Paper Weight Converter (GSM and lb)

Paper weight is quoted in grams per square metre almost everywhere except North America, where it is given in pounds. The trouble is that a pound figure alone is not a weight per area. It is the weight of a ream of 500 sheets of the grade’s own standard sheet size, and each grade uses a different size. A printer in Chicago who asks for 80 lb cover and a designer in Berlin who specifies 250 gsm may be asking for nearly the same sheet, but only a conversion that knows the grade can show it.

Why gsm is simpler

Grams per square metre is a direct measure of weight per area, so it compares any two papers regardless of size or type. A 120 gsm sheet is twice as heavy per area as a 60 gsm sheet. Pounds depend on the basis size, which is not the size you buy: bond paper is measured at 17 by 22 inches although it is sold as letter or legal sheets. That is why the grades need separate conversion factors.

The formula

A ream is 500 sheets of the basis size. Its weight in pounds is converted to grams with 453.59, then divided by the total area of those sheets in square metres. For bond, 17 × 22 inches is 374 square inches, or 0.2413 m². So 20 lb bond is 20 × 453.59 ÷ (500 × 0.2413) = 75.2 gsm. The page applies the same formula to each grade and shows the answer to one decimal.

A worked example

Press Sample for 80 gsm in the bond grade. The page shows 21.3 lb and the result line 80 gsm = 21.3 lb bond. The table below converts the same paper to every grade: 21.3 lb bond, 54 lb text, 29.6 lb cover, 44.2 lb index, 36.5 lb bristol and 49.2 lb tag. Switch to “pounds to gsm” and enter 20 with the bond grade: the result is 75.2 gsm. With the cover grade, 20 lb is 54.1 gsm, and with the text grade, 20 lb is 29.6 gsm. Those three answers for the same number show the problem.

Reading the table

The grade names mirror trade usage. Bond, writing and ledger are office and letterhead papers. Text, book and offset papers are used for books, brochures and flyers. Cover is the heavier stock for covers and cards. Index, bristol and tag are stiff boards for index cards, art and signs. When a US supplier says 100 lb text, divide by the text factor: 100 lb text is about 148 gsm, a typical brochure weight, while 100 lb cover is about 270 gsm, a postcard stock.

Practical use

When you move a job between countries, convert from the grade the printer names and not from a generic table. For proofs and quotations quote both, such as 80 lb text (118 gsm). Remember that weight does not give thickness or stiffness: coated gloss and uncoated paper of the same gsm differ in caliper, which matters for a book spine, and the book spine calculator takes the type into account.

Edge cases

Mills round their labels, so a sheet sold as 20 lb bond may be 75 gsm on the pack, 75.2 by exact conversion, or 20 lb with a slightly different measured weight. Weights outside 1 to 2000 are not accepted. The table follows the exact figure you enter, so converting an already rounded number gives a rounded result.

Limits

The tool converts weights using the six grades listed. It does not cover Japanese kg ream weights, newsprint grades or label stock. Everything is computed in your browser.

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