Unit & Measurement Tables
Paper Weight Table
Paper weight is the one measurement where the unit does not determine the quantity. A pound of paper is 500 sheets at a size the paper industry chose for that grade, and the sizes differ: bond uses 17 × 22 inches, text uses 25 × 38, cover uses 20 × 26. So 100 lb of text paper and 100 lb of cover stock are not the same paper — they are not even close, differing by 82% in actual weight per square metre.
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That is why the metric system won here. Grams per square metre is an absolute measurement: it says how much one square metre of the sheet weighs, regardless of what the mill decided a ream should be. A 120 gsm sheet is 120 gsm whether it is a book page, a brochure or a business card. The pound system cannot say that, because the same number carries a different meaning per grade.
The conversion factors are not arbitrary either. Each is the weight of one pound divided by the area of 500 sheets at that grade's basis size. Bond at 17 × 22 inches gives 3.7597 gsm per pound; text at 25 × 38 gives 1.4801; cover at 20 × 26 gives 2.7041. Multiply the basis weight by the factor for its grade and you have the metric equivalent.
The practical consequence is that paper specifications are ambiguous unless the grade is stated. When a printer asks for 100 lb stock, the correct response is to ask which kind — because 100 lb text is a sturdy book page, 100 lb cover is a business card, and 100 lb bristol is something in between. The tables below give all three side by side so the difference is visible.
Basis sizes and the lb-to-gsm factor for each paper type
| Paper type | Basis size (in) | Basis area (m²) | lb to gsm factor |
|---|---|---|---|
| Bond / writing | 17.00 × 22.00 | 0.2413 | 3.7597 |
| Text / book | 25.00 × 38.00 | 0.6129 | 1.4801 |
| Cover | 20.00 × 26.00 | 0.3355 | 2.7041 |
| Index | 25.50 × 30.50 | 0.5018 | 1.8080 |
| Bristol | 22.50 × 28.50 | 0.4137 | 2.1928 |
A 500-sheet ream of 20 lb bond weighs 20 lb at 17 × 22 inches. The same 20 lb at 25 × 38 inches covers twice the area, which is why text paper needs a different factor.
Pounds to grams per square metre, by paper type
| Pounds | Bond / writing | Text / book | Cover | Index | Bristol |
|---|---|---|---|---|---|
| 20 | 75.2 | 29.6 | 54.1 | 36.2 | 43.9 |
| 24 | 90.2 | 35.5 | 64.9 | 43.4 | 52.6 |
| 28 | 105.3 | 41.4 | 75.7 | 50.6 | 61.4 |
| 32 | 120.3 | 47.4 | 86.5 | 57.9 | 70.2 |
| 50 | 188.0 | 74.0 | 135.2 | 90.4 | 109.6 |
| 60 | 225.6 | 88.8 | 162.2 | 108.5 | 131.6 |
| 70 | 263.2 | 103.6 | 189.3 | 126.6 | 153.5 |
| 80 | 300.8 | 118.4 | 216.3 | 144.6 | 175.4 |
| 100 | 376.0 | 148.0 | 270.4 | 180.8 | 219.3 |
| 110 | 413.6 | 162.8 | 297.5 | 198.9 | 241.2 |
| 120 | 451.2 | 177.6 | 324.5 | 217.0 | 263.1 |
The same number means different paper. 100 lb is 148 gsm as text, 219 gsm as bristol and 270 gsm as cover — a difference of 82%.
Grams per square metre back to pounds, by paper type
| gsm | Bond / writing | Text / book | Cover | Index | Bristol |
|---|---|---|---|---|---|
| 70 | 18.6 | 47.3 | 25.9 | 38.7 | 31.9 |
| 80 | 21.3 | 54.0 | 29.6 | 44.2 | 36.5 |
| 90 | 23.9 | 60.8 | 33.3 | 49.8 | 41.0 |
| 100 | 26.6 | 67.6 | 37.0 | 55.3 | 45.6 |
| 120 | 31.9 | 81.1 | 44.4 | 66.4 | 54.7 |
| 150 | 39.9 | 101.3 | 55.5 | 83.0 | 68.4 |
| 170 | 45.2 | 114.9 | 62.9 | 94.0 | 77.5 |
| 200 | 53.2 | 135.1 | 74.0 | 110.6 | 91.2 |
| 250 | 66.5 | 168.9 | 92.5 | 138.3 | 114.0 |
| 300 | 79.8 | 202.7 | 110.9 | 165.9 | 136.8 |
| 350 | 93.1 | 236.5 | 129.4 | 193.6 | 159.6 |
80 gsm office paper is 21.3 lb bond, which is why the ream in the copier says 20 lb: the trade rounds to the nearest standard basis weight.
What the common weights are used for
| gsm | lb bond equivalent | Typical use |
|---|---|---|
| 60 to 70 | 16 to 19 | Newspaper, cheap paperbacks |
| 80 | 21 | Standard office paper, copiers, letters |
| 90 | 24 | Premium office paper, resumes |
| 100 to 120 | 27 to 32 | Book interiors, brochures |
| 150 to 170 | 40 to 45 | Flyers, posters, light card |
| 200 to 250 | 53 to 66 | Postcards, business cards |
| 300 to 350 | 80 to 93 | Heavy card, covers, packaging |
The bond column is approximate because bond is not the natural reference for heavy stock. For anything above 170 gsm the trade quotes card in gsm and cover in lb.
How to Use These Tables
- Converting a US spec to metric: identify the grade first, then multiply. 80 lb text is 80 × 1.4801 = 118 gsm.
- Converting metric to US: divide by the grade factor. 250 gsm cover is 250 / 2.7041 = 92.5 lb cover.
- Buying office paper: 20 lb bond and 75 gsm are the same paper, and 24 lb bond is 90 gsm. Both labels appear on the same ream.
- Specifying a business card: 300 to 350 gsm, which is 111 to 129 lb cover. A 100 lb cover card at 270 gsm is on the light side.
- Comparing book paper: a 118 gsm text sheet (80 lb) is standard for trade paperbacks; 148 gsm (100 lb) is a heavier stock used for art books.
Common Questions
Why is 100 lb paper not simply 100 pounds per something?
Because the pound refers to 500 sheets at a basis size that depends on the grade. Text paper is measured at 25 × 38 inches and cover at 20 × 26, so 100 lb means 148 gsm for text and 270 gsm for cover.
Is 80 gsm the same as 20 lb?
Almost. 20 lb bond works out to 75.2 gsm and 80 gsm is 21.3 lb bond, so they are the same grade of office paper with the trade rounding to standard basis weights.
What is the difference between lb text and lb cover?
Only the basis size used to define the pound. Cover uses 20 × 26 inches, which is smaller than text at 25 × 38, so a cover pound is worth 1.83 text pounds.
What weight is a business card?
Usually 300 to 350 gsm, which is 111 to 129 lb cover. Anything below 250 gsm feels flimsy for a card that gets handled.
Does higher gsm always mean better paper?
No. Weight measures mass per area, not quality. A 100 gsm sheet with good fibre and coating outperforms a 120 gsm sheet that is loosely formed, and brightness and opacity are separate specs.
What is caliper and how does it relate to gsm?
Caliper is thickness in thousandths of an inch, and it correlates with gsm but is not determined by it. A bulky uncoated sheet can be thicker than a coated sheet of the same weight.
More Unit & Measurement Tables
Sources
- ISO 536 — Paper and board, determination of grammage — International Organization for Standardization
- TAPPI T 410 — Grammage of paper and paperboard — Technical Association of the Pulp and Paper Industry
- NIST Handbook 130 — Uniform laws and regulations, paper labeling — National Institute of Standards and Technology
Filed with Unit Conversion calculations.