Concrete volume calculator
PlumberMate AU · Australian trade tools
Calculate the geometric volume of a rectangular pour, then estimate bags using your chosen allowance.
Geometric volume
0.200 m³
0.220 m³ with allowance · 21 nominal 20 kg bags
How concrete quantity is calculated
Volume (m³) = length (m) × width (m) × depth (mm) ÷ 1,000. An ordering allowance increases this volume; bag quantities round up to a whole bag.
Worked example: 2 m × 1 m × 100 mm = 0.200 m³. With 10% allowance, 0.220 m³ requires 20 bags at the assumed 0.011 m³ yield per 20 kg bag.
Assumptions to check on site
The pour is a single rectangular shape with uniform depth. The 0.011 m³ bag yield is a nominal estimating assumption, not a universal product specification. Check the bag packaging, excavation dimensions and actual site conditions.
Australian plumbing FAQs
How many 20 kg bags for a 2 m × 1 m × 100 mm slab?
Volume is 2 × 1 × 100 mm ÷ 1,000 = 0.200 m³. With a 10% ordering allowance that becomes 0.220 m³, which at the assumed yield of 0.011 m³ per 20 kg bag rounds up to 20 bags.
Is 0.011 m³ per 20 kg bag a guarantee?
No, it is a nominal estimating figure used by this calculator. Actual yield is printed on the bag and varies between products and manufacturers, so check the packaging or the supplier’s data sheet before you order.
Do I have to add an ordering allowance?
That is your decision. Uneven excavation, spill, compaction and depth variation are the usual reasons tradies order above the geometric volume. Enter the percentage you judge appropriate and the calculator rounds bags up to a whole bag.
Can I use it for an L-shaped or stepped pour?
Split the area into rectangles of uniform depth, calculate each one separately and add the volumes together. For irregular or complex shapes, a ready-mix supplier calculator is usually a better fit.
Does it tell me which concrete strength or mix to order?
No. Strength class, mix design and slump are separate decisions that depend on the job and the applicable standard, so confirm them with the supplier and the project requirements.
Educational and estimating aids only. For plumbing, drainage or gas work, verify assumptions against current Australian standards, manufacturer instructions and local requirements. These tools do not replace professional assessment.