Ordering too little concrete means stopping work mid-pour, which can create a cold joint — a weak line in the finished slab. Ordering too much wastes money and leaves you with a problem of disposal. Getting the calculation right before you order is essential, and it is simpler than most people think.
Use our free Concrete Slab Calculator to get your volume instantly, or follow the steps below.
The Formula
Concrete volume is calculated as:
Volume (m³) = Length (m) × Width (m) × Depth (m)
Note that depth is usually given in millimetres on drawings but must be converted to metres for this calculation. A 150mm slab depth = 0.15m.
What Slab Depth Do I Need?
The right slab depth depends on the intended use:
| Application | Typical Depth |
|---|---|
| Path / footway (foot traffic only) | 75–100 mm |
| Domestic driveway (cars) | 100 mm |
| Garage floor or shed base | 100 mm |
| Garden room or extension base | 150 mm |
| Driveway (HGV / heavy vehicles) | 150–200 mm |
For any slab carrying structural loads — a house extension, a garage with a heavy vehicle, or anywhere with poor ground conditions — consult a structural engineer before proceeding.
Worked Example
You want to pour a concrete base for a garden room. The base is 5m × 4m and needs to be 150mm (0.15m) deep.
Volume = 5 × 4 × 0.15 = 3.0 m³
Add 10% for wastage, spillage and uneven ground: 3.0 × 1.10 = 3.3 m³
You would order 3.5 m³ from a ready-mix supplier (they typically supply in 0.5 m³ increments).
Ready-Mix vs Site-Mixed Concrete
For volumes over about 0.5 m³, ready-mix concrete delivered by truck is almost always more economical and produces a more consistent result than site-mixing by hand or with a small mixer. For smaller volumes, bagged concrete mixes are convenient but significantly more expensive per cubic metre.
A standard C25 mix (25 N/mm² at 28 days) is suitable for most domestic slabs. Specify C30 for driveways subject to heavy use or salt exposure from de-icing products.
How Many Bags of Concrete Do I Need?
If you are using bagged concrete mix (such as Postcrete or Rapid Set), a standard 20kg or 25kg bag typically yields around 0.01–0.012 m³ of mixed concrete. For the 3.3 m³ example above, you would need roughly 275–330 bags of 25kg mix — making ready-mix the far more practical choice for anything beyond a small post hole.
Do the Calculation in Seconds
Our Concrete Slab Calculator handles the volume calculation in both metric and imperial, adds your chosen wastage allowance, and shows how many 25kg bags you would need as an alternative to ready-mix — all in one click.
