Two sheets of paper can appear very similar in size and color on a workbench. Yet, when you lift them, flex them, and place them on a scale, you find significant differences. That is because two basic paper properties, basis weight and thickness, can vary quite a bit from one another. Both relate to the construction of the sheet, but they are distinct measures.
Basis weight, often referred to as grammage, refers to the mass of paper per unit area. Typically expressed in grams per square meter, it compares how much paper there is for a given area. A sample of paper with more mass per unit area than another has a higher basis weight. Thickness, on the other hand, is the distance from one side of the sheet to the other, which can be measured with a micrometer or a thickness gauge. Heavier does not mean thicker, nor does thick mean heavier.
You can quickly illustrate this with a few sheets of paper. Select several samples of similar dimensions, for example, office paper, packaging paper, and a heavy uncoated sheet. If your digital scale has sufficient precision, weigh out sections of equal area. Compare the thickness of those pieces with a micrometer or thickness gauge. Note the two values separately. This experiment is not to determine whether one sheet is superior to the others, but rather to show that mass and caliper can differ significantly.
There is a connection between basis weight and thickness. It provides some insight into bulk and apparent density. If a sheet is relatively thick for its basis weight, it has more bulk. If a sheet carries a similar mass in a smaller thickness, it is denser. Depending on fiber furnish, refining, pressing, and calendering, the relationship between thickness and basis weight can vary. Increased pressing compacts the wet web, while calendering can flatten and smooth the finished sheet. In both cases, the sheet becomes thinner without necessarily decreasing its mass per unit area at the same rate.
Novices often make the mistake of assessing a sample by touch. A piece of paper may appear thick due to stiffness or bulk. But that cannot substitute for an accurate measurement of basis weight. Furthermore, the treatment of the surface can affect the way a sample feels in the hand. When you measure the mass, area, and thickness independently, you can avoid confusing those impressions with actual paper characteristics.
This understanding comes in handy when considering different classes of paper. Paper for packaging, printing, and specialty applications may be formulated to achieve certain grammage, bulk, smoothness, opacity, and strength characteristics. No universal rule links thickness to basis weight because the ideal sheet structure varies depending on its end use. For beginners, the key is to identify each characteristic separately. Then you can try to understand why the paper is behaving the way it does.
When you encounter two sheets that appear similar, do not simply compare them visually. Measure or compare their area, mass, and thickness. Describe what each result means. You will soon realize that you are detecting differences you could not see with the naked eye. One sheet may be significantly thicker without having a corresponding increase in basis weight.