Here is a helpful online calculator. Often I get asked how much, say a 100 lb paper is in grams per square meters.
It is quite a simple calculation:
1lb. of Text paper = 1.48 gsm. Multiply each pound of text paper by 1.48.
1 lb. of Cover paper = 2.708 gsm. Multiply each pound of cover paper by 2.708.
But if you punch in your numbers here its all done for you.
If you wonder why you would even need to do this calculation. Well for example a Chinese printer might give you a quote in gsm. But your friendly local printer was speaking about the paper weight in lb.