Paper Weight Explained: GSM vs US Basis Weight (lb)
Why GSM is the universal way to measure paper weight, how it differs from US basis weight in pounds, and how to convert between them for copy paper, cardstock and more.
Paper Weight Explained: GSM vs US Basis Weight (lb)
Two print shops sent me quotes for the same job last spring. One said "100 lb text," the other said "150 gsm." I assumed they were proposing different papers — turns out they were describing the same sheet. That mismatch is the whole problem with paper weight: the rest of the world counts it one way, the United States counts it another, and the two numbers don't line up unless you know the trick.
This is a short guide to reading paper weight correctly. By the end you'll know what GSM actually measures, why US basis weight in pounds is so slippery, and how to move between the two without a chart.
GSM is an absolute measure
GSM stands for grams per square metre. It is exactly what it says: take a sheet of that paper one metre wide and one metre tall, put it on a scale, and read the grams. That's the GSM number.
The reason GSM has become the international standard is that it depends on nothing except the paper itself. It does not care whether the sheet is cut to A4, to Letter, or to a billboard. A square metre of 80 gsm copy paper weighs 80 grams whether you're in Tokyo, Berlin or São Paulo. Because the measure is absolute, two papers with the same GSM carry the same weight per area, full stop — which makes comparison trivial. Higher GSM generally means a thicker, stiffer sheet, with the usual caveat that coating and fibre can shift the feel a little at the same weight.
Here's where everyday papers land on the GSM scale:
- 35–55 gsm — newsprint and airmail paper, deliberately thin
- 80 gsm — standard office copy paper, the stuff in every printer tray
- 90–120 gsm — letterhead and quality stationery
- 130–170 gsm — flyers, brochures, magazine pages
- 200–250 gsm — light cardstock, postcards
- 250–350 gsm — business cards, folders, greeting cards
Once you have that scale in your head, "200 gsm" instantly tells you "stiff postcard," no decoding required.
Why US basis weight in pounds is confusing
US basis weight is the weight, in pounds, of a 500-sheet ream — but cut to a base sheet size that is different for every paper grade. That single design choice is the source of all the confusion.
Bond and writing paper are weighed on a 17 × 22 inch base sheet. Text and book paper use 25 × 38 inches. Cover stock uses 20 × 26 inches. Index uses 25.5 × 30.5 inches. So when someone says "20 lb," the pounds refer to whatever base size that grade happens to use. The same physical sheet of paper carries a different pound label depending on which grade you weigh it as.
The consequence catches people out constantly: 20 lb Bond is roughly 75 gsm, while 20 lb Cover is roughly 54 gsm. Same number on the label, two genuinely different papers. You cannot compare pound figures across grades, and you cannot convert a pound figure to GSM at all unless you also know the grade. There is no universal pounds-to-GSM factor, because the base sheet size is baked into every conversion.
This is exactly why GSM keeps winning. One number, no grade lookup, no asterisk.
Converting between GSM and pounds
The conversion is just unit bookkeeping. The formula is:
gsm = lb × 1406.5 ÷ (width × height of the grade's base size, in inches)
The 1406.5 is the constant that reconciles pounds-per-ream-of-500 with grams-per-square-metre. Plug in the base size for the grade you care about and you get a clean answer.
Worked example. A print shop quotes "100 lb text" for a brochure. Text stock is weighed on 25 × 38 inches, so the base area is 950 square inches.
gsm = 100 × 1406.5 ÷ 950 = 140,650 ÷ 950 ≈ 148 gsm
So 100 lb text is about 150 gsm — the very brochure paper I mentioned at the top. Run the reverse for a card: a 110 lb cover sheet (base size 20 × 26 = 520 sq in) works out to 110 × 1406.5 ÷ 520 ≈ 298 gsm, a proper business-card weight.
The arithmetic is simple but the base sizes are easy to fumble, which is what the Paper Weight GSM Calculator is for: pick the grade, type a GSM or a pound value, and the matching number updates live so you never have to remember whether Cover is 20 × 26 or 26 × 20. If you also need to juggle the millimetre, inch and centimetre dimensions that show up on sheet specs, the Unit Converter handles those alongside it.
What GSM means for everyday papers
Most of the time you don't need a grade at all — you just want to know whether a sheet is flimsy, normal or stiff, and GSM answers that directly.
The copy paper in your office is almost certainly 80 gsm. It folds easily, feeds through any printer, and a 500-sheet ream of A4 weighs about 2.5 kg. Step up to 100 gsm and the paper feels noticeably more premium without becoming card. Around 120 gsm you're into "this is clearly a nice flyer" territory.
Cardstock is the other anchor point. A typical business card is about 300 gsm — roughly four times the weight per area of copy paper, which is why it stands up on a desk instead of flopping over. Greeting cards, folder covers and postcards live in the 250–350 gsm band. If you want something that feels substantial but still runs through a home printer, 200–250 gsm is usually the practical ceiling.
The single-sheet weight matters more than people expect, especially for postage. An 80 gsm A4 sheet weighs about 4.99 g, because A4 is 0.06237 square metres and 80 × 0.06237 ≈ 4.99. Bump to 100 gsm and the same sheet is about 6.24 g. For a mailing that has to stay under a postage band, those grams add up fast across pages and an envelope — worth checking before you print a thousand copies rather than discovering it at the counter.
The short version
GSM is grams per square metre, an absolute weight that means the same thing everywhere and needs no grade. US basis weight is pounds per 500-sheet ream cut to a grade-specific base size, which is why the same pound number describes different papers and why you can never compare pound figures across grades. To convert, you need the grade and the formula gsm = lb × 1406.5 ÷ (base width × height in inches). And for a quick mental map: copy paper is 80 gsm, a nice flyer is 130, and a business card is 300.
Once you measure everything in GSM, the two print quotes that looked like different papers turn out to be the same sheet — and you can argue about price instead of thickness.
Made by Toolora · Updated 2026-06-13