Paper density measures a paper product's mass per unit volume. The density can be calculated by dividing the grammage of paper (in grams per square metre or "gsm") by its caliper (usually in micrometres, occasionally in mils).
In countries that did not adopt the metric system, paper density is often measured in basis weight. This measuring system expresses the density of paper by weighing a set number of so-called basis sheets. These are the sheets produced when the continuous paper product made by a paper mill is first cut into pieces. Basis sheets are usually 17.5 inches by 22.5 inches. Sheets of this size are then cut and trimmed into four sheets of standard 8.5" x 11" office paper or two sheets of 8.5" x 14" legal paper.
