How the calculation works
Volume in cubic metres = length × width × thickness in metres. Convert to litres by multiplying by 1,000, then add your extra-material allowance. Premixed bags = rounded up (total litres ÷ labelled yield in litres per bag). This uses finished concrete yield, not dry bag weight.
Worked example
Example: a 4 m × 3 m slab at 100 mm thickness needs 1.20 m³ before allowance, or 1.32 m³ with 10% extra. If one premix bag makes 10 L of finished concrete, that is 132 bags. For a larger slab, ask local suppliers about ready-mix delivery.
Before you order
A 10 L yield is only an editable example: check your specific premix bag label. Thickness and ground preparation depend on intended loads and site conditions. Structural foundations and load-bearing work need appropriate professional guidance.
Questions about this estimate
Can I use bag weight instead of yield?
No. Two bags with the same dry weight may produce different finished volumes. Use the manufacturer’s stated mixed yield in litres.
Does this specify the right slab thickness?
No. It estimates quantity only. The correct design depends on soil, loading, reinforcement and intended use.