How are landscape materials calculated?
Most are a layer: area times depth. With volume and density you get tons, and with the sales unit, bulk bags or bags. Paths and driveways add compacted base; pavers add bedding and joint sand.
The cost calculators add your price per ton, yard or bag plus delivery.
What depths are typical?
Decorative rock 1½–2 in; gravel paths 2 in; mulch 2–3 in; compost top-dressing ½–1 in; topsoil for lawns 4–6 in; driveway base 6–10 in compacted.
Always lay landscape fabric under gravel and rock, and compact any base layer in lifts before the top layer goes on.
When do landscape jobs need an engineer?
Retaining walls over about 4 ft, or with surcharge, need engineering: soil and water pressure grow with the square of height. The pressure calculator helps you pre-size and understand the loads.
Good drainage behind the wall is what extends its life most.