Use our free concrete slab calculator to instantly work out how much concrete you need for any slab, pad, footing or driveway. Enter your dimensions below and get results in cubic metres or cubic yards — plus ready-mix bag counts.
Concrete Slab Calculator
Bag counts include 10% wastage allowance. 1 m³ ≈ 2,400 kg of concrete.
How to Use the Concrete Slab Calculator
- Choose your unit system — select Metric (metres/mm) or Imperial (feet/inches).
- Enter length and width — the overall dimensions of your slab in metres or feet.
- Enter depth/thickness — in millimetres (metric) or inches (imperial). A standard domestic slab is 100 mm (4 in).
- Click Calculate — you’ll instantly see the volume of concrete required plus how many ready-mix bags to buy.
Standard Concrete Slab Thickness Guide
Choosing the right thickness is critical for strength and longevity. Use this guide as a starting point — always consult a structural engineer for load-bearing applications.
| Application | Metric Thickness | Imperial Thickness |
|---|---|---|
| Footpath / garden path | 75 mm | 3 in |
| Domestic garage floor | 100 mm | 4 in |
| Driveway (cars) | 100–125 mm | 4–5 in |
| Driveway (heavy vehicles) | 150 mm | 6 in |
| Commercial floor slab | 150–200 mm | 6–8 in |
| Structural / load-bearing | 200 mm+ | 8 in+ |
Frequently Asked Questions
How much concrete do I need for a 4m × 3m slab at 100mm thick?
A 4 m × 3 m × 0.1 m slab requires 1.2 m³ of concrete (plus wastage). Our calculator adds a 10% wastage buffer automatically, so you would order approximately 1.32 m³ or purchase 66 × 20 kg bags.
What is the difference between a concrete slab and a screed?
A concrete slab is a structural element — typically 100 mm+ thick — used for floors, driveways and foundations. A screed is a thinner (25–75 mm) finishing layer poured on top of a slab to create a smooth surface. This calculator is designed for full concrete slabs.
Should I add reinforcement (rebar or mesh)?
Yes, for most residential and commercial slabs. Steel reinforcement mesh (SL72 or SL82 in Australia; A142 or A193 in the UK) is placed at mid-depth of the slab to control cracking. For driveways and structural slabs, rebar at 200 mm centres is common. Always check local building codes.
How accurate is this calculator?
The calculator gives a precise theoretical volume based on your inputs. In practice, allow 5–10% extra for uneven sub-base, spillage and over-pour — our calculator automatically adds 10% when calculating bag quantities. For large pours (1 m³+), ordering ready-mixed concrete by the cubic metre from a batching plant is usually more economical than bagged product.