You can have the best of both worlds: when I'm fixing my daughter's curtain rail, I use mm to measure distance from the window frame because if I use fractions

You can have the best of both worlds: when I'm fixing my daughter's curtain rail, I use mm to measure distance from the window frame because if I use fractions of an inch I'm far more likely to bugger up the maths. But if I want to tell someone my height, I will use feet and inches as those are more human-scale and convenient than metric. For packaging and products there seems no logical reason to not cite both, although the EU mandated metric-only