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Soil Amendment Calculator

Enter your bed area and amendment depth to find the volume needed.

How it is calculated

The bed area is in square feet and the amendment depth in inches. Volume in cubic feet is ft³ = area × depth ÷ 12, because the depth is converted from inches to feet. Cubic yards are yd³ = ft³ ÷ 27, since one cubic yard holds 27 cubic feet. The tool also estimates bags of 1.5 ft³ each as bags = ⌈ft³ ÷ 1.5⌉, rounded up to whole bags. Area and depth must be above zero. Results are rounded to two decimals. The figure is the loose volume to spread; it does not add extra for settling, uneven ground, or for amendment that is mixed in and compacts, so many gardeners order a little more.

Worked example

A raised-bed area of 120 sq ft, to be covered with 3 inches of compost. Cubic feet: 120 × 3 ÷ 12 = 30.00 ft³. Cubic yards: 30 ÷ 27 = 1.11 yd³. Bags: 30 ÷ 1.5 = 20 bags of 1.5 ft³, so exactly 20 are needed. If you buy in bulk, ask the yard for about 1.1 cubic yards; ordering 1.25 leaves a small margin.

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Common mistakes

  • Entering the area in square metres or the depth in centimetres. The fields are in square feet and inches, so convert first (1 m² is about 10.76 ft²; 1 cm is about 0.39 in).
  • Entering the depth in feet. A depth of 3 means 3 inches here; typing 0.25 for the same 3 inches would give only one twelfth of the volume.
  • Calling the result cubic metres. It is in cubic yards (27 ft³), and a cubic yard is about 0.76 m³.
  • Ignoring the bag size. The bag count assumes 1.5 ft³ bags; if your bags hold 1 or 2 ft³, divide the cubic feet by your own bag volume.
  • Applying the depth to the whole yard when only the beds are amended. Measure just the planted beds, not the paths between them.

About this calculator

Adding compost, aged manure, or another soil amendment to a garden bed only helps if it is applied at an adequate depth across the whole bed — too shallow and it barely mixes into the root zone, too deep wastes material and money. Amendment volume is a simple area-times-depth calculation, but it needs a unit conversion because garden area is measured in square feet and amendments are typically sold by the cubic yard.

Bed area in square feet and the depth of compost you want to work in, in inches, are converted to cubic yards, the unit bulk compost and soil yards price and deliver by (cubic feet and a bag count are shown too), so you order the right amount for the season's beds.

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Comments (1)

  • Editorial team

    Depth here means the layer you spread, not how deep you dig it in. For 200 sq ft at 2 inches the tool gives 33.33 ft³, 1.23 yd³ or 23 bags, because the bag count rounds up from 22.2. At that size it is worth pricing 23 bags against 1.23 yd³ in bulk, which is often cheaper per cubic foot but may carry a minimum load. Do you buy by the bag or by the load?

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