One ream of 20-pound letter-sized paper weighs 5 pounds, and a single letter-sized sheet of 20-pound paper weighs 0.16 ounces (4.5 g), which is equivalent to 72 g/m2. The most common density of A4 paper is 80 g/m 2. The precise origins of the dimensions of US letter size paper (8.5 Ã 11 in) are not known.