To convert US basis weight in pounds to gsm, multiply by a factor that depends on the paper grade: 3.760 for bond, 1.480 for text, 2.704 for cover and 1.627 for kraft. So 80 lb text is 118.4 gsm and 20 lb bond is 75.2 gsm. There is no single lb-to-gsm factor, because a pound of basis weight is defined against a different sheet size for each grade. The converter below applies the right factor for the grade you choose.
Outside North America, paper is quoted in gsm (grams per square metre, also called grammage and measured to ISO 536). For grades and what they are used for, see packaging paper grades explained.
Why does the lb to gsm factor change with the grade?
US basis weight is the weight in pounds of a ream of paper cut to the grade's standard basis sheet size. Bond is weighed as 500 sheets of 17 x 22 inches, text as 500 sheets of 25 x 38 inches, and cover as 500 sheets of 20 x 26 inches. A bigger basis sheet makes the pound figure bigger for the same paper, so 20 lb bond and 50 lb text are nearly the same paper (75.2 gsm and 74.0 gsm).
The conversion is: gsm = lb × 453.592 ÷ ream area in m², where 453.592 is grams per pound. Kraft, bag, newsprint and tag papers use a ream of 3000 square feet. Linerboard and corrugating medium are quoted in pounds per 1000 square feet (MSF) instead of per ream.
Conversion factors by grade
| Grade (US name) | Basis size | Ream area (m²) | gsm = lb × factor |
|---|---|---|---|
| Bond / Writing | 17 x 22 in, 500 sheets | 120.64 | 3.760 |
| Text / Book / Offset | 25 x 38 in, 500 sheets | 306.45 | 1.480 |
| Cover | 20 x 26 in, 500 sheets | 167.74 | 2.704 |
| Bristol | 22.5 x 28.5 in, 500 sheets | 206.85 | 2.193 |
| Index | 25.5 x 30.5 in, 500 sheets | 250.89 | 1.808 |
| Tag | 24 x 36 in, 500 sheets (3000 sq ft) | 278.71 | 1.627 |
| Kraft / Bag / Newsprint | 3000 sq ft ream | 278.71 | 1.627 |
| Linerboard / Corrugating medium | lb per 1000 sq ft (MSF) | 92.90 | 4.882 |
To go the other way, divide gsm by the factor: lb = gsm ÷ factor. Tag and kraft share a factor because 24 x 36 inches x 500 sheets is also 3000 square feet. Linerboard is a weight per 1000 square feet, not per ream, so its factor is larger.
Common lb weights converted to gsm
| Grade | Common weights converted to gsm |
|---|---|
| Bond / Writing | 16 lb = 60.2 gsm ; 20 lb = 75.2 gsm ; 24 lb = 90.2 gsm ; 28 lb = 105.3 gsm ; 32 lb = 120.3 gsm |
| Text / Book / Offset | 50 lb = 74.0 gsm ; 60 lb = 88.8 gsm ; 70 lb = 103.6 gsm ; 80 lb = 118.4 gsm ; 100 lb = 148.0 gsm |
| Cover | 65 lb = 175.8 gsm ; 80 lb = 216.3 gsm ; 100 lb = 270.4 gsm ; 110 lb = 297.5 gsm |
| Bristol | 67 lb = 146.9 gsm ; 80 lb = 175.4 gsm ; 100 lb = 219.3 gsm ; 110 lb = 241.2 gsm |
| Index | 90 lb = 162.7 gsm ; 110 lb = 198.9 gsm ; 140 lb = 253.1 gsm |
| Tag | 100 lb = 162.7 gsm ; 125 lb = 203.4 gsm ; 150 lb = 244.1 gsm |
| Kraft / Bag / Newsprint | 30 lb = 48.8 gsm ; 40 lb = 65.1 gsm ; 50 lb = 81.4 gsm ; 60 lb = 97.6 gsm ; 70 lb = 113.9 gsm ; 80 lb = 130.2 gsm |
| Linerboard / Corrugating medium | 26 lb = 126.9 gsm ; 33 lb = 161.1 gsm ; 42 lb = 205.1 gsm ; 69 lb = 336.9 gsm ; 90 lb = 439.4 gsm |
GSM to lb lookup table
| gsm | Bond lb | Text lb | Cover lb | Kraft / Tag lb | Liner lb/MSF |
|---|---|---|---|---|---|
| 40 | 10.6 | 27.0 | 14.8 | 24.6 | 8.2 |
| 50 | 13.3 | 33.8 | 18.5 | 30.7 | 10.2 |
| 60 | 16.0 | 40.5 | 22.2 | 36.9 | 12.3 |
| 70 | 18.6 | 47.3 | 25.9 | 43.0 | 14.3 |
| 80 | 21.3 | 54.0 | 29.6 | 49.2 | 16.4 |
| 90 | 23.9 | 60.8 | 33.3 | 55.3 | 18.4 |
| 100 | 26.6 | 67.6 | 37.0 | 61.4 | 20.5 |
| 120 | 31.9 | 81.1 | 44.4 | 73.7 | 24.6 |
| 150 | 39.9 | 101.3 | 55.5 | 92.2 | 30.7 |
| 170 | 45.2 | 114.9 | 62.9 | 104.5 | 34.8 |
| 200 | 53.2 | 135.1 | 74.0 | 122.9 | 41.0 |
| 250 | 66.5 | 168.9 | 92.5 | 153.6 | 51.2 |
| 300 | 79.8 | 202.7 | 110.9 | 184.3 | 61.4 |
| 350 | 93.1 | 236.5 | 129.4 | 215.1 | 71.7 |
Values are rounded to one decimal. Use the column that matches the grade named in the specification.
Worked examples
80 lb text to gsm. 80 × 1.480 = 118.4 gsm. Text weights are often rounded to 118 gsm in catalogues.
Kraft paper lb to gsm. A 50 lb kraft is 50 × 1.627 = 81.4 gsm. For comparison, the paper bag GSM guide says machine-made kraft carrier bags commonly use paper of about 70 to 130 gsm, which is about 43 to 80 lb on the 3000 sq ft basis.
Linerboard. A 42 lb liner means 42 lb per 1000 sq ft, which is 42 × 4.882 = 205.1 gsm.
100 gsm in US terms. 100 gsm is 26.6 lb bond, 67.6 lb text and 61.4 lb kraft. Always ask which basis the lb figure refers to.
Can you convert gsm to caliper?
No. Caliper is thickness, and thickness depends on how bulky the sheet is, not on weight alone. The relation is: caliper (µm) = gsm × bulk (cm³/g). Bulk is a property of the grade and finish, and many uncoated packaging papers fall roughly between 1.2 and 1.6 cm³/g, while dense papers such as glassine are lower and folding boxboard is higher. The table shows the effect: the same 100 gsm gives 100 to 160 µm across that bulk range. Take caliper from the mill's data sheet. The measurement terms are explained in packaging paper grades explained.
| gsm | Bulk 1.0 cm³/g | Bulk 1.2 cm³/g | Bulk 1.4 cm³/g | Bulk 1.6 cm³/g |
|---|---|---|---|---|
| 60 | 60 µm | 72 µm | 84 µm | 96 µm |
| 80 | 80 µm | 96 µm | 112 µm | 128 µm |
| 100 | 100 µm | 120 µm | 140 µm | 160 µm |
| 120 | 120 µm | 144 µm | 168 µm | 192 µm |
| 170 | 170 µm | 204 µm | 238 µm | 272 µm |
| 250 | 250 µm | 300 µm | 350 µm | 400 µm |
What to write in a specification
- Quote gsm as the main figure, and add the lb figure with its grade in brackets if your team uses it, for example "118 gsm (80 lb text)".
- Add caliper in µm when stiffness or machine feeding matters.
- Say which grade the lb figure belongs to. "80 lb" alone is ambiguous between text and cover.
- Check the gsm for carrier bags against the load and construction advice in the paper bag GSM guide.