How much concrete do I need for a 20m × 10m slab?

A 20m × 10m slab covers 200 square metres. At 100 mm thick that is 20.00 cubic metres of concrete, or 22.00 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 30.00 cubic metres, 33.00 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 × 10m slab?

About 2,000 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 × 10m slab?

3 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 × 10m slab usually for?

Warehouse bay or long access slab. 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.

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.

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.

Concrete for a 20m × 10m Slab

Two hundred square metres in a two-to-one rectangle is a warehouse bay, an access road slab, a long yard, or a machinery apron. Twenty metres is long enough that thermal movement becomes a real design consideration rather than a footnote — a twenty metre run of concrete moves several millimetres between summer and wint...

SLAB DIMENSIONS

LENGTH (metres)

WIDTH (metres)

THICKNESS (millimetres)

CONCRETE PRICE ($/m³, optional)

Enter your local ready-mix price for an estimate.

RESULTS

VOLUME (exact)

20.00 m³

WITH 10% WASTE — order this amount

22.00 m³

25 KG BAGS

2000

≈ 0.01 m³ per 25 kg bag

ESTIMATED MATERIAL COST

$3,300

at $150/m³ + 10% waste

SLAB VISUALISER (not to scale)

SLAB SUMMARY

Surface area200.00 m²

Thickness100 mm

Volume exact20.0000 m³

Volume +10%22.0000 m³

25 kg bags2000 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 10 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 20.00 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 × 10m.

WORKED EXAMPLE

A 20m × 10m slab at 100 mm thick. The area is 20 × 10 = 200 m². Multiplying by the 0.1 m thickness gives 20.00 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 22.00 m³. That is about 52.8 tonnes. In 25 kg bags, at roughly 0.01 m³ of finished concrete per bag, that is 2,000 bags. From a truck holding 8 m³ it is 3 loads. At 150 mm instead, the same area needs 30.00 m³, or 33.00 m³ with waste — half again the concrete for half again the depth.

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Last updated: July 28, 2026 · Formula verified by EagleCalculator team · Eagle-eyed accuracy for every calculation.