Clothing sizes
tend to be 1-2 sizes (the number) different here from what they are in the US, but like so much with clothing - it
depends on the store, the manufacturer, sometimes even the individual garment as there doesn't seem to be a true standard size any longer (lots of vanity sizing etc). So for instance, if your wife wears a size 10, say, in the US - she might wear a size 12 or even 14 here in the UK. However, like I said, this varies a lot from shop to shop, garment to garment, etc.
Shoe sizes are usually 2 down here - I wore a size 9 in the US, here I'm a size 7 in shoes.
I find with both of those, it's best just to try the thing on & go from there!
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Most of the metric conversions (weight, liquid, feet/inches, temperature, etc), you can easily find conversions by doing a quick Google search. That's what I usually do, just use Google & pull up an instant converter where I put in the measure I know & it brings up the equivalent metric one.