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.

A worked example: one sheet, four different numbers

Take a single real sheet weighing 250 gsm — a fairly heavy cardstock, the kind you might find on the thicker end of a craft-shop shelf. Run that one gsm figure through the basis-weight formula for each grade and you get four completely different-looking pound numbers for the exact same paper: about 92.5 lb if it were sold as cover stock, about 168.9 lb if it were sold as text, about 66.5 lb if it were sold as bond, and about 138.3 lb if it were sold as index. Nothing about the paper has changed between those four figures — only the basis size used to weigh a notional ream of it. That is the entire trick of the pound system laid bare: pick a different grade to weigh the same sheet against, and the number on the label can more than double or less than halve, with no change to what you are actually holding. If you only remember one thing from this article, make it that a pound figure without a grade attached is not really a measurement — it is half of one.

It also explains why 250 gsm lands in the “heavy cardstock / cover” band on a plain-language scale: whatever grade it is sold under, a sheet that heavy behaves like sturdy card, and that behaviour — not the label — is what should guide your project.

Decoding the abbreviations on a listing

Paper listings compress the grade into a short code, and it helps to know what you are looking at. “T” or “Text” means text/book stock — the grade behind flyers, brochures, and most patterned craft paper. “C”, “Cover”, or sometimes “CS” (cardstock) means the cover grade used for card bases, invitations, and covers. “Index” is the stiff grade behind index and file cards. “Bristol” is a smooth board grade popular for illustration and technical drawing. “Tag” covers tags, tickets, and sturdy board stock, and plain “Bond” (sometimes “Writing” or “Ledger”) is ordinary copy and letterhead paper. Seeing “80 lb” on its own tells you almost nothing; seeing “80 lb Cover” or “80T” tells you exactly what shelf you are looking at.

Matching weight to feel, without a scale

Once you know the gsm, you can predict roughly how a sheet will behave even before you touch it. Under 50 gsm is tissue-weight — translucent, and it tears if you look at it sideways. From about 50 to 100 gsm is ordinary writing and copy weight, the paper in every office printer. From 100 to 150 gsm is heavy text, moving into light card — noticeably stiffer, the top end of what still folds easily by hand without scoring. From 150 to 250 gsm is properly cardstock territory: postcards, light card bases, and the point at which folding without scoring starts to crack the surface. From 250 to 350 gsm is heavy cardstock and cover weight, the range greeting-card bases live in, where scoring stops being optional. Above 350 gsm you are into board, which behaves less like paper and more like thin binder’s board — it needs deep scoring or a hinge rather than a simple fold. These bands are exactly the plain-language categories our paper weight converter reports alongside the numeric conversion, so you get both the figure and what it means in one look.

A full reference table, built from the same numbers

If you want to see this laid out for every common named weight at once — every “20 lb bond,” “80 lb cover,” and “110 lb index” you are likely to meet while shopping, each with its gsm figure, its plain-language category, and what it typically suits — our paper weight reference lays all of it out in one table, generated from the exact same conversion the calculator above uses, so the two never disagree.

Where a manufacturer's own figure can drift from the formula

Run the formula yourself and you may occasionally land a pound or two away from what a manufacturer prints on their own spec sheet, and that is normal rather than a sign you did the arithmetic wrong. Basis weight is officially defined against an exact ream of exactly 500 sheets at an exact basis size, but real production runs are not laboratory-perfect: a mill’s actual output varies a little sheet to sheet, and companies round their published figure to a tidy number rather than publishing the true average to one decimal place. Treat a converted figure as accurate to within a pound or a couple of grams per square metre, which is close enough for every craft use in this guide, and reach for the physical sheet itself — not the label — whenever an exact thickness genuinely matters, such as fitting stock into a printer’s specified weight range. If a project genuinely depends on an exact figure — matching an existing print run precisely, say — ask the supplier for their measured value rather than relying on the printed nominal weight alone; most paper merchants can provide it on request, and it will always be more reliable for that one specific batch than a converted estimate.

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.