Can the word 'in' be used to show equality?

Question

Take these two sentences:

1. You're supposed to drink up to your body weight in grams of water.

2. The average woman uses her height in lipstick every 5 years

I can understand that in the first, you're supposed to drink as much water(in grams) as your weight,

i.e. if your weight is 120lbs, drink 120 grams

And in the second if the person's height is 169cm, then they use that much lipstick every 5 years.

But I haven't found a definition for this kind of usage. So I was wondering is the word 'in' meant to show equality or correlation?

I'm pretty sure that it is, but would like some confirmation.

Answer 1

The preposition in serves to introduce the measure - unit, scale, instrument or system used to judge the weight (or height) concerned.

The unit could as easily be from the imperial system (pounds and ounces - feet and inches) or any other object whose size and weight are well understood.

One can estimate people's weight in carob seeds or their height in lego blocks. In simply introduces the item or scale used.

Answer 2

Your first sentence does not actually say what you say that it means. Should it read as follows?

You are supposed to drink as much water in grams as your body weight in pounds

"in" is commonly used to define the units of measurement that are relevant. So : ...water in grams...; weight in pounds; temperature in Fahrenheit; how much is that in dollars? - and so forth.

But when you are actually measuring something then 'of' is used: add three ounces of salt; heat to a temperature of 35 degrees; 120 grams of drinking water, please.