Understanding Paper Weights: GSM vs LB

Buying paper should be simple, and then you notice the numbers. One shop sells a card base as “216 gsm,” another calls what looks like the same sheet “80 lb cover,” and a third stocks “110 lb index” that is somehow thinner than both. None of it is a trick; the two systems just measure weight in different ways, and once you see how, the labels stop being intimidating.

What gsm actually measures

GSM stands for grams per square metre. It is the weight of a single sheet of the paper cut to exactly one metre by one metre. That is all it is — a mass tied to a fixed area. Because the area never changes, gsm lets you compare any two papers directly: 120 gsm is always heavier per sheet than 100 gsm, whether the paper is Japanese, Italian, or from the recycling bin. Ordinary copier paper sits around 80 gsm, a greeting-card base is usually 216 to 300 gsm, and heavy display board climbs past 350 gsm. Learn those few landmarks and you can judge an unfamiliar paper from its gsm alone.

Why pounds are more confusing

The US “pound” system measures the weight of a full ream — 500 sheets — but at that paper’s basis size, and here is the catch: every grade of paper uses a different basis size. Bond and writing paper are weighed at 17 by 22 inches. Text and book paper use 25 by 38 inches. Cover stock is weighed at 20 by 26 inches, and index at a different size again. So “80 lb” means “a ream of 500 sheets at this grade’s basis size weighs 80 pounds” — and because the sheets being weighed are different sizes, the same physical thickness of paper gets a completely different pound number depending on which grade it is sold as.

That is why an 80 lb text and an 80 lb cover feel nothing alike. The text paper is flimsy and the cover is a sturdy card, yet both are “80 lb,” because they were weighed on differently sized sheets. It also explains the index paradox: a 110 lb index is roughly 199 gsm, lighter than a 100 lb cover at about 270 gsm, even though 110 sounds bigger than 100.

Converting between the two

Because pounds depend on the grade, you can only convert once you know which grade you are dealing with. The relationship is gsm = pounds × 1406.14 ÷ (width × height of the basis size in inches). You rarely need to do that by hand — our paper weight converter does it for every grade and shows the same sheet expressed across all of them — but it is worth understanding so the answer makes sense. A few useful anchors: 20 lb bond is about 75 gsm, 65 lb cover is about 176 gsm, 80 lb cover is about 216 gsm, and 70 lb text is about 104 gsm.

Which number should you shop by?

When you have the choice, shop by gsm. It is unambiguous, it compares directly across every brand and country, and it will not surprise you the way pound figures can. If a paper is only listed in pounds, always note the grade alongside it — “80 lb” on its own tells you almost nothing, while “80 lb cover” tells you it is a proper card weight.

Weight is not the whole story, though. Two papers at the same gsm can feel quite different depending on their caliper (actual thickness) and how the fibres are packed: a bulky, uncoated sheet can be thicker than a dense, coated one of the same weight. Weight tells you how much fibre is there; it does not tell you how stiff, opaque, or smooth the sheet will be. For most projects, though, gsm gets you to the right shelf, and a quick feel of a sample settles the rest.

The short version

GSM is an absolute measure of weight per area, easy to compare and hard to misread. Pound basis weight is a ream weight tied to each grade’s own basis size, which is why the numbers jump around between bond, text, cover, and index. Keep a couple of gsm landmarks in your head, note the grade whenever you see a pound figure, and let a converter handle the arithmetic when you need to cross-check a supplier’s label.