To determine paper weight, manufacturers either weigh one ream, 500 sheets, of paper or one sheet of paper.

Weights may be expressed in grams per square meter, gsm, or pounds per ream, lb. Thicker paper weighs more, so when looking for a thick paper for watercolor or other art project choose one with a higher weight.

There are a few different sorts of projects where paper weight will be important. If you are creating a book, the cover needs to be thick enough that it is as durable as you need. The inner pages, if they are just text need to be just heavy enough that the ink won’t soak through when printed on both sides. Anytime the paper will be saturated, it should be of appropriate weight. Paper construction projects need to be mindful of weight in regards to durability and type of fastener (i.e. glue) to be used. Origami or other folding projects will generally want thin paper. When making cards or envelops, too heavy of paper will cause strange, lumpy folds so test out a few weights before you purchase in bulk.

If you are indeed planning on painting your paper, it may first need to be stretched to prevent buckling. When paper absorbs water, it may dry in a way that you do not desire. If you want your paper to dry flat and smooth and it is less than 356 gsm or 260 lb, it may need to be stretched. You can experiment with a small bit of paper by painting it as you normally would and see how it dries. If it buckles, you should stretch it. Stretching paper is not terribly difficult, but it is tricky. Basically you soak the whole sheet of paper, smooth it against a flat board, use gummed tape to fasten the edges to the board, and allow the paper to dry evenly. The next time the paper is wet, it will not buckle or warp. The advantage of user paper over 356 gsm or 260 lb is that you never have to stretch it, it can withstand several applications of paint and glaze, and it will be more durable.