When converting lumber sizes the nominal dimensions (2x) or (4x) should NOT be converted, rather the actual sizes like 1.5"x3.5" should be soft converted to 38x89mm.
2x4 = 38x89mm
2x6 = 38x140mm
2x8 = 38x184mm
2x10 = 38x235mm
2x12 = 38x286mm
Soft vs. Hard conversion - soft conversion is simply converting inch-pound units to the nearest equivalent metric unit. Panel products for example are typically 4'x8' which would convert to 1220x2440mm. Hard conversion is when the metric units are rounded to a rational unit. In the preceding example hard conversion for panels would be 1200x2400.
To convert from board feet to cubic meters; according to ASTM E380, multiply board feet by 0.002359737.
MAT 120 Metric and Wood for more information.