A cubic yard is the volume of a cube one yard on each side. With 3 feet per yard, it is 3 × 3 × 3 = 27 cubic feet. NIST lists it as 0.7645549 m³. Mulch, soil, gravel and ready-mix concrete in the US are typically sold by the cubic yard.
Formula
yd³ = ft³ ÷ 27
yd³ = (length ft × width ft × depth ft) ÷ 27
m³ = yd³ × 0.7645549
Example
A garden bed 12 ft long, 4 ft wide and 0.5 ft (6 in) deep needs 12 × 4 × 0.5 = 24 ft³. 24 ÷ 27 = 0.889 yd³. Two cubic yards would be 2 × 27 = 54 ft³ or 2 × 0.7645549 = 1.529 m³.
Common mistakes
Dividing by 3 instead of 27 overstates the volume by a factor of nine. Depth is also easy to enter in inches: 6 inches is 0.5 feet. The cubic yard calculator and mulch calculator handle both steps; how many feet in a yard explains the 3-to-27 jump.