What is GSM in paper?
GSM is grammage: grams per square meter. It describes mass per unit area and is more directly comparable across paper categories than U.S. basis-weight labels.
GSM is universal; “lb” depends on paper category
GSM is grams per square metre. U.S. basis weight is the weight of a 500-sheet basis ream whose sheet dimensions depend on the paper category. That is why an 80 lb text sheet and an 80 lb cover sheet are not the same stock.
| Category | Basis size used by the converter |
|---|---|
| Bond / writing | 17 × 22 in |
| Text / book | 25 × 38 in |
| Cover | 20 × 26 in |
| Index | 25.5 × 30.5 in |
Use the numbers from your actual job
The guide explains the decision; the linked calculator gives you the value for your own file or production assumptions.
GSM measures mass per unit area
GSM means grams per square metre. Because it is based on a square metre, the value describes the paper’s mass per area regardless of the finished sheet size.
GSM is not thickness
Caliper measures physical thickness. Coating, fiber, compression and bulk can make two papers with the same GSM have different thicknesses and stiffness.
U.S. basis pounds require a paper category
Bond/writing, text/book, cover and index use different basis-sheet dimensions. A pound value therefore cannot be converted accurately to GSM without knowing the category.
Use the measurement that controls the decision
Use GSM for cross-category mass-per-area comparisons, caliper for thickness/binding constraints, and manufacturer stiffness or machine specifications when feedability is the real concern.
Practical answer
GSM is grammage: grams per square meter. It describes mass per unit area and is more directly comparable across paper categories than U.S. basis-weight labels.
When the decision affects a paid print run, book upload, client order or expensive material, keep the calculation with the job notes and verify any external specification again immediately before production.