Concrete Slab Calculator

About Concrete Slab Calculator

Concrete Slab Calculator works out how much concrete a rectangular slab needs from three inputs: slab length, slab width and slab thickness. A 3 m x 3 m slab poured at 100 mm thick contains 0.9 m3 of concrete, which becomes 0.99 m3 once the default 10 percent waste allowance is added, or roughly 83 bags of 25 kg mix. The same slab in imperial terms, 10 ft x 10 ft at 4 inches, works out to about 1.36 cubic yards with waste included, which is 62 bags of 80 lb mix or 82 bags of 60 lb mix. Thickness is the input that moves the result most, because volume scales linearly with it: taking a slab from 100 mm to 150 mm adds 50 percent to the order. Typical residential slab thicknesses are 100 mm (4 inches) for patios, paths and shed bases, 125 to 150 mm (5 to 6 inches) for anything carrying a vehicle, and 200 mm (8 inches) or more for structural slabs where an engineer specifies the depth and reinforcement. Results appear in metric and imperial at the same time, so you can compare a ready mix quote priced per cubic metre against bag prices quoted per 80 lb bag without converting anything by hand.

How to use Concrete Slab Calculator

  1. Choose metric or imperial input
  2. Enter length, width, and depth of your slab
  3. See volume and bag count in both metric and imperial instantly

Frequently Asked Questions

How do I calculate concrete for a slab?
Multiply slab length by slab width by slab thickness, keeping every measurement in the same unit. A 4 m x 3 m slab at 100 mm thick is 4 x 3 x 0.1 = 1.2 m3. The calculator handles the unit conversion for you, so you can enter length and width in metres with thickness in centimetres, or length and width in feet with thickness in inches.
How thick should a concrete slab be?
A patio, path or shed base is normally 100 mm (4 inches). A driveway or any slab carrying a car should be at least 125 to 150 mm (5 to 6 inches). Structural slabs and slabs on soft ground usually start at 200 mm (8 inches) and need reinforcement specified by an engineer.
How many bags of concrete for a 10x10 slab?
A 10 ft x 10 ft slab at 4 inches thick holds 1.23 cubic yards, or 1.36 cubic yards with a 10 percent waste allowance. That is about 62 bags of 80 lb mix or 82 bags of 60 lb mix. At that volume, ready mix is usually cheaper and far less work than mixing bags.
Does the calculator include reinforcement or a sub base?
No. It calculates the concrete volume only. Mesh, rebar, the compacted hardcore sub base and any sand blinding are separate materials, and the sub base sits below the slab so it does not reduce the concrete volume.
How much waste should I allow on a slab?
Ten percent is the default and covers spillage, slightly over dug ground and forms that are not perfectly level. On a well formed slab over a flat sub base you can drop to 5 percent. On uneven or freshly excavated ground, 15 percent is safer than running short mid pour.

Related Tools

Also Available As