calculatemygarden
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Garden Soil Calculator

Enter a bed's area and depth to see cubic yards for delivery or the bags to buy.

Wheelbarrow of dark compost beside a half filled wooden raised bed
Units
What are you doing?
Bed shape
ft
ft
in

Maryland Extension: 2 to 4 in, mixed into the top 4 in.

Price per
$

Optional.

Compost to order

1.85cu yd

50 cu ft, or 25 bags of 2 cu ft.
Area
200sq ft
Cubic feet
50cu ft
Cubic yards
1.85cu yd
Litres
1,416L
1.5 cu ft bags
34
2 cu ft bags
25
3 in20 ft
New compost on top of the bed; the dashed line is how deep to mix it in. Not to scale.

Spread the compost evenly and fork it into the top 4 in of soil. A bed of 200 sq ft takes 1.23 to 2.47 cu yd across the 2 to 4 in that Maryland Extension suggests.

How the calculation works

The soil you need is the area you cover times the depth of the new layer. Work in feet for the area and inches for the depth, then convert.

Cubic yards = area (sq ft) × depth (in) ÷ 12 ÷ 27

For a rectangle the area is length × width; for a circle it is π × radius². For a bed with an odd outline, split it into rectangles and circles, add their areas and use the "Known area" option. Bags are rounded up to whole bags.

A worked example

The page opens with a 20 by 10 foot vegetable bed getting 3 inches of compost.

  • Area: 20 × 10 = 200 square feet.
  • Volume: 200 × 3 ÷ 12 = 50 cubic feet.
  • In cubic yards: 50 ÷ 27 = 1.85 cubic yards.
  • In bags: 25 bags of 2 cu ft, or 34 bags of 1.5 cu ft.

Switch to "Adding soil" and the depth changes to 6 inches: the same bed then needs 100 cubic feet, 3.7 cubic yards or 50 bags of 2 cu ft. A round bed 10 feet across covers 78.5 square feet, so 3 inches of compost is 19.6 cubic feet, about 0.73 cubic yards.

How much compost to add

University of Maryland Extension's advice for improving existing garden soil is to add 2 to 4 inches of compost and mix it into the top 4 inches with a garden fork or spade. That is why the compost layer starts at 3 inches. Across the 2 to 4 inch range, the 200 square foot bed takes 1.23 to 2.47 cubic yards.

Topsoil, garden soil and compost

These names mean different things on a bag or a delivery ticket.

  • Topsoil is mineral soil: sand, silt and clay particles. Maryland Extension says good topsoil is dark and crumbly with an earthy smell, and to avoid soil that is very sandy or clayey, smells foul, or shows grayish mottling or a chalky, sticky or rough texture. Topsoil sales are not regulated in Maryland, so ask the supplier for soil test results and where the soil came from.
  • Bagged "organic soil" or garden soil often contains no topsoil at all. Maryland Extension notes these products are typically mixes of wood waste, bark, compost and peat moss.
  • Compost is decomposed organic matter, used to improve the soil you have.

To raise a low area or build up a bed look for real topsoil; to improve an existing bed, Maryland Extension's advice is compost mixed in. For a raised bed, Maryland Extension suggests blending compost with purchased topsoil at 1 : 2 or 1 : 1; the raised bed soil calculator splits that mix for you.

Bags or bulk

Bulk soil and compost are sold by the cubic yard, bagged products by the cubic foot. NIST's tables put 27 cubic feet in a cubic yard, so one cubic yard is 13.5 bags of 2 cu ft or 18 bags of 1.5 cu ft. Enter the price per bag or per cubic yard to see what the job costs, and try both to compare.

Next questions

  • How much mulch goes on top? Use the mulch calculator, which works the same way with mulch depths.
  • What about pots and planters? The potting soil calculator handles round pots, window boxes and grow bags.
  • Metric? Switch units at the top of the page. The calculator then works in square metres and centimetres and gives cubic metres and litres; pick 50 L in the bag list.