Paper Weight Converter

A pound of paper is not a pound of paper

US paper is sold by basis weight: the weight in pounds of five hundred sheets at the paper’s basis size. The catch is that the basis size is different for every grade, and the grade is not part of the number.

A spread of 2.5× from a single number — the difference between flimsy letterhead and stiff card. Which is why “65 lb paper” without a grade is not a specification, and why this converter will not answer without one.

The logic is consistent once seen. Text stock is counted over a 950 in² sheet and cover over a 520 in² one, so the same physical paper accumulates a bigger pound figure as text than as cover. The number is not describing the paper; it is describing a ream of a particular sheet.

Two grades even share a basis size — tag and newsprint are both 24 × 36″ — so the basis size does not identify the paper either. Only the grade name and the number together mean anything.

The conversion constant, 1406.139, is exact rather than measured: the pound has been exactly 453.59237 g and the inch exactly 25.4 mm since 1959, so the whole chain is definitional. Every figure on this page inherits that.

20 lb bond and 50 lb text are the same sheet

The best-known consequence, and the one that trips people buying paper for the first time: ordinary office copier paper is 20 lb bond and 50 lb text and 75 gsm. All three describe one product.

Fifty is two and a half times twenty, and the paper is identical — because a text basis sheet is two and a half times the area of a bond one. A bigger pound number is not a heavier paper unless the grade is the same.

The reverse trap is the expensive one. A print shop quoting “80 lb” could mean 118 gsm text or 216 gsm cover, and those are a brochure page and a brochure cover. If a quotation names a pound figure with no grade, it has not specified the paper.

There is a small piece of history behind the bond size that makes it easier to remember: a 17 × 22″ basis sheet is exactly four US Letter sheets. That is where the office standard comes from — a ream of Letter is a quarter of a ream of basis sheets, so 20 lb bond means a Letter sheet weighs exactly a hundredth of a pound.

Grammage says nothing about thickness

Grammage is mass per square metre. Caliper is thickness. They are related by how densely the paper is made, and that varies enormously between paper types — so there is no gsm-to-microns factor, and any chart offering one has picked a paper class without telling you.

2.4× in thickness at one grammage. This is why a glossy magazine feels dense and thin while a novel of the same page weight feels substantial: coating and calendering compress the sheet, and a high-bulk book paper is chosen deliberately so that a short book looks like a real one.

The arithmetic is pleasantly simple once the units line up. One gram per square metre at a bulk of one cubic centimetre per gram is exactly one micrometre thick, so caliper in microns is just grammage times bulk. A point — the US trade unit — is a thousandth of an inch, so 250 µm is 9.8 pt.

Pages per inch follows from caliper, remembering that each leaf carries two pages. It is the number a book designer actually needs, because it determines the spine: a 300-page book on 80 gsm offset comes out at about 15.6 mm, and the same book on a high-bulk stock at 21.6. Both are the same words on the same weight of paper.

Why A4 sheet weights come out round

A0 is defined as one square metre, with sides in the ratio √2 so that halving it gives the same proportions. Every subsequent size is exactly half the last. So A4 is exactly one sixteenth of a square metre — not approximately, exactly.

Which means 80 gsm A4 weighs exactly 5 g, and a ream of it exactly 2.5 kg. 100 gsm A4 is exactly 6.25 g. These are not conveniences someone rounded to; they fall out of the definitions.

US Letter is 3.5% smaller than A4, so the same paper gives a lighter sheet and nothing lands on a round number. That difference is usually invisible and occasionally expensive: a document close to a postage threshold can cross it purely by being printed on A4 rather than Letter, and a courier’s weight band does not care why.

The practical habit this supports is counting postage by sheets. At 80 gsm, an A4 sheet is 5 g and a standard envelope about 5 g, so ten sheets and an envelope is 55 g — comfortably inside a 100 g bracket, and the arithmetic can be done without a scale. None of that works in the US system, which is a fair part of why paper specifications elsewhere migrated to grammage and stayed there.

Reading a paper spec without being caught

A few rules that cover most of the ways this goes wrong.

A pound figure with no grade is not a specification. Ask which grade before quoting, ordering or comparing. “80 lb” is a brochure page or a brochure cover depending on an answer nobody gave you.

Do not compare pound figures across grades. 80 lb text is lighter than 65 lb cover, which looks wrong and is not. Convert both to grammage first; that is the only common ground.

Grammage does not tell you how a paper feels. Ask for caliper too, especially for anything that will be folded, bound or read through. Two 200 gsm sheets can differ by two and a half times in thickness.

Opacity is a separate property again. Heavier usually shows through less, but filler content and coating matter as much as weight, and a well-made 90 gsm book paper can be more opaque than a cheap 100 gsm one. If show-through is the worry, that is a question about opacity, not about grammage.

Weigh a sample when a threshold is close. Mill tolerance on grammage is a few per cent and paper takes up moisture from the air, so a calculation that lands within a per cent or two of a postage bracket or a binding limit is not a decision — it is a reason to put a real sheet on a real scale.

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Sources and methodology

The metric side of this page rests on two ISO standards: grammage is defined as mass per unit area independently of sheet size, and the A-series is built on A0 being exactly one square metre. Those two together are why a metric paper spec is unambiguous and its sheet weights come out round. The conversion constant is exact for a third reason — the pound and the inch have both been defined rather than measured since 1959.

• ISO 216 — Writing paper and certain classes of printed matter: trimmed sizesInternational Organization for Standardization · verified 2026-09-07 · That A0 is defined as one square metre with sides in the ratio √2 and each subsequent size is the previous halved — which is what makes an A4 sheet exactly 1/16 m² and 80 gsm A4 exactly 5 g
• ISO 536 — Paper and board: determination of grammageInternational Organization for Standardization · verified 2026-09-07 · Grammage as mass per unit area in grams per square metre, measured independently of sheet size — the property that makes the metric figure unambiguous where a basis weight is not
• International yard and pound agreement (1959)National Institute of Standards and Technology · verified 2026-09-07 · The pound as exactly 453.59237 g and the inch as exactly 25.4 mm, which together make the basis-weight constant 1406.139 exact rather than a measured conversion factor