How this instrument works
Most of the world weighs paper the straightforward way: grammage, in grams per square meter (gsm), tells you directly how much a given area of the sheet weighs, independent of sheet size. The US paper trade instead uses 'basis weight' — the weight in pounds of a ream (500 sheets) of that paper cut to its grade's own traditional basic size — which sounds simple until you notice the basic size itself changes by paper type: Bond paper's basic size is 17×22 inches, Text/Book paper's is a larger 25×38 inches, Cover stock's is 20×26 inches, and so on for Index, Tag and Bristol grades.
That changing basic size is exactly why two papers with the same physical thickness and even the same gsm can carry very different basis-weight numbers on the label — a 20 lb Bond and a 50 lb Text can be nearly the same actual paper, because Text's much larger 25×38 basic sheet weighs proportionally more at 500 sheets even though each individual sheet is comparably thick. The basis weight number is a US printing-industry convention, not a direct physical measurement, and this calculator's job is converting between the two systems for a given grade.
The conversion itself is plain dimensional arithmetic, not an industry 'trust me' factor: gsm tells you grams per square meter, the basic sheet's area in square inches converts to square meters using the internationally exact 1 inch = 0.0254 meters, 500 sheets at that per-sheet weight gives the ream's total mass in grams, and dividing by the exact 453.59237 grams-per-pound conversion gives the answer in pounds — every step traceable back to an exact, defined unit conversion rather than an approximation.
- Enter the paper's Grammage (gsm, g/m²) — usually printed on the ream wrapper or spec sheet, especially for paper sourced or sold outside the US.
- Select the matching Paper grade (basic sheet size) — Bond/Writing, Text/Book, Cover, Index, Tag or Bristol — since each grade's basis weight is calculated against a different traditional basic sheet size.
- Read US basis weight (lb per 500-sheet ream) for the pounds figure that US print-shop specs and paper labels would quote for that same paper.
- Double-check the grade before comparing basis weight across paper types — the same gsm gives a different pound figure on Text/Book stock than on Bond, purely from the basic sheet size.
Worked example — 90 gsm manuscript paper on Text/Book stock
A print shop receives a manuscript specified at 90 gsm and needs the equivalent US basis weight for a Text/Book grade job, whose basic sheet size is 25×38 inches (950 square inches). Basis weight = 90 × 950 × 0.0254² × 500 ÷ 453.59237 ≈ 60.80 lb.
That comes out as a '60 lb Text' paper on a US spec sheet — a genuinely common weight for novels and manuscripts printed in the US, which is why 90 gsm (the everyday international standard for many book-interior stocks) and 60 lb Text end up being two labels for essentially the same paper.
Questions
Why does the same gsm give a different pound number depending on the grade I pick?
Because 'basis weight' isn't the weight of a fixed physical thing — it's the weight of 500 sheets cut to that specific grade's own traditional basic size, and the basic size varies by grade (374 in² for Bond, 950 in² for Text/Book, and so on). A larger basic sheet weighs more at 500 sheets even at identical gsm, so the same paper genuinely carries a different basis-weight number depending on which grade's convention you're measuring it against.
Is 20 lb Bond the same paper as ordinary 75-80 gsm copier paper?
Very close — 20 lb Bond corresponds to almost exactly 75 gsm under this formula (solving it in reverse), and everyday copier paper commonly sold internationally at 75 or 80 gsm is essentially the same product US suppliers label '20 lb.' It's one of the most commonly cited gsm-to-basis-weight equivalences in the printing trade for exactly this reason.
What are the six basic sheet sizes this calculator uses?
Bond/Writing at 17×22 in, Text/Book at 25×38 in, Cover at 20×26 in, Index at 25.5×30.5 in, Tag at 24×36 in, and Bristol at 22.5×28.5 in — these are the standard US printing-industry basic sizes each grade's basis weight is traditionally calculated against, and picking the wrong grade for a given paper will give a basis-weight figure that doesn't match what's printed on its actual US label.
Why does the US use basis weight instead of just grammage like most of the world?
Historical convention — the US paper industry standardized on ream weight (basis weight) well before gsm became the international norm, and the trade never fully switched over, so American paper specs, print-shop order forms and mill catalogs still commonly quote basis weight in pounds even as most of the rest of the world works in gsm. This calculator exists specifically to bridge that gap.
Is basis weight the same thing as paper thickness (caliper)?
No — basis weight (or gsm) measures mass per area, while caliper measures physical thickness per sheet, and the two aren't always proportional; a denser paper can be thinner than a fluffier paper of the same weight. This site's separate Paper Thickness Calculator handles caliper specifically, computed from a measured stack rather than from weight.