How to Estimate Concrete Volume

Measurement Checklist

• Choose slab, footing, or post-hole mode before measuring.
• For a slab, measure length and width in metres and thickness in millimetres.
• For a footing, measure length, width, and depth in metres.
• For post holes, measure diameter in millimetres, depth in metres, and count the holes.
• Enter a waste percent to apply after the base volume.
• Use ready-mix output for cubic metres or enter bag yield in m3 per bag for bag rounding.
• Enter optional bag price only if you want a multiplication by your own typed price.

Worked Example

1. Slab length 4 m, slab width 3 m, slab thickness 100 mm.
2. Convert thickness: 100 mm = 0.1 m.
3. Base volume = 4 x 3 x 0.1 = 1.2 m3.
4. With 10% waste, waste-adjusted volume = 1.2 x 1.1 = 1.32 m3.
5. When bag yield is not entered, ready-mix volume is shown as 1.32 m3.
6. If bag yield is 0.018 m3, bags required = ceil(1.32 / 0.018) = 74.

Common Mistakes

• Entering slab thickness in metres when the slab mode expects millimetres.
• Using slab mode for a long trench-shaped footing with a separate depth value.
• Measuring post-hole radius but entering it as diameter.
• Forgetting that post-hole volume multiplies by the hole count.
• Treating volume output as structural design guidance. The calculator estimates volume only.

Assumptions and Limitations

• Slab volume = length x width x thickness in metres.
• Footing volume = length x width x depth.
• Post-hole volume = pi x radius squared x depth x hole count.
• Waste is applied after the base volume is calculated.
• Bag counts are rounded up to whole bags when a bag yield is entered.
• The calculator does not specify slab design, reinforcement, sub-base, load capacity, permits, or site preparation.