How to Calculate Gravel, Soil, and Mulch

Measurement Checklist

• Choose gravel, topsoil, mulch, or custom material.
• Review or edit the density in kg/m3.
• Split the project into rectangular zones.
• Measure each zone length and width in metres.
• Measure intended depth in millimetres.
• Enter waste percent after deciding how much allowance to include for uneven grade or spreading variation.
• Choose loose delivery or enter bag capacity in litres for whole-bag rounding.

Worked Example

1. One gravel area is 5 m x 2 m with a depth of 50 mm.
2. Area = 5 x 2 = 10 m2.
3. Depth 50 mm = 0.05 m, so raw volume = 10 x 0.05 = 0.5 m3.
4. With 10% waste, waste-adjusted volume = 0.5 x 1.1 = 0.55 m3.
5. Using the gravel preset density of 1600 kg/m3, tonnes = 0.55 x 1600 / 1000 = 0.88 t.
6. With 40 L bags, bags required = ceil(0.55 / 0.04) = 14.

Common Mistakes

• Entering depth in metres when the calculator expects millimetres.
• Using the preset density without checking whether a different density assumption is more appropriate for the material being planned.
• Measuring a curved or irregular bed as one large rectangle instead of splitting it into closer rectangular zones.
• Forgetting that litre-bag output uses volume, while tonnes output uses the editable density.
• Treating density presets as supplier guarantees. The module labels them as planning assumptions.

Assumptions and Limitations

• Each zone area = length x width.
• Zone volume = area x depth after converting millimetres to metres.
• Raw volume is the sum of all zone volumes.
• Waste is applied after raw volume is calculated.
• Tonnes = waste-adjusted m3 x density kg/m3 / 1000.
• Bag counts are rounded up to whole bags when bag capacity is entered.
• Actual density and delivery packaging vary by material, moisture, compaction, and supplier.