0000 m³

Volume +10%33.0000 m³

25 kg bags3000 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 20 by 15 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 30.00 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 20m × 15m.

WORKED EXAMPLE

A 20m × 15m slab at 100 mm thick. The area is 20 × 15 = 300 m². Multiplying by the 0.1 m thickness gives 30.00 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 33.00 m³. That is about 79.2 tonnes. In 25 kg bags, at roughly 0.01 m³ of finished concrete per bag, that is 3,000 bags. From a truck holding 8 m³ it is 5 loads. At 150 mm instead, the same area needs 45.00 m³, or 49.50 m³ with waste — half again the concrete for half again the depth.

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Last updated: July 27, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.