> If you are building things in the US/UK/Canada then you are using feet and inches.
In the UK you are more likely to be using millimetres if working from any kind of design. Working in inches and feet would depend on your age and perhaps whether you are working on an older property that was designed in inches.
In the UK your designs and plans will be in mm, but your materials will vary between metric-rounding-of-imperial, (2400x1200mm) [1] imperial-but-labeled-in-metric (1220x2440mm) [2] and imperial-in-one-dimension-metric-in-the-other (38x144x2400mm) [3]. Wallpaper? 10m x 520mm for sure [4] but doors? 1981x762mm [5]. Cement? Back to metric, with 10kg and 25kg bags [6].
You'll also probably measure long distances in miles but short distances in meters; and weigh your ingredients in grams but your body in stone.
I guess what I'm saying is if you want a rationally designed measurement system, don't copy us Brits and especially don't copy our builders.
In the UK you are more likely to be using millimetres if working from any kind of design. Working in inches and feet would depend on your age and perhaps whether you are working on an older property that was designed in inches.