Concrete Slab Calculator — Volume and Bags Needed
Getting your concrete volume right before you mix avoids the worst DIY outcome — running short mid-pour. Enter your slab dimensions and the bag size you are using to find the exact cubic metres and bag count.
How we calculate
Volume = length × width × depth (m). Bags = ⌈volume with waste ÷ bag volume⌉.
Worked example
A 3 m × 2 m slab at 100 mm depth = 0.6 m³. With 10% waste: 0.66 m³. At 12 L/bag (0.012 m³): ⌈0.66 ÷ 0.012⌉ = 55 bags.
Common mistakes
- Mixing too little at once and ending up with cold joints — pour the whole slab in one go if possible.
- Not accounting for depth variation on uneven ground — measure depth at several points and use the average.
- Forgetting reinforcement mesh reduces the concrete volume slightly but not enough to skip the waste allowance.
- Ordering premix bags and forgetting you also need a plasticiser or fibres for structural slabs.
Frequently asked questions
How many bags of concrete do I need for a patio slab?+
A 3 m × 2 m patio at 100 mm depth (0.6 m³) with 10% waste needs about 55 standard 25 kg bags. Always pour the entire slab in one go to avoid cold joints, so have all bags on site before you start.
How deep should a concrete slab be?+
A garden path or patio needs at least 75–100 mm. A driveway or area with vehicle traffic needs 150 mm. Structural slabs for buildings are typically 150–200 mm or more, depending on the load and soil conditions.
How much concrete does a 25 kg premix bag make?+
A standard 25 kg bag yields approximately 12 litres (0.012 m³) of mixed concrete. Some brands vary — always check the label. For large pours over 1 m³, ready-mix delivered by truck is usually more cost-effective.
Do I need reinforcement in a concrete slab?+
For a garden path or patio, mesh reinforcement is optional but recommended to prevent cracking. For driveways and structural slabs, reinforcement is essential. Mesh or rebar displaces a small amount of concrete but not enough to meaningfully change your bag count.