Soil Calculator

Buying soil is a volume problem dressed up as a gardening problem. Every result here starts from a footprint area multiplied by a uniform depth: length times width for a rectangular bed, pi times the square of half the length for a circular one, and half of length times width for a triangular corner bed. Depth entered in inches is divided by twelve because the whole calculation is carried in feet. The step that trips people up is the last conversion, since bulk suppliers sell by the cubic yard and a cubic yard is three feet cubed, or 27 cubic feet. A 4 by 8 foot bed filled 10 inches deep therefore needs 26.7 cubic feet, which is almost exactly one yard. Weight matters as much as volume once you have to move the material. The figure of 75 pounds per cubic foot used here is reasonable for moist screened topsoil, so a cubic yard runs to roughly 2,000 pounds, at or above the payload of most half-ton pickups. Light soilless blends heavy in compost, peat or coir can weigh half that, while wet clay fill can exceed 100 pounds per cubic foot. Order 10 to 20 percent more than the geometric answer. Fresh material settles as irrigation closes the pore space, the organic fraction decomposes over the first season, and every bed looks shallower after the first heavy rain than it did on delivery day.

Selling soil by the cubic yard is a legacy of the American aggregate trade, where the yard has long been the standard truck-and-scoop unit for sand, gravel and fill, and garden centres simply inherited the convention. The scientific vocabulary underneath came from the United States Department of Agriculture, which began its national soil survey in 1899 under Milton Whitney and later codified the sand-silt-clay texture triangle that still defines what counts as a loam. The idea of building a bed rather than amending a field is older than it looks. Nineteenth-century Parisian market gardeners raised deep manure-rich beds under glass to force early crops, and that culture maraichere was carried into English horticulture and then to California by Alan Chadwick, who founded the student garden at UC Santa Cruz in 1967. John Jeavons codified the biointensive version in How to Grow More Vegetables in 1974, and Mel Bartholomew popularised shallow framed beds filled with premixed media through Square Foot Gardening and its PBS series in the early 1980s. The bagged mixes that made those beds practical are mid-century inventions: the UC system published by Kenneth Baker in 1957, and the Cornell Peat-Lite mixes of James Boodley and Raymond Sheldrake, which replaced field soil with peat, perlite and vermiculite of predictable weight and porosity.