Volume in cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Divide by 27 for cubic yards, which is how bulk suppliers sell. The extra percentage covers settling, uneven ground and spillage. Bagged mulch and soil usually come in 2 cubic foot bags, so bags = cubic feet ÷ 2, rounded up. Weight uses typical densities per cubic yard: mulch about 0.45 tons, topsoil 1.1, gravel 1.4 and sand 1.35. Wet material weighs more, so treat weight as a guide and check with your supplier.
Example: a 20 × 10 ft bed covered 3 in deep needs 200 × 3 ÷ 12 = 50 cu ft, or 1.85 cubic yards. With 5% extra that is 52.5 cu ft, or 27 bags of 2 cu ft.
Two to three inches suppresses weeds and holds moisture without smothering roots. Keep it a few inches away from tree trunks and plant stems.
One cubic yard covers about 108 sq ft at 3 inches deep, 162 sq ft at 2 inches or 324 sq ft at 1 inch.
Two to three inches works for a garden path over landscape fabric; driveways usually need 4 to 6 inches of compacted base plus a top layer.
See also: concrete calculator · square footage calculator · volume converter