The local currency unit.
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volume in units of gallons, liters, cubic feet, cubic meters etc
I would use the metric unit is inches
I would use a postal scale to weigh the letter. Once I knew the weight, I could express it in any unit of force, but 'grams' would probably result in the most convenient number.
Feet, Yards,Miles Centimetres, metres, kilometres Light years
Use fraction to express a number less than one