Paper weights, and the table nobody agrees on
Two paper systems, one of them measuring a quantity that changes depending on what the paper is for. Here is the conversion table, and the honest reason it can never be exact for every sheet on it.
Ask for "80 pound paper" and you have not said anything until you also say what kind. The American basis-weight system weighs a ream of paper at its basis size, and the basis size is different for every grade. Text stock is measured at 25 × 38 inches, cover stock at 20 × 26, index at 25.5 × 30.5. So 80 lb text and 80 lb cover are not close: the cover is roughly twice the substance.
The metric system does not have this problem. Grams per square metre is grams per square metre, whatever the sheet is for.
| Grade | Basis size (in) | 80 lb in gsm | 100 lb in gsm |
|---|---|---|---|
| Text / book | 25 × 38 | 118 | 148 |
| Cover | 20 × 26 | 216 | 270 |
| Bristol | 22.5 × 28.5 | 163 | 203 |
| Index | 25.5 × 30.5 | 146 | 180 |
| Tag | 24 × 36 | 148 | 185 |
Important
When a printer and a designer disagree about paper, this table is usually the reason, and neither of them is wrong. Say gsm and the disagreement disappears.
Weight is not thickness
Two sheets at the same gsm can differ in thickness by a third, because bulk depends on how much air is left in the sheet. An uncoated antique stock is bulky; a coated gloss sheet at the same weight is thin and dense, because calendering has pressed the air out.
That ratio is the bulk, and it is what actually decides how thick the finished book is:
| Stock | 120 gsm, 200 leaves | Feel |
|---|---|---|
| Coated gloss | 18 mm | thin, slick, heavy in the hand |
| Uncoated wove | 24 mm | the default trade book |
| Antique / bulky | 30 mm | a thin book made to look substantial |
Show-through is the number that matters for text
A text block printed on both sides of a light sheet reads through, and the ghost of the reverse side sits between the lines. Below about 90 gsm on an uncoated stock it is visible on any page with an illustration; below 70 it is visible on plain text.
Tip
Ask for a dummy — an unprinted copy bound to the exact page count and stock — before committing a run. It costs almost nothing and it is the only way to know what the thing will feel like. Every number above is a prediction; the dummy is the measurement.
The one that catches people out
Paper has a grain direction, the way the fibres lie. A book must be bound with the grain running parallel to the spine, or the pages fight the fold, refuse to lie flat, and warp along the edge as humidity changes. It is invisible in a spec sheet, it costs nothing to get right, and it is unfixable once printed.