Soil Calculator

How Garden Soil Volume Is Calculated

Whether filling a raised bed, pot, or garden trench, calculating soil volume prevents both running short and wasting money on excess material.

Volume Formulas:

Rectangular bed: V = Length × Width × Depth

Circular bed: V = π × Radius² × Depth

Worked Examples:

Raised bed (2.4m × 1.2m × 0.3m deep):

• V = 2.4 × 1.2 × 0.3 = 0.864 m³ = 864 liters

Large circular pot (diameter 60cm, depth 40cm):

• Radius = 0.30 m
• V = π × 0.09 × 0.40 = 0.113 m³ = 113 liters

Converting to Bags:

• 864 litres bare is 864 / 50 = 17.3, so 18 bags of 50 L
• But soil compacts after the first watering, so add 10%: 950 litres, which is 20 bags
• The bag counts in the result panel already include that 10%, because the number you want is the number to buy, not the number the hole would hold if nothing settled

Soil Mix for Raised Beds (Mel’s Mix): A proven formula by Mel Bartholomew for productive raised bed gardening:

• 1/3 blended compost (5+ types mixed)
• 1/3 peat moss or coconut coir
• 1/3 coarse vermiculite

Soil Settling: Freshly filled beds settle 15–25% over the first growing season as organic matter decompresses and microbes break it down. Plan for this by overfilling initially, or top-dress with compost each year.

Soil Weight Reference:

• Potting mix: ~400–500 kg/m³
• Garden soil (loam): ~1,200–1,400 kg/m³
• Saturated clay: ~1,600–1,800 kg/m³

For raised beds and containers, always use a lightweight mix rather than heavy garden soil. Loam that behaves perfectly well in the ground compacts into a brick once it is boxed in.