Topsoil calculator
Enter the area and the depth of soil you want to add. Spreading 4 inches over a 20 × 10 ft area takes about 2.47 cubic yards, or 89 bags of 0.75 cu ft before extra.
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How the calculator works
Length × width × depth gives cubic feet; divide by 27 for cubic yards, the unit landscape suppliers sell by. Bagged topsoil is labelled in cubic feet; check the size on the bag you are buying.
Why add extra
Loose soil settles once it is spread, raked and watered, and ground is rarely perfectly level. The default 10% covers that; increase it for very uneven ground or if you plan to firm the soil down.
Bulk or bags?
One cubic yard equals 36 bags of 0.75 cu ft or 27 bags of 1 cu ft. For anything over about a cubic yard, a bulk delivery is usually cheaper and less work than carrying bags, if you have space for the pile.