Concrete Calculator

Work out the volume of concrete a slab needs, in cubic metres or cubic yards, before you order from the plant.

Formula and method

Volume is calculated as area multiplied by thickness. The waste percentage is added afterwards to cover spillage, uneven sub-base and over-dig, and the result is rounded to two or three decimals.

How to measure the inputs

Measure the slab length and width at the widest points.
Measure the thickness in several places and use the largest value.
Convert everything to the same unit before entering the figures.

Worked example

Slab length 13.12 ft; Slab width 9.84 ft; Thickness 5.91 in; Waste 6 % → Concrete volume: 2.50 yd³

Practical tips

Have everything ready before the truck arrives: concrete waits for nobody.
Order from one plant and one batch for a consistent mix.
Do not add extra water on site, it reduces strength.

Assumptions

The slab is a rectangular prism with a uniform thickness.
The sub-base is already prepared and level to the required tolerance.
Concrete grade, reinforcement, curing and joints are not part of this estimate.

Frequently asked questions

How much extra concrete should I order?

Order 5–10 % more than the calculated volume. Running out mid-pour creates cold joints and can be far more expensive than a small surplus.

What thickness should a concrete slab be?

That is a structural decision. Garden paths are often 75–100 mm, driveways 100–150 mm and structural slabs more; follow your design.

Does this include reinforcement?

No. Mesh, fibres and bar reinforcement are separate. Use the rebar calculator for bar quantities and weights.

Can I use it for footings and columns?

Yes, if you enter the plan area and the depth or height. For a strip footing, enter its length as the slab length and its width as the slab width.

Related calculators

Estimate only

Results are material quantity estimates only. They are not structural design, and they are not a substitute for local building codes, engineer approval or manufacturer instructions. Structural sizing — bar diameters, spacing, foundation depth and load-bearing capacity — must be confirmed by a qualified engineer for your site conditions.

Browse by category

Concrete