The USA is a big place and in relatively recent history is used to choosing what it does without considering how the rest of the world does what it does. Nowhere is this more obvious than in the units that are used to measure everything from mass and time to distance and magnitude.
The USA uses a modified Imperial system of measurements, and they are in good company – Liberia and Myanmar use the same system. The remaining benighted 190 or so countries use a little known metric system. Americans traveling virtually everywhere abroad struggle with unit conversion. I encourage every traveler to train themselves to think in metric, and convert back to modified Imperial when required. On this site I use only metric units.
Mass: A kilogram (kg) is 2.20 Pounds. Think of it as 2+ pounds. There are a 1,000 grams (g) in a kilogram. Think of 100g as 4oz – a stick of butter.
Distance: A kilometer (km) is 0.62 of a mile. Think of it as 2/3 of a mile. There are 1,000 meters (m) in a kilometer. Think of 1m as a little more than a yard.
Volume: A litre (l) is 2.11 US pints. Think of it as 2+ pints, or slightly over a US quart. so a gallon is just a bit less than 4l. There are a 1,000 milliliters in a liter. Think of 30ml as a fluid ounce, or the size of a ‘shot’.
Magnitude: In very many countries a comma is used where in the USA a decimal point would be employed. For example 300.10 in the USA translates to 300,10 elsewhere.
Currency notation: In the USA it is conventional to use the dollar sign ($). However, there are many countries that use a ‘dollar’ (origin from the Thaler). Learn the international acronyms of currencies like the US Dollar (USD), the Euro (EUR), the Swiss Franc (CHF). I will use ‘$’ to mean USD.
Time: Very many places, especially transportation timetables, use a 24hour (‘military time’) clock instead of the familiar 12hour time. Thus 3.20pm is 15.20. I still occasionally show up at the wrong time.
Dates: Nearly everywhere uses the obviously superior DD/MM/YY, or DD.MM.YY format instead of the US MM/DD/YY scramble. Thus 05/06/20 is May 6th in the US, but June 5th in France. This can be confusing when booking something on an app, so think and check twice before clicking the ‘buy’ button.
Building floors: Not strictly a unit of measurement. But in most of the world the ’ground floor’ is designated the ’0’ floor, and thus the US 2nd floor is elsewhere known as the 1st floor.
In my limited experience nothing marks me out as a provincial tourist more than when I am not fluent in the metric system.