What next?
Concrete is one part of the job. These cover the surfaces that follow it.
What your result means
The headline figure is the volume of finished concrete, including your waste allowance. Bag counts assume a standard pre-mixed ballast product — a 20 kg bag yields roughly 0.011 m³ once mixed, which is why the bag count climbs so quickly. Weight uses 2,400 kg per cubic metre, the usual density for normal-weight concrete.
Why this one is different
The volume arrives in cubic metres, cubic yards and cubic feet at once, because the slab is measured in one and quoted in another. A weight figure and a 20 kg bag count sit beneath, which is the decision the number actually feeds: mixing on site by the bag, or ordering a ready-mix load by the cube.
Running out mid-pour is far worse than ordering too much
Concrete has to be placed and finished as one continuous operation. A gap while you fetch more creates a cold joint — a visible line that is also a structural weak point. That is why every trade adds a waste allowance rather than ordering the exact calculated figure.
Ten percent is a reasonable default on a flat, well-formed slab. Uneven ground, deep footings or awkward access justify more, because the sub-base absorbs more than you expect.
How it works
Volume is length × width × depth, converted to metres first so the result is in cubic metres. The waste allowance is applied on top, then the total is converted to cubic yards, cubic feet, weight and bag counts using standard yields.
How to use this calculator
- Choose the units you measured in.
- Enter length, width and depth — depth is usually the smallest figure.
- Set how many identical pours if you have several footings the same size.
- Add a waste allowance of around 10%.
- Compare the bag count against a ready-mix order.
Formula
All dimensions converted to metres. Weight assumes 2,400 kg/m³. Yields: 20 kg bag ≈ 0.011 m³, 25 kg bag ≈ 0.0138 m³.
Example calculation
A slab 4 m × 3 m at 100 mm thick with 10% waste:
With 10% waste = 1.32 m³
≈ 1.73 yd³, about 3.2 tonnes
≈ 120 bags of 20 kg — between 1 and 2 m³ it is a genuine toss-up
Frequently asked questions
How thick should a slab be?＋
A garden shed base or footpath is typically 75–100 mm. A driveway carrying cars usually needs 100–150 mm over a compacted sub-base, often with mesh reinforcement. Anything structural should be specified by an engineer.
At what point should I order ready-mix?＋
Most people switch above about one to two cubic metres. Beyond that the bag count becomes impractical, mixing consistency suffers, and the pour takes long enough to risk a cold joint.
Does the calculator include the sub-base?＋
No. Hardcore, sand blinding and any damp-proof membrane are separate. Calculate the sub-base as its own layer using the same length and width with its own depth.
What mix should I use for a domestic slab?＋
A general-purpose C20 to C25 mix suits most paths, sheds and patios. Driveways and anything taking vehicle loads usually call for C30 or above, and the supplier will advise on the specification for your use.
How much does concrete weigh?＋
Roughly 2.4 tonnes per cubic metre. A modest 3 by 3 metre slab at 100 mm thick is nearly a cubic metre, so almost two and a half tonnes of material to place in one session.
Do I need reinforcement mesh?＋
For thin domestic slabs on a well-prepared sub-base, often not. Mesh is normally specified where the slab carries vehicles, spans variable ground, or where crack control matters, and its position within the slab depth is what makes it effective.
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Assumptions & limitations
Every figure here comes from a simplified model. Keep these limits in mind when reading your result:
- Assumes a rectangular pour of uniform depth — irregular shapes should be split into rectangles and added.
- Bag yields are typical figures; check the manufacturer’s stated yield, which varies by product.
- Weight assumes normal-weight concrete at 2,400 kg/m³.
- Excludes reinforcement, formwork, sub-base and any over-dig.