Paper weights are horrible in the imperial system. Not only is it per “ream” (and that reams are non-standardized in themselves), but “20lbs” can mean different things depending on the kind of paper you’re using. And it doesn’t help that there are conflicting references online. For example this site:
https://bollorethinpapers.com/en/basic-calculators/gsm-basis-weight-calculator
says:
gsm/1.48 = basis weight in lbs
Which … somewhat works for “Text / Book weight” (whatever that means) except when it doesn’t and, well, that’s just one of six different types of Imperial scales according to that table:
I mean I thought fathoms, miles per gallons, yards, feet, and inches were weird, but that’s just beyond insanity. I think the worst part is this is still very much in use in Canada - I would understand this would survive as an historical artifact documented in Wikipedia, but this is what you prominently see on labels here in Montréal (except when they decide not to and put GSM instead).
It’s ridiculous. But so is life in a multicultural society sometimes. 
The handy conversions I’ve started internalizing are:
- 90gsm / 60lbs book: somewhat normal paper
- 148gsm / 100lbs book: somewhat thin photography paper
- 216gsm / 80lbs cover: cardboard-y, not quite thick enough for sturdy continuous use
- 240gsm / 90lbs cover: cardboard, target weight for my calendar project
I still have to figure out the rest, to be honest. Notice how the left column grows regularly while the right column is irregular?
What I don’t get is what “bond, text/book, cover, bristol, index, tag” comes from… What do they mean? How do people use that kind of stuff?
Gah.