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That's meaningless if you don't tell us what the base size is.
I think the base size is 25" x 38", but I'm not 100% sure. If it is 25" x 38" and the basis weight is 20 pounds, the grammage is : (20/500 x 453.59)/(25 x 38 x 0.0254²) = 29.6 g/m².
On the Japanese page, it says India paper is between 20 g/m² and 40 g/m².