Paper weight explained: gsm, pounds and thickness

Paper weight is one of the numbers printed on packets of art and photo paper. It tells you how much a set area of the paper weighs, which gives a first idea of how thick the sheet will be. This page explains the two systems in use, grams per square metre and pounds, how to convert between them, and how weight relates to thickness. It belongs to the guide to art prints and paper, which covers the other choices behind a print.

Grams per square metre

Most of the world gives paper weight as grams per square metre, written g/m² and often shortened to gsm. It is the mass of a single sheet one metre wide and one metre long. Markus Kuhn, who maintains a guide to international paper sizes at the University of Cambridge, says this figure is commonly called grammage in both English and French.

Grammage works the same for any size of sheet. Kuhn's guide gives an example. A0 paper has an area of one square metre, and A4 is one sixteenth of A0. Ordinary office printer paper weighs 80 g/m², so one A4 sheet weighs 5 grams.

The same method works for a print. A 24 × 36 inch sheet has an area of 0.557 square metres, because one inch is exactly 25.4 millimetres, as the National Institute of Standards and Technology confirms. At 200 g/m², a sheet that size would weigh 111.5 grams. At 300 g/m², it would weigh 167.2 grams.

The pound system

In North America, many papers are sold by basis weight in pounds. Basis weight is the mass of 500 sheets, called a ream, at a basic size. Kuhn's guide points out the difficulty: to understand the number, you need to know the number of sheets and the width and height of the basic sheet, and these change with the type of paper.

The US Government Publishing Office sets the paper standards for federal printing. Its specification lists the basic sizes for reams of 500 sheets:

The last column is a calculation. It divides the mass of one pound, 453.59237 grams, by the area of 500 basic sheets. The National Institute of Standards and Technology gives the pound as exactly 0.45359237 kilograms in Handbook 44.

Why "80 lb" can mean two papers

Because the basic size changes, the same number of pounds describes different papers in different grades. Here are the results for common labels:

• 20 lb bond = 75.2 g/m²
• 80 lb text = 118.4 g/m²
• 110 lb index = 198.9 g/m²
• 80 lb cover = 216.3 g/m²

So 80 lb cover paper has 1.83 times the grammage of 80 lb text paper. Kuhn's guide gives a second warning. For newsprint, weighed as 500 sheets of 24 × 36 inches, each pound equals 1.63 g/m², so 20 lb newsprint is a lighter paper than 20 lb bond.

When a paper packet gives both figures, trust the g/m² figure for comparing papers. When it gives only pounds, find the grade first, then convert with the table above.

Weight and thickness

Weight and thickness are separate measurements. Thickness is also called caliper. In North America it is often given in mils. The National Institute of Standards and Technology says one mil is exactly 0.001 inch, or 0.0254 millimetres, and that the mil is used by some US industry sectors for measuring thickness.

The Government Publishing Office gives both basis weight and thickness for every grade it buys. Within one grade, a heavier paper is thicker. Its uncoated text paper runs:

Across grades, the finish changes the picture. At the same 80 lb basis weight, the office lists:

• uncoated text at 6.3 mils,
• matte-coated text at 4.9 mils,
• gloss-coated text at 3.6 mils.

The gloss-coated sheet carries the same mass in 57 percent of the thickness of the uncoated one. The office defines density as weight divided by thickness, and a coated paper is denser. For a buyer, this means a coated photo paper can be thinner than an uncoated art paper of the same grammage. The page on matte and glossy paper explains how those coatings are built.

Heavier grades: cover and index

Art and photo papers are heavier than text paper, so the cover and index grades give a closer comparison. The Government Publishing Office lists these thicknesses at 80 lb cover weight, which is 216.3 g/m² for every cover paper below:

The uncoated laid-finish cover is the thickest of the five at the same weight, and each coated cover is thinner than the plain uncoated cover. At one weight, the five papers span 7.0 to 9.6 mils. So two papers with the same grammage can differ in thickness depending on their surface, and a packet that lists thickness tells you more than one that lists weight alone.

Index paper is weighed on a different basic size again. The office lists index paper at 110 lb as 9.0 mils thick. That is 198.9 g/m² by the conversion above. Its 100 percent cotton index paper is listed at 125 lb and 10 mils, which is 226.0 g/m².

Why grammage is easier to use

Kuhn's guide explains why the metric system uses grammage. In the ISO 216 system, the long side of every A sheet is 1.4142 times the short side, so the sides cannot all be round numbers of millimetres. The standard instead gives the sheets round areas: A0 is one square metre, A1 half of that, and so on. Because paper is sold by g/m², the mass of any stack of sheets follows from a single multiplication.

Kuhn's guide gives a postal example: at 80 g/m², 16 A4 pages weigh 80 grams, and with an envelope estimated at 20 grams the letter reaches the 100 gram limit for the next postal fee. In the pound system the same sum needs the ream size of the grade first.

How paper weight is tested

The Government Publishing Office names the test methods its suppliers must use. They come from TAPPI, the technical association of the pulp and paper industry. Basis weight is measured by method T-410 and thickness by method T-411. For density, the office measures thickness in centimetres by T-411 and weight in grams per square centimetre by T-410, then divides one by the other.

Its specifications also allow a tolerance on weight. For many grades it allows 5 percent above or below the stated basis weight. So two packets with the same label can differ by a few percent.

Reading an art paper packet

Paper weight is one line among several on a packet of art or photo paper. Read it together with the others:

1. Grammage in g/m², the clearest figure for comparing papers.
2. Thickness, in mils, microns or millimetres, if the maker gives it.
3. Surface, such as matte, glossy or textured.
4. Fibre, such as cotton or wood pulp. The page on archival paper and inks explains what makes paper last.
5. Coating and ink, such as a porous coating for pigment ink.

Weight describes mass per area only, so ask about each of the other lines as well.

What this means for a royal pet portrait

Purrealism art prints come in four sizes, 8 × 10, 12 × 16, 18 × 24 and 24 × 36 inches, on matte or glossy paper, and you choose them in the portrait studio. The larger the sheet, the more it matters how it is handled before framing. A 24 × 36 inch sheet has 10.8 times the area of an 8 × 10 sheet, so at the same grammage it weighs 10.8 times as much. The page on storing unframed prints explains how to support a large sheet.

If you choose the digital download and print it yourself, the page on printing a digital download covers what to ask a print shop, including the paper. To see finished portraits before choosing, visit the gallery.