Why "80 lb paper" is not an answer
If a printer, a supplier or a client tells you a job is on "80 lb stock", you still know almost nothing. Eighty pound text is about 118 gsm — a decent brochure sheet, slightly heavier than good letterhead. Eighty pound cover is about 216 gsm — stiff card you could make a folder from. The number is the same. The paper is not remotely the same.
This happens because US basis weight measures 500 sheets of a paper's basic size, and the basic size is different for every grade. Bond is weighed at 17 × 22 inches. Text is weighed at 25 × 38. Cover at 20 × 26. These are historical mill dimensions — nobody sells finished paper at 25 × 38 inches, and you will never see a sheet that shape. The figure on the label is the weight of five hundred sheets of a size that exists only in a reference table.
Metric weight sidesteps every bit of this. Gsm is grams per square metre, and a square metre is a square metre regardless of grade, finish or the size the sheet is cut to. A 150 gsm paper is always heavier than a 120 gsm paper. There is nothing further to ask.
The formula
gsm = lb × 1406.5 ÷ (basic width × basic height in inches)
lb = gsm × (basic width × basic height) ÷ 1406.5
1406.5 is the constant that reconciles 500 sheets, square inches and grams per square metre. It falls out of the unit conversion — 1 lb = 453.59 g, 1 m² = 1550 in², so 453.59 × 1550 ÷ 500 = 1406.1, rounded to 1406.5 by convention in the trade.
The five basic sizes
| Grade | Basic size (in) | Area (in²) | Multiply lb by… |
|---|---|---|---|
| Bond / Writing / Ledger | 17 × 22 | 374 | 3.760 |
| Text / Book / Offset | 25 × 38 | 950 | 1.480 |
| Cover | 20 × 26 | 520 | 2.705 |
| Index | 25.5 × 30.5 | 777.75 | 1.808 |
| Tag | 24 × 36 | 864 | 1.628 |
| Bristol | 22.5 × 28.5 | 641.25 | 2.193 |
A worked example
Take 20 lb bond, the standard office copier paper in the United States. Bond's basic size is 17 × 22 inches, an area of 374 in².
gsm = 20 × 1406.5 ÷ 374 = 28130 ÷ 374 = 75.2 gsm
Which is exactly right — office copier paper in metric countries is sold as 75 or 80 gsm. Now try 80 lb cover, using cover's 20 × 26 basic size, an area of 520 in²:
gsm = 80 × 1406.5 ÷ 520 = 112520 ÷ 520 = 216.4 gsm
And 80 lb text, on the 25 × 38 basic size, 950 in²:
gsm = 80 × 1406.5 ÷ 950 = 112520 ÷ 950 = 118.4 gsm
Same "80 lb", 216 gsm versus 118 gsm. That is the whole problem in three lines of arithmetic.
What each weight actually feels like
| gsm | Feel | Typical use |
|---|---|---|
| 35–55 | Very thin, translucent | Newsprint, airmail, tracing paper |
| 60–75 | Light, slightly see-through | Budget copier paper, paperback interiors |
| 80–100 | Standard office feel | Copier paper, everyday documents |
| 100–120 | Noticeably better, opaque | Letterhead, CVs, quality stationery |
| 120–170 | Light card, holds a fold | Flyers, single-sheet brochures, posters |
| 170–250 | Definite card, stiff | Folded brochure covers, menus, tags |
| 250–350 | Rigid card | Postcards, business cards, softcover book covers |
| 350–450+ | Heavy board | Premium business cards, luxury packaging |
Weight is not thickness
Two sheets at identical gsm can measure noticeably different thicknesses. The variable is bulk — how much air the fibre structure traps — and it is controlled by the pulp, the coating and how hard the paper was pressed at the mill. A coated gloss sheet is calendered flat and dense; an uncoated bulky novel paper of the same weight is visibly thicker and feels softer.
Publishers use this deliberately. Choosing a high-bulk stock lets a 180-page novel present a respectable spine instead of looking like a pamphlet. It is also why the spine width calculator asks for a paper stock rather than a gsm figure: spine width depends on caliper, and caliper cannot be derived from weight alone.
The thickness estimate our converter shows is a rough guide based on typical bulk factors — roughly 1.3 for uncoated and 0.9 for coated stock. Treat it as an indication only. For anything where thickness matters, ask your supplier for the actual caliper or pages-per-inch of the specific stock.
Which grade should you ask for?
- Bond / writing — letterheads, forms, office documents. Sold 16 to 32 lb.
- Text / book / offset — book interiors, brochures, flyers. Sold 50 to 100 lb.
- Cover — book covers, business cards, postcards, folders. Sold 60 to 130 lb.
- Index — index cards, dividers, sturdy but uncoated. Sold 90 to 140 lb.
- Tag — hang tags, tickets, strong and tear-resistant. Sold 100 to 250 lb.
- Bristol — cards, covers, technical drawing board. Sold 67 to 140 lb.
When ordering, the safest habit is to give both figures: "100 lb text, that's 148 gsm." It removes all ambiguity, and any printer worth using will appreciate that you know the difference.