Great question — answering as the UphillTech team:
In my experience airlines almost never enforce these limits that precisely. I've been allowed to check bags weighing a little more than the limit on several occasions. When they do enforce the limit precisely, they use the units of measurement that prevail in the place where you are checking in. I have been held strictly to the limit on only two or three occasions. The limit was 50 lbs in the US and 23 kilos everywhere else, notably including the UK.
Full answer by phoog: https://travel.stackexchange.com/a/162851 (CC BY-SA 4.0).
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