← 🏠 Homeowners and DIYers toolkit
Concrete Slab Calculator
How much concrete to order for a slab, path, base or footing.
Part of the 🏠 Homeowners and DIYers toolkit, with more tools made for the job.
Use this instead of length and width.
Leave empty for 10%. Guides often say 5 to 10%.
The volume of mixed concrete one bag makes, from the bag.
Type the size and thickness, and the volume you need appears here.
Was this tool useful?
One click, no sign-up.
Enter the length, width and thickness of the slab. This shows the volume of concrete in cubic metres and cubic yards, with an allowance for spillage and uneven ground.
Concrete is ordered by volume, and the maths is just length times width times thickness. The part that catches beginners out is units, because thickness is usually in centimetres or inches while length and width are in metres or feet. Suppliers deliver in whole units, so the tool rounds up, and it adds an allowance because the ground is never perfectly flat and some is always spilled.
How it’s worked out
- Area is length times width, or the area you type directly.
- Volume is the area times the thickness, with the thickness converted to the same unit as the rest.
- An extra allowance is added. Guides often suggest 5 to 10 percent, and the tool uses 10 unless you change it.
- The volume is shown in cubic metres, cubic yards and cubic feet. A cubic yard is 27 cubic feet, and a whole-yard figure is shown because suppliers deliver whole units.
A 4 m by 3 m slab 10 cm thick is 1.2 cubic metres. With 10 percent extra it is 1.32 cubic metres, which is about 1.7 cubic yards, so a supplier would ask for 2.
Common mistakes
- Mixing units. Thickness in centimetres or inches has to be converted before you multiply.
- Assuming the ground is flat. A dip of a couple of centimetres across a big area adds up quickly.
- Forgetting the base. A slab usually sits on a compacted sub-base, which is a separate material.
- Ordering by guess. Running short mid-pour leaves a cold joint, so order slightly more.
Questions
How thick should a slab be?
It depends on the use and the ground. A path, a shed base and a driveway need different thicknesses, so follow the maker's or a professional's advice for yours.
Should I use ready-mix or bags?
For small jobs, bags are practical. For larger areas, ready-mix delivered by truck is common. The tool gives the volume, and you can type the size of one bag to see how many you need.
Does it include reinforcement?
No. Mesh, bars and edge formwork are separate. Ask a supplier or builder what your slab needs.
Is this for a house foundation?
No. Foundations and anything structural need design by a qualified person and approval under your local building rules. This tool is for the volume of simple slabs, paths and bases.
Estimates only
These are estimates and general information, not building, safety, legal or professional advice. Materials, coverage, waste and rules vary by product, project and place, so check the maker's instructions and your local rules, and use a qualified person for gas, electrical and structural work.
Rain stopped play. Officially research.