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From what I know, a preposition "per" is commonly used for the units of measurement containing fractions. For example, kilogram/liter [kg/L] can be safely pronounced as "kilogram per liter"; but can you also use the following spoken variants that one would use for the numerical fractions, such as:

  • kilogram by liter;
  • kilogram over liter;
  • kilogram into liter,

or anything else? Or is X per Y the only correct form in this case?

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X per Y is the only correct form.

'Per' is used when expressing rates, prices, or measurements to mean "for each". You are basically saying: For every liter, I need 1 kilogram of such and such. So kilogram per liter. You are driving 60 miles in one hour, so 60 miles per hour.

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    We also use a slash to indicate division, so "dollars / population" might be intended to mean "dollars divided by population". You have to know the intent when you read it. we might well calculate liters per kilometer by dividing the liters used by kilometers traveled. So you really need to observe the context to know how to read it. Like if someone wrote, "To calculate liters/kilometer we compute liters/kilometer", that should be read, "To calculate liters per kilometer we compute liters divided by kilometers." – Jay Jul 11 '17 at 13:26

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