Behind the numbers: Understanding paper measurement terms

The following five terms appear on most Hammermill® packaging and sell sheets, and you probably understand them all at a basic level. But if you don’t, or you’d just like to learn some fun and quirky details, read on. 

Basis Weight

Basis weight sounds simple, but it can be confusing. Paper is separated into basic sizes,” which relate to its end use. Copier and printer paper (called bond paper) is cut from a basic size of 17” x 22”. So when you buy 20-lb. bond, the 20 lbs. refers to the weight of 500 sheets measuring 17” x 22”. When you divide these 17” x 22” sheets by four, you end up with four 500-sheet reams, each measuring 8.5” x 11” and weighing 5 lbs. (20 lbs. divided by 4). Other basic paper sizes include book/offset, Bristol, cover, index and tag. 


These letters stand for grams per square meter, which is another way to list a paper’s weight. The “gsm number” puts all paper on the same playing field. By removing the confusion of “basis weight” and “basic size,it allows you to compare papers more easily. For example, a 20 lb. bond sheet and a 50 lb. offset sheet are both 75 gsm, which means that one square meter of each paper weighs the same. However, these papers may have completely different calipers (thickness measurement) and smoothness numbers. Most copiers and printers specify the gsm range that is best for optimum performance. 


Brightness is the measure of a paper’s reflectiveness, on a scale of 0 to 100. The higher the number, the brighter/more reflective the paper. (e.g., 98 is brighter than 92.) Brightness is measured with a very specific wavelength of blue light (457 nanometers), a standard used by all paper companies.


Brightness and whiteness sound interchangeable. They’re not. Whiteness is measured with the entire spectrum of visible light—not just blue. Why? White light contains all the colors of the spectrum, and for that reason, whiteness is more closely aligned with how we see. You can also think of whiteness as the shade of the paper. A higher whiteness tends to be more blue where a lower whiteness has less blue.


Like brightness and whiteness, opacity is measured on a scale of 0 to 100. Opacity tells you how much light passes through the paper—or rather, how much light doesn’t pass through it. For example, a paper that has an opacity of 94 reflects 94% of the light that hits it, allowing only 6% of that light to pass through it. The higher a paper’s opacity, the less show-through you’ll see when printing, which is why higher opacity papers are better for two-sided printing.