SMACNA is the most common standards to be used in the United States. British Standards are commonly used in Europe. Hoewver, the thickness of the duct thickness is not the only consideration during the design, you need to look into the tightness requirements(leakage) as well. Some of the States in the US set up their standards which is more strigent than SMACNA, you need to check the local codes. OSHPD, Medical Facilities and Laboratories also have tighter requirements on both material thickness and tightness.