Concrete Slab Calculator
Calculate the volume of concrete needed for a slab and how many 20 kg bags you need.
How to use this tool
- Enter slab length, slab width and slab thickness in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your concrete volume and the full breakdown beneath it.
Calculate how much concrete you need for a slab, path, or pad.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Concrete volume (m³) = length (m) × width (m) × thickness (cm) ÷ 100
20 kg bags = ⌈ volume (m³) ÷ 0.01 ⌉
volume = length × width × (depth / 100)bags_20kg = ceil(volume / 0.01)
How it works
This calculator computes the volume of concrete required for a rectangular slab by multiplying its length, width, and thickness (converted from centimetres to metres). It then converts that volume into the number of standard 20 kg bags needed, using the approximation that one 20 kg bag yields roughly 0.01 m³ of mixed concrete.
The bag count is rounded up to ensure you have enough material. Actual yield varies slightly by brand and water-to-cement ratio; for large pours, ready-mix concrete is more economical and consistent than bagged product.
Worked example
- Slab: 3 m long × 3 m wide × 10 cm thick.
- Convert thickness: 10 cm ÷ 100 = 0.10 m.
- Volume = 3 × 3 × 0.10 = 0.9 m³.
- Bags = ⌈0.9 ÷ 0.01⌉ = ⌈90⌉ = 90 bags.
Concrete volume = 0.9 m³; 20 kg bags needed = 90
Common mistakes to avoid
- Entering thickness in metres instead of centimetres — a 10 cm slab entered as 10 m produces a volume 100x too large.
- Ordering exactly the calculated volume with no margin — concrete suppliers round to the nearest 0.25 m3; over-ordering by 5-10% avoids a costly short pour.
- Assuming one 20 kg bag yields 0.01 m3 for all mix types — high-strength pre-mix bags may yield slightly less; always check the bag specification.
Key terms
- Concrete volume
- The quantity of mixed concrete required, measured in cubic metres (m³).
- 20 kg bag yield
- A standard 20 kg bag of dry-mix concrete produces approximately 0.01 m³ of finished concrete when mixed with water.
- Slab thickness
- The depth of the concrete pour; residential slabs are typically 10–15 cm thick depending on load requirements.
- Ready-mix concrete
- Concrete delivered pre-mixed by truck, measured in cubic metres; more practical and cost-effective than bagged product for volumes above ~1 m³.
Frequently asked questions
- How many bags of concrete do I need for a slab?
- Calculate volume in m³ = length × width × thickness (m). A 20 kg bag covers about 0.01 m³, so divide your volume by 0.01 and round up.