Understanding Paper Weight and Grammage
Paper weight is measured using different systems around the world, which can make comparing paper stocks difficult. The most straightforward measurement is grammage, expressed in grams per square metre (g/m², also written GSM). Grammage measures the actual mass of a single square metre of paper. Because a square metre is a constant area in every country and for every grade of paper, grammage allows you to compare any two papers directly.
In contrast, other regional systems rely on the weight of a bulk stack of sheets cut to a specific size. When you need to translate between these systems, the Paper Weight Converter performs the calculations to show how a single sheet of paper is described across different standards.
How US Basis Weight Works
The US basis weight system does not measure a single square metre. Instead, a US basis weight is the weight in pounds of 500 sheets (a standard ream) cut to that specific grade's basic size.
Because different types of paper are manufactured for different uses, each printing grade has its own unique basic size:
- Bond: 17 × 22 in (431.8 × 558.8 mm)
- Text: 25 × 38 in (635 × 965.2 mm)
- Cover: 20 × 26 in (508 × 660.4 mm)
- Bristol: 22.5 × 28.5 in (571.5 × 723.9 mm)
- Index: 25.5 × 30.5 in (647.7 × 774.7 mm)
- Tag / Newsprint: 24 × 36 in (609.6 × 914.4 mm)
Because these basic sizes vary so much, a sheet of "80 lb text" and a sheet of "80 lb cover" are entirely different papers. A cover sheet is measured using a much smaller total area than a text sheet. Consequently, the same physical paper will carry a much lower pound rating when classified as cover than when classified as text. For example, a standard 100 lb cover sheet is equivalent to 183 lb text, 150 lb index, 123 lb bristol, 166 lb tag, and 72 lb bond.
The Japanese Ream Weight System
The Japanese paper industry uses a ream weight system. A Japanese ream weight is the weight in kilograms of 1,000 sheets of paper cut to a named format.
The four primary Japanese formats are:
- Shiroku: 788 × 1091 mm
- Kiku: 636 × 939 mm
- A-ban: 625 × 880 mm
- B-ban: 765 × 1085 mm
While this system is conceptually similar to the US basis weight system, it uses a 1,000-sheet count and metric dimensions. Additionally, Japanese trade tables traditionally round the final ream weight to the nearest half kilogram. The Paper Weight Converter calculates and displays the unrounded value, which means the result may differ from a printed commercial table by up to 0.5 kg.
Mathematical Formulas and Constants
Every paper weight system relies on a single reference sheet size and sheet count. This allows a single master formula to convert any system back to grammage, where W is the weight in grams, N is the number of sheets, and A is the area of one reference sheet in square metres:
g/m² = W ÷ (N × A)
- Grammage: This is the base case where
N = 1andA = 1. - US Basis Weight: Calculated using 500 sheets (
N = 500) of a basic size measured in inches. Since 1 lb = 453.59237 g and 1 in = 0.0254 m exactly, the formula for a basic size ofw × hinches simplifies to:g/m² = lb × 1406.1392 ÷ (w × h)(Note: Paper suppliers sometimes round this constant to 1406.5, which is 0.026% high. This tool uses the exact physical constant.) - Japanese Ream Weight: Calculated using 1,000 sheets (
N = 1,000) of a format measured in metres. For a format ofw × hmetres, the formula simplifies to:g/m² = kg ÷ (w × h)
Reference Values for Common Papers
You can verify your calculations or check a supplier's paper stock against these standard equivalent values:
- 20 lb bond = 75.2 g/m²
- 80 lb text = 118.4 g/m²
- 65 lb cover = 175.8 g/m²
- 80 lb cover = 216.3 g/m²
- 100 lb cover = 270.4 g/m²
- 110 lb index = 198.9 g/m²
- 67 lb bristol = 146.9 g/m²
- 100 lb tag = 162.7 g/m²
- Shiroku 90 kg = 104.7 g/m²
Why Weight Cannot Determine Thickness
A common point of confusion is trying to convert paper weight directly into thickness (caliper). Caliper in points, mils, or microns, and pages per inch (PPI), describe how thick a sheet is, not how much it weighs.
Two papers with the exact same grammage can differ in thickness by 50% or more. This variation is caused by manufacturing processes like coating and calendering (pressing the paper between rollers), which change the paper's density and bulk. Because there is no mathematical factor that turns weight into thickness, you must read the caliper directly from the paper supplier's technical data sheet.
How to Use the Converter
To perform a conversion, enter a numeric value in the Weight field. The tool accepts decimals formatted according to your local settings, using either a dot (e.g., 118.4) or a comma (e.g., 118,4).
Select the starting system in the Written as dropdown and the target system in the Convert to dropdown. You can use the Swap the two systems button to reverse the two systems, or click "Load example" to populate the fields with a pre-loaded calculation.
If you select "A sheet you define", you can input custom parameters under Your reference sheet:
- Sheet width and Sheet height: Must be between 1 mm and 5,000 mm (0.04 in and 197 in).
- Sheets weighed: Must be a whole number between 1 and 100,000.
- Weight given in: Choose either lb or kg.
Defining a custom sheet with the dimensions 788 × 1091 mm, 1,000 sheets, and the weight in kg will yield the exact same results as selecting the Shiroku preset.
Processing and Limits
All calculations run locally in your browser. Nothing you type into the tool is uploaded to BroBroGo.
The tool accepts weights up to 100,000 in any system. Conversions run at full precision and are rounded to one decimal place for display, though values under 1 retain four digits. If the calculated grammage falls outside the standard printing range of 10 to 1000 g/m², the tool will display a warning but will still output the exact conversion without clamping the value.
Frequently Asked Questions
Why does one GSM value give a different pound number for each paper?
A US pound rating is the weight of 500 sheets cut to that grade’s basic size, and each grade has its own basic size: text is 25 × 38 in, cover only 20 × 26 in. A cover sheet is measured on far less area, so the same paper carries a much lower pound number as cover than as text. Grammage sidesteps this because a square metre is a square metre in every grade.
How do I know which grade my paper is?
It is whatever the supplier printed next to the number. “80 lb text” and “80 lb cover” are different papers, and a bare “80 lb” is ambiguous. Copier and letterhead stock is usually bond, brochures and book pages are text, business cards and folders are cover, tab dividers are index, and shipping tags and newsprint share the 24 × 36 in basic size.
Can I convert GSM to thickness in points or microns?
Not from the weight alone — the ratio between the two changes with every paper. Read the caliper in points, mils or microns straight off the stock’s data sheet, or use its pages-per-inch figure when you are working out how thick a book will be. With only a sample in hand, measure a stack of 100 sheets with a micrometer and divide by 100.
What is a ream weight and how does it relate to GSM?
Japanese mills quote a ream weight: the weight in kilograms of 1,000 sheets at a named sheet format such as Shiroku (788 × 1091 mm) or Kiku (636 × 939 mm). It is the same idea as a US pound rating with a different sheet count and format, so this converter treats it the same way. Trade tables round it to the nearest half kilogram; the value here is unrounded.