Concrete Slab Calculator

Slabs are the most common concrete pour and the easiest to get wrong, usually by a couple of inches of depth nobody checked. Enter your length, width and thickness below for the volume in m³ and cubic yards, the weight, and how many bags it takes.

Concrete slab with formwork — length, width and thickness

Using the calculator

Measure length and width at the formwork, not the string line. They drift.
Set the thickness — the table below covers the usual applications.
Choose a wastage figure. 5% on a compacted, screeded sub-base. 10% if the ground is uneven or you are pouring against soil.
Hit Calculate for volume, weight and bag counts across every size.

Worked example. A shed base 3 m × 2.4 m at 100 mm thick: 3 × 2.4 × 0.1 = 0.72 m³. Add 5% and you are ordering 0.76 m³ — around 66 bags of 25 kg premix, or about 1.9 tonnes of concrete on the ground.

Slab thickness guide

Application	Thickness	Notes
Garden path / patio	75–100 mm (3–4 in)	On 100 mm compacted hardcore
Shed / greenhouse base	100 mm (4 in)	C20 is plenty
Driveway (cars)	100–125 mm (4–5 in)	C25 or C30, mesh worth adding
Driveway (vans, trucks)	150 mm (6 in)	Mesh or structural fibres
Garage / workshop floor	150 mm (6 in)	Thicken the edges
House floor slab	150–200 mm (6–8 in)	To the engineer’s drawing

The formula

Volume = Length × Width × Thickness
27 ft³ = 1 yd³ · 1 m³ = 1.308 yd³ = 35.31 ft³

L-shaped or awkward slabs split into rectangles. Work out each piece separately and add them together — or run the calculator once per section and total the results.

What actually makes a slab last

Depth gets the attention, but three other things decide whether you are looking at hairline cracks or a repair job in five years.

The sub-base. 100 mm of compacted MOT Type 1 or crushed stone, whacked properly. Concrete is strong in compression and weak in tension; if the ground underneath settles, the slab bends and bending is what cracks it.
Control joints. Cut them every 3 to 4 m in both directions, to a quarter of the slab depth. Concrete will crack as it shrinks. Joints decide where.
Curing. Keep it damp for the first week. Concrete does not dry — it reacts with water, and a slab that dries out early never reaches the strength you paid for.

Frequently asked questions

How much concrete for a 10×10 slab?

At 4 inches thick, 1.23 cubic yards or 0.94 m³ — about 56 bags of 80 lb premix, or 81 bags at 25 kg. Add 5% and call it 1.3 yards when you order.

How much concrete for a shed base?

A 2.4 × 1.8 m (8 × 6 ft) base at 100 mm needs roughly 0.43 m³, about 38 bags of 25 kg. A 3 × 2.4 m base comes to 0.72 m³. Use C20 on 100 mm of compacted hardcore.

What mix should I use for a slab?

C20 (1:2:4) for paths, patios and shed bases. C25 to C30 for driveways and garage floors. The mix ratio calculator turns either into cement, sand and gravel quantities.

Do I need mesh in my slab?

A 100 mm slab carrying foot traffic generally does not. Driveways, garages and anything over about 10 m² benefit from A142 mesh or polypropylene fibres. Neither compensates for a poorly compacted sub-base.

Can I pour a slab in sections?

Yes, with proper joints between them. Large slabs need control joints anyway. What you want to avoid is an unplanned cold joint because the second load turned up 40 minutes late.

How long before I can use it?

Foot traffic after 24 to 48 hours. Vehicles after seven days minimum. Full design strength at 28 days. Keep it wet during that first week and it will be stronger for it.