Paper weight explained: GSM, basis weight and choosing paper

Paper weight is measured in grams per square metre (gsm, g/m²) in most of the world: everyday copy paper is about 80 gsm, and business cards are often 300 gsm or more. In North America paper is sold by basis weight in pounds, which depends on the paper grade, so 20 lb bond is about 75 gsm while 80 lb text is about 118 gsm. The paper weight converter translates between gsm and every common US grade.

What gsm means

Grammage, or gsm, is simply how much one square metre of the paper weighs. Because it is per square metre, it does not depend on the sheet size, which makes papers easy to compare. It also gives a neat trick with ISO sizes: an A0 sheet is one square metre, and each size down halves the area, so an A4 sheet is one sixteenth of a square metre. A sheet of 80 gsm A4 therefore weighs 5 g, and a 500-sheet ream about 2.5 kg. A4 vs US Letter: the two paper sizes and how to print between them explains how the ISO sizes are built, and the paper sizes reference lists their dimensions.

US basis weight

The US system weighs a ream of 500 sheets of a grade's basic size, an uncut sheet that differs from grade to grade:

• Bond (writing and copy paper): 17 × 22 in;
• Text or book (offset and brochure paper): 25 × 38 in;
• Cover (card stock): 20 × 26 in;
• Index: 25.5 × 30.5 in.

Because the basic sheets are different sizes, the same pound number means different papers. 20 lb bond, 50 lb text and 28 lb cover all come out at around 74 to 76 gsm, while "80 lb" could mean 118 gsm text or 216 gsm cover. That is the main source of confusion when ordering paper across borders, and the reason I made the converter show every grade side by side.

Converting between them

Each grade has its own factor, worked out from its basic size: gsm = pounds × 3.76 for bond, × 1.48 for text, × 2.70 for cover and × 1.81 for index. Common equivalents:

• 20 lb bond ≈ 75 gsm; 24 lb bond ≈ 90 gsm; 28 lb bond ≈ 105 gsm; 32 lb bond ≈ 120 gsm;
• 80 lb text ≈ 118 gsm; 100 lb text ≈ 148 gsm;
• 65 lb cover ≈ 176 gsm; 80 lb cover ≈ 216 gsm; 100 lb cover ≈ 270 gsm.

Going the other way, standard 80 gsm copy paper is about 21 lb bond or 54 lb text, and 300 gsm card is about 111 lb cover. Suppliers often round these to the nearest familiar grade, so expect small differences between a spec sheet and a calculation.

Choosing a paper weight

• 70 to 80 gsm: everyday printing and copying.
• 90 to 100 gsm: letters, CVs and reports that should feel a little more substantial.
• 120 to 170 gsm: flyers, posters and brochure pages.
• 200 to 250 gsm: light card for menus, covers and postcards.
• 300 to 400 gsm: business cards, invitations and greeting cards.

Before buying heavy stock, check your printer's maximum paper weight in its specifications, usually given in gsm; some printers take heavier card only through a rear or manual feed. For layouts, the print sheet tool arranges images on a page, and the business card generator produces cards ready to print on card stock. Print resolution matters as much as paper, which What DPI you actually need for printing covers.

Weight versus thickness

Gsm measures mass, not thickness. Thickness, called caliper, is measured separately in microns or thousandths of an inch (points), and it depends on how dense the paper is. Coated papers are denser, so a 300 gsm coated card is usually thinner than an uncoated card of the same weight, though it can feel just as stiff. If thickness matters, for a card that must fit a slot or an envelope, ask for the caliper rather than relying on the weight.

Weight also affects postage. A letter’s cost depends on its total mass, so switching a 10-page mailing from 80 gsm to 120 gsm paper adds half again to the paper weight. With A4 sheets, the gsm divided by 16 gives the weight of one sheet in grams, which makes it quick to check whether a mailing will stay under a postal weight band.