It can be confusing if you are purchasing from a territory where they measure paper density differently.
Pound, lbs, gsm, gms and kgs are all used to measure paper heft – and not always accurately.
The two most common paper weight units are pounds, or just lbs. and gsm, or grams per square meter. However, it is not without problems. With paper weight listed in lbs it is referring to a ream of that paper at that size and is not a true static density measurement. Typically smaller paper follows the rough rule of thumb below. As paper size gets larger it is thrown out more and more since a ream of A6 will weigh less than a ream of A2 (for example).
They roughly translate with some basic functions.
Going from gsm to lbs. 66%, or 2/3rds.
Example: 150gsm paper is 100lbs.
Going from lbs. to gsm. Add 50%.
Example: 100lbs paper is 150gsm.
With these simple calculations you are armed to choose paper easily. Ultimately the lbs weight of paper is not a good measurement of paper density compared to gsm.