A concrete calculator estimates the volume of concrete required for a rectangular pour based on the length, width and depth of the area. It converts the result to cubic metres, cubic yards and the approximate number of pre-mixed concrete bags needed. This is useful for planning small concrete projects such as garden slabs, footings, steps and driveways.

1. Enter the length, width and depth of the area.
2. Select the unit of measurement.
3. Click Calculate to see the volume and bag count.

Slab: 3m × 3m × 0.1m = 0.9 m³
≈ 53 × 60 lb bags or ≈ 40 × 80 lb bags
Add 5–10% for waste and spillage.

Always add 5–10% to the calculated volume to account for waste, spillage and uneven sub-base. For large pours (over 1 m³), ready-mix concrete delivered by truck is usually more economical than bags. The bag estimates are approximate — check the manufacturer's coverage data for the specific product you are using.

• Not adding a waste factor — always order 5–10% more than the calculated volume.
• Mixing units — ensure all dimensions are in the same unit before calculating.
• Forgetting to account for reinforcement (rebar) which slightly reduces the concrete volume needed.

How many bags of concrete do I need for a 3m × 3m slab at 100mm depth?

3 × 3 × 0.1 = 0.9 m³. You would need approximately 53 × 60 lb bags or 40 × 80 lb bags, plus 5–10% extra for waste.

Should I use bags or ready-mix?

For volumes under about 0.5 m³, bags are practical. For larger pours, ready-mix concrete delivered by truck is usually more economical and produces a more consistent mix.