How the calculation works

The calculator works out the volume of each bed, adds your settling allowance, then splits the total by your mix.

Rectangular bed: volume = length × width × depth.
Round bed: volume = π × (diameter ÷ 2)² × depth.
Units: depth in inches is divided by 12 to get feet. 27 cubic feet make one cubic yard, and one cubic foot is 28.32 litres.
Bags: each ingredient's volume is divided by the bag size and rounded up to a whole bag.

Worked example

A 4 × 8 ft bed filled 12 inches (1 foot) deep holds 4 × 8 × 1 = 32 cubic feet, or 1.19 cubic yards. Adding 10% for settling gives 35.2 cubic feet. With a 70% topsoil and 30% compost mix you need 24.64 cubic feet of topsoil (17 bags of 1.5 cu ft) and 10.56 cubic feet of compost (6 bags of 2 cu ft).

In metric, a 120 × 240 cm bed filled 30 cm deep holds 864 litres. With 10% extra that is 950 litres: 14 bags of 50 L topsoil and 6 bags of 50 L compost for a 70/30 mix.

Which soil mix should you use?

There is no single right answer, which is why the calculator lets you pick. Iowa State University Extension calls equal parts topsoil, organic matter (such as compost) and coarse sand an excellent mix, and offers about 70% topsoil and 30% compost as a simpler option. For a bed on a lawn, University of Maryland Extension suggests compost and topsoil at 1:2 or 1:1, and for a bed on a hard surface or heavily compacted ground it suggests compost and a soilless growing mix in equal parts. Topsoil quality varies: Maryland Extension notes that topsoil sales are not regulated there and suggests asking the seller where the soil comes from and for soil test results, and Iowa State advises making sure bought-in topsoil is free of plant diseases and contaminants such as lead and pesticides.

How deep should you fill a raised bed?

For a bed on a hard surface or heavily compacted ground, University of Maryland Extension suggests at least 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for peppers, tomatoes and squash. A bed on a lawn can be shallower, because roots grow through the bed into the ground below: Maryland says that if the bed is at least 6 inches deep you can cut the grass and weeds at ground level and fill it straight away.

Bags or bulk?

Bulk soil and compost are sold by the cubic yard (or cubic metre). One cubic yard equals eighteen 1.5 cu ft bags, so for large or several beds it is worth comparing the price per cubic foot of bulk delivery with bags.

Common questions

How many bags of soil do I need for a 4 × 8 raised bed?: A 4 × 8 ft bed filled 12 inches deep holds 32 cubic feet. With 10% extra for settling that is 35.2 cubic feet: 24 bags of 1.5 cu ft, or 18 bags of 2 cu ft. A bed filled 6 inches deep needs half as much.
How many bags make a cubic yard?: One cubic yard is 27 cubic feet, so it equals 27 bags of 1 cu ft, 18 bags of 1.5 cu ft, 13.5 bags of 2 cu ft or 9 bags of 3 cu ft. If you need more than about a cubic yard, compare the price of bulk delivery with bags.
Why add extra for settling?: Loose, compost-rich mixes pack down once they are watered, so a bed filled exactly to the top can end up short. The calculator adds 10% by default as a rule-of-thumb margin; it is not a figure from the sources below. Set it to 0 if you only want the exact volume.
Can I use this for round beds or several beds at once?: Yes. Choose "Round" for circular beds and enter the diameter. Use "Add another bed" for beds of different sizes, and the "How many" box for identical beds.

Sources

Last checked and updated: October 31, 2026.