By Tim King · Updated 19 September 2026

Concrete Slab Calculator

Cubic Metres, Cubic Yards and Bags from Length, Width and Thickness

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Units

Length (metres)

Width (metres)

Thickness (e.g. 100mm)

Waste allowance (%)

Result

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How the Concrete Slab Calculator works

This concrete slab calculator works out how much to order. Type the length, width and thickness of the slab and the readout gives its area, the concrete volume, the amount to order once waste is added, how much that concrete weighs and how many bags it is. In imperial mode the dimensions accept feet and inches written as 12'6", and the volume comes back in cubic yards, which is how ready-mix is sold in North America. The table gives the bag count for each common bag size.

Concrete slab calculator: order once, pour once

Run short mid-pour and you get a cold joint, a weak line where fresh concrete meets concrete that has begun to set. Order too much and you pay for a part-load you must dispose of. The volume is just length × width × thickness, but mixed units, forgotten waste and an uneven sub-base are where DIY pours go wrong.

The formula

Volume = length × width × thickness, with all three in metres (giving m³) or feet (giving ft³, which divide by 27 for cubic yards).

Order = volume × (1 + waste % ÷ 100). Weight is taken at 2,400 kg per m³, or 150 lb per ft³. Bags are the order divided by the yield printed on the sack: about 9 litres for a 20kg bag of premix, and 0.45 ft³ for a 60lb bag.

The thickness field accepts 100mm, 10cm or 4in, so there is no unit conversion by hand.

Worked examples

The first example is the default 4 × 3 m base at 100 mm. That is 12 m² × 0.1 m = 1.200 m³. With 10% waste the order is 1.32 m³, which weighs about 3.17 tonnes and comes to 147 bags of 20kg.

The second example is a US shed pad, 12'0" × 10'0" at 4 inches: 120 ft² × 0.333 ft = 40 ft³, or 1.48 yd³. Add 10% and the order is 1.63 yd³, which is 98 bags of 60lb or 74 of 80lb and weighs about 3.3 US tons.

How many bags for common slab sizes?

Shed, patio and garage bases at the two usual thicknesses: Concrete for common metric slab sizes, with 10% waste added to the order

Slab	Thickness	Volume	Order	20kg bags
2 × 2 m	100 mm	0.400 m³	0.44 m³	49
2 × 2 m	150 mm	0.600 m³	0.66 m³	74
2.4 × 1.8 m	100 mm	0.432 m³	0.48 m³	53
2.4 × 1.8 m	150 mm	0.648 m³	0.71 m³	80
3 × 2 m	100 mm	0.600 m³	0.66 m³	74
3 × 2 m	150 mm	0.900 m³	0.99 m³	111
3 × 3 m	100 mm	0.900 m³	0.99 m³	111
3 × 3 m	150 mm	1.350 m³	1.49 m³	166
4 × 3 m	100 mm	1.200 m³	1.32 m³	147
4 × 3 m	150 mm	1.800 m³	1.98 m³	221
3.6 × 3.6 m	100 mm	1.296 m³	1.43 m³	159
3.6 × 3.6 m	150 mm	1.944 m³	2.14 m³	238
5 × 3 m	100 mm	1.500 m³	1.65 m³	184
5 × 3 m	150 mm	2.250 m³	2.48 m³	276
5 × 4 m	100 mm	2.000 m³	2.20 m³	245
5 × 4 m	150 mm	3.000 m³	3.30 m³	367
6 × 3 m	100 mm	1.800 m³	1.98 m³	221
6 × 3 m	150 mm	2.700 m³	2.97 m³	331
6 × 4 m	100 mm	2.400 m³	2.64 m³	294
6 × 4 m	150 mm	3.600 m³	3.96 m³	441
6 × 6 m	100 mm	3.600 m³	3.96 m³	441
6 × 6 m	150 mm	5.400 m³	5.94 m³	661

How to read the result

Slab area prices mesh, membrane and sub-base.
Concrete volume is the exact figure before waste.
Order with waste is what to ask a supplier for. Ready-mix is sold in steps of 0.25 or 0.5 m³, or quarter or half yards, so round up to the next step.
Bags gives the 20kg or 60lb count. The table under the result shows the other bag sizes.
Approximate weight matters for a trailer or a van's payload.

What thickness to use

75–100 mm (3–4 in) for garden shed bases, paths and light patios on a compacted sub-base.
150 mm (6 in) for driveways, garages and anything a car or van will stand on, usually with steel mesh.
Under the slab, put about 100 mm of compacted hardcore or crushed stone. The gravel calculator sizes that layer.

Bags or ready-mix?

Hand mixing suits up to about half a cubic metre. Above about 1 m³, ready-mix or a volumetric mixer is usually cheaper once your time is counted, and gives one continuous pour. Check minimum loads and whether a lorry can reach the site.

Tips and pitfalls

Measure inside the formwork, not the outside of the boards.
Check the depth in several places. A sub-base 10 mm low across 12 m² takes an extra 0.12 m³, which is the whole 10% allowance gone. Use 15% on rough ground.
Don't pour in a frost. Concrete that freezes before it has set loses much of its strength. Keep it covered and damp for the first week.
Imperial inputs. Type 12'6" for twelve feet six inches. A bare number means feet for length and width, so type 4in for the thickness.

For round holes, use the concrete post hole calculator. For footings and steps, use the concrete calculator, and for a timber alternative the decking calculator.

FAQ

How much concrete do I need for a shed base?

Multiply length by width by thickness in metres. A 3 by 2 metre base at 100mm is 0.6 cubic metres, about 0.66 with waste, which is 73 bags of 20kg premix or a small ready-mix delivery. Enter the base size and the calculator gives the figure and bag counts.

How thick should a concrete slab be?

100mm (4 inches) for sheds, paths and patios; 150mm (6 inches) for driveways, garages and anything vehicles will stand on. Thicker slabs in poor ground or under heavy loads should be designed by an engineer, usually with mesh reinforcement.

How many bags of concrete make a cubic metre?

About 111 bags of 20kg premix, 89 of 25kg or 56 of 40kg, since a 20kg bag yields roughly 9 litres. In imperial, a cubic yard is about 60 bags of 60lb or 45 of 80lb. The table shows the count for your slab for each size.

How do I convert cubic metres to cubic yards?

One cubic metre is 1.308 cubic yards, and one cubic yard is 0.765 cubic metres. The imperial mode reports both cubic feet and cubic yards, so a slab measured in feet and inches can be ordered from a supplier working in either unit.

How much does a concrete slab weigh?

About 2,400 kg per cubic metre, or 150 pounds per cubic foot. A 4 by 3 metre slab at 100mm weighs close to 2.9 tonnes before waste. The readout gives the weight for the ordered volume so you can judge access and ground bearing.