Concrete Slab Calculator

About the Concrete Slab Calculator

A concrete slab — a patio, a shed base, a driveway, a garage floor — is the most common concrete job there is, and the calculation behind it is just a box: length times width times thickness.

What makes a slab calculator worth using is everything around that multiplication. The thickness you choose sets most of the cost, so it is worth knowing what the job actually needs. Uneven ground takes more concrete than a perfect box would. And the order you place has to be rounded up to what a supplier will deliver.

This concrete slab calculator covers all of it. It gives the volume with waste, the ready-mix order and bag count, an optional cost, and shows what one more inch or 25 millimetres of thickness would add.

How to Use the Concrete Slab Calculator

Choose feet and inches or metres and millimetres.

Enter the slab's length and width (feet or metres) and its thickness (inches or millimetres).

Adjust the waste allowance if you want — 10% is the default.

Optionally, enter the price per cubic yard or cubic metre from your ready-mix supplier to see the cost of the concrete.

How the Volume Is Worked Out

  area      =  length × width
  volume    =  area × thickness
  to order  =  volume × (1 + waste %), rounded up to the next 0.25

Step-by-Step Example

A 5 by 4 metre patio, 100 mm thick, at 120 per m³.

  Area:        5 × 4               =  20 m²
  Volume:      20 × 0.100          =  2.00 m³
  With waste:  2.00 × 1.10         =  2.20 m³
  Order:       2.25 m³ of ready-mix
  Cost:        2.25 × 120          =  270, concrete only

In imperial, a 20 by 12 foot slab 4 inches thick is 80 cubic feet, 2.96 cubic yards, and 3.26 yards with waste — an order of 3.5 yards.

Choosing the Thickness

Thinner than about 4 inches, slabs crack easily and are rarely worth pouring. Thicker slabs, reinforcement and a better sub-base are needed on soft ground and for heavy loads, and building codes or a structural engineer may set the thickness for structural floors.

What an Extra Inch Costs

Volume is directly proportional to thickness, so the arithmetic is simple and the effect is large:

  4 in → 5 in   =  +25% concrete
  5 in → 6 in   =  +20% concrete
  100 mm → 125 mm  =  +25%

That is why the calculator shows what one more inch or 25 millimetres would add. It is worth choosing thickness for the job the slab will do rather than adding a margin "just in case", which the waste allowance already covers.

Why the Waste Allowance Matters for Slabs

The ground under a slab is never perfectly level. If it dips by just half an inch on average under a 4-inch slab, that is 12.5% more concrete than the box calculation says — before any spillage, or any movement of the formwork as the concrete is poured.

A 10% allowance is standard for slabs. Running short is far worse than having a little left over: the extra batch arrives after the first has started to set, leaving a weak cold joint across the slab.

Beyond the Concrete

The concrete is only part of a slab. Most also need:

A sub-base of compacted hardcore or gravel, usually 4 to 6 inches deep, for support and drainage.

Formwork to hold the edges.

A damp-proof membrane under floors inside buildings.

Reinforcement — mesh or rebar — for driveways, larger slabs and weaker ground.

Control joints cut or formed every few metres to control where cracking happens.

These are not included in the concrete volume, and together they can cost as much as the concrete itself.

Planning the Pour

A slab is poured in one continuous operation, so the order has to be right and the site has to be ready.

Check access for the truck. A ready-mix truck is heavy and needs a firm route close to the pour. If it cannot get near, a concrete pump or barrows and extra labour will be needed, and that changes the cost.

Have enough people. Concrete starts to stiffen within an hour or two, faster on a hot day. A slab of a few cubic metres needs several people to spread, compact and level it before it sets.

Watch the weather. Very hot, cold or wet conditions all affect how concrete cures. Frost can damage fresh concrete, and heavy rain can spoil the surface.

Keep it damp while it cures. Covering the slab or keeping it moist for the first week helps it reach its full strength and reduces surface cracking.

Understanding Your Result

Concrete needed is the total including waste.

Volume gives the figure in cubic yards and feet, or cubic metres.

Area is the slab's surface area.

Ready-mix order rounds up to the supplier's quarter.

Premix bags gives the bag alternative.

Cost multiplies the order by your price, if entered.

Worth knowing shows the effect of more thickness, or warns when a slab is too thin.

When Should You Use This Calculator?

Planning a patio, path or shed base.

Pricing a driveway, where thickness matters most.

Comparing thicknesses before deciding.

Before calling a supplier, to know the order to place.

Common Mistakes

Treating 4 inches as 0.4 feet. It is a third of a foot.

Skipping the waste allowance. Uneven ground alone can use 10%.

Pouring too thin. Under 4 inches cracks easily.

Adding thickness "just in case". Each inch adds 20–25% to the cost.

Forgetting the sub-base. It is not concrete, but the slab depends on it.