Planning a slab, footing or fence post? This free concrete calculator works out the volume in cubic metres and how many 20 kg bags you’ll need. Enter your length, width and slab depth — with 10% wastage included — and get an instant estimate.
Concrete Calculator
How much concrete do you need? Enter your slab below – results in cubic metres & 20kg bags.
Over 1 m³? A premix truck is usually cheaper than bags.
Estimates only — one 20kg bag yields about 0.01 m³ of concrete. Confirm with your supplier before ordering.
How to use the calculator
- Measure the slab in metres and enter the length and width, plus the depth in centimetres — 10 cm is standard for a residential slab.
- Keep the 10% wastage box ticked to allow for spillage and uneven ground, then press Calculate.
- If your volume is over 1 m³, compare bag prices with a premix truck — it’s usually cheaper for larger pours.
Related calculators: Gravel Calculator, Sand Calculator, Paver Calculator
FAQs
How many 20 kg bags of concrete are in a cubic metre?
One 20 kg bag yields about 0.01 cubic metres, so a cubic metre needs roughly 100 bags before wastage — around 110 with the 10% allowance. For anything over a cubic metre, a premix truck is usually cheaper than bags.
How deep should a concrete slab be?
A standard residential slab is 100 mm (10 cm) thick. Driveways and areas carrying vehicles should be at least 100–125 mm with steel reinforcement. Always check your local building requirements.
Should I add extra concrete for wastage?
Yes — allow 10% extra for spillage, uneven subgrade and over-excavation. The calculator includes this by default; untick the box if you’ve measured your formwork precisely.