Converts paper weight between gsm and pounds using the correct parent sheet for each grade, and shows what the same figure means in all of them.
A pound figure is meaningless without the grade
Basis weight is the weight of five hundred sheets of a paper at its parent size. The trap is that the parent size is not the same for every kind of paper. Text and book stock is 25 × 38 inches; cover is 20 × 26; bond is 17 × 22; index is 25.5 × 30.5.
So 80 lb text is 118 gsm and 80 lb cover is 216 gsm. Same number, nearly twice the paper. Index, bristol, tag and bond all give different answers again — six distinct papers sharing one label.
The mirror of this is just as confusing. 20 lb bond and 50 lb text are the same sheet, about 75 gsm — it is ordinary office paper, sold under two names depending on which counter you buy it at.
The constant is 1406.1392, and the charts use 1406.5
The conversion is gsm = lb × 1406.1392 ÷ (parent width × parent height in inches), and that constant is not arbitrary. It falls straight out of 453.59237 grams per pound, five hundred sheets to the ream, and 0.00064516 square metres per square inch.
The figure the trade uses is 1406.5. It is wrong — it implies an inch of 25.3967 mm, and an inch is 25.4 mm by definition. The discrepancy is 0.0257 per cent, which sounds negligible until you round: it flips about one whole-gsm answer in eleven by exactly one gram.
This is why a merchant's printed chart and an online calculator disagree on a handful of values, and both look authoritative. This converter uses the exact constant and tells you when the answer is one of the ones where the trade constant would have differed.
A grade you are told does not exist
PrintNinja's own page on paper weight — a page titled around weight, coating and parent sheet — states that there is no equation to convert paper types to weight. That is not true. There is an exact equation; it needs one more input than people expect, which is the grade.
The reason the claim persists is that the question is usually asked without the grade, and without the grade it genuinely has no single answer. That is a different thing from having no equation.
Weight still will not tell you thickness
Grammage measures mass per unit area, and thickness is a separate property. Three text stocks at 118 gsm can differ by more than fifty per cent in bulk depending on how heavily they are calendered.
For anything where thickness matters — a book spine, a folded piece, a box — you need the caliper or the pages-per-inch figure, which the mill publishes separately. The spine calculator on this site takes that figure directly, and it is the only one I have found that does.
Asked often
What does 80 lb paper mean?
It depends entirely on the grade. 80 lb text is 118 gsm, 80 lb cover is 216 gsm, 80 lb index is 145 and 80 lb bond is 301. The converter shows all of them at once.
Why do converters disagree by 1 gsm?
Most use the trade constant 1406.5, which implies an inch of 25.3967 mm. The exact constant is 1406.1392. The difference flips roughly one whole-gsm answer in eleven.
Is heavier paper thicker?
Not reliably. Grammage is mass per area. Three 118 gsm text stocks can differ by over 50% in thickness depending on calendering.