Guides
Raised bed and container questions answered with the same arithmetic the calculators use.
On open ground, nothing but a layer of plain cardboard to smother the grass. In a bed deeper than about 18 in, fill the bottom with logs, branches and leaves: in a 4 × 8 ft bed 24 in deep, a 12 in layer of them saves about 35.8 cu ft of soil, 18 bags of 2 cu ft. Skip the gravel layer: in a bed on soil it does not improve drainage.
A 5 gallon grow bag takes about 21 litres, or 0.7 cu ft, of potting mix once you allow for settling, so one 2 cu ft bag of mix fills 2 of them. A 10 gallon bag takes about 1.5 cu ft. Use potting mix, not garden soil.
Twelve inches suits most vegetables, and it takes about 35.8 cu ft of soil in a 4 × 8 ft bed (18 bags of 2 cu ft). Six to 8 in is enough for lettuce, greens and herbs; tomatoes, squash, potatoes and long carrots do better at 18 in or more, or in a 12 in bed over loosened ground.
A cubic yard of moist topsoil and compost blend weighs about 1,854 lb, close to a ton. Bulk sand and loam is about 2,360 lb, compost about 1,264 lb, and potting mix only 758 to 843 lb. Water matters too: every 10% of the volume that is water adds 169 lb, so the same yard is several hundred pounds heavier after rain. A yard is 27 cubic feet, or 14 bags of 2 cu ft.
For practical purposes, yes. Both names are used for container growing media, and most contain no soil at all: peat moss or coir with perlite or vermiculite, sometimes bark fines, plus lime, a wetting agent and a little fertilizer. A bag sold as potting soil is more likely to include compost or soil and be denser. What is not the same is garden soil or topsoil, which packs down in a pot. Read the ingredient list rather than the name.