Hong Life CalculatorsUnderstanding the paper weight converter — concept and principle
This converter turns US paper "pound" weights into grammage (g/m²) and between the US paper grades: bond, text, cover, index, bristol and newsprint. It helps when a US printer quotes "20 lb bond" or "80 lb cover" and you think in g/m², or when comparing papers across grades.
How it is calculated
- US basis weight = weight in pounds of 500 sheets of the grade's basic sheet size.
- g/m² = lb × 453.59237 ÷ (500 × sheet area in m²).
- Basic sizes and factors: bond 17 × 22 in (3.760), text/book 25 × 38 in (1.480), cover 20 × 26 in (2.705), index 25.5 × 30.5 in (1.806), bristol and tag 22.5 × 28.5 in (2.192), newsprint 24 × 36 in (1.627).
- Other grades = g/m² ÷ their factor.
The principle
Each US paper grade has its own basic sheet size, chosen long ago for its typical use. A ream of 500 basic sheets is weighed, so the same paper gets a different "lb" number in each grade: 20 lb bond and 50 lb text are the same paper, because text's basic sheet is much larger. Grammage avoids the confusion by using mass per square metre, the same for every grade. Converting is a matter of dividing the ream weight by the ream's area.
Worked example
Default inputs: 20 lb bond.
- Bond sheet area = 17 × 22 in = 374 in² = 0.2413 m²; 500 sheets = 120.6 m².
- g/m² = 20 × 453.59 ÷ 120.6 = 75.2 g/m².
- Text = 75.2 ÷ 1.480 = 50.8 lb; cover = 75.2 ÷ 2.705 = 27.8 lb.
- Index = 41.6 lb; bristol and tag = 34.3 lb; newsprint = 46.2 lb.
| Paper weight | Grammage (g/m²) |
|---|---|
| 5 | 18.8 |
| 10 | 37.6 |
| 15 | 56.4 |
| 20 | 75.2 |
| 25 | 94 |
| 30 | 112.8 |
| 40 | 150.4 |
How to read the result
Compare papers using g/m²; it is the international measure used everywhere outside the US. Common office paper of 75–80 g/m² is about 20–21 lb bond. Remember that grammage measures weight, not stiffness or thickness: a dense paper can be thinner than a bulky one of the same grammage.
Common mistakes
- Comparing lb figures from different grades directly.
- Treating "lb" as the weight of one sheet.
- Assuming higher grammage always means a thicker sheet.
- Forgetting that some suppliers round conversions.
Frequently asked questions
What is 20 lb paper in gsm?
20 lb bond is about 75 g/m².
Why is 20 lb bond the same as 50 lb text?
The two grades use different basic sheet sizes, so the same paper has different pound numbers.
What is 80 lb cover in gsm?
About 216 g/m² (80 × 2.705).
Sources: Valmet, "Table of ream sizes for common paper, board and tissue grades" (2018) · NIST Special Publication 811 (2008), Appendix B (1 lb = 453.59237 g; 1 in = 25.4 mm)