Understanding Paper Weight, Bulk and Volume

The one idea that unlocks the rest

Weight and thickness are different properties, and paper is sold on weight.

Think of two loaves of bread of identical weight, one a dense rye and one an airy white. Same weight on the scales, very different height on the board. Paper works the same way: the fibres can be pressed tightly together or left loose and full of air, and how tightly they are pressed is a decision the mill makes independently of how much fibre goes in.

Everything below follows from that. Grammage measures how much fibre. Bulk measures how loosely it is packed. Multiply them and you get thickness.

Here is what it means in practice. Stora Enso publishes its Lux Cream book paper at 80 g/m² in four different bulks:

Every row is 80 g/m². Only the bulk changes.

One weight of paper, four thicknesses, a range of nearly forty per cent. A 300-page novel printed on the first is about 17 mm thick; on the last it is about 24 mm. Same weight of paper, same postage, same cost per tonne. A quite different object on a shelf.

That choice is available to you, and it is normally made by default because nobody mentions it.

Grammage: the weight

Grammage is the mass of one square metre of the paper, in grams. It is written gsm or g/m², and it is defined by an international standard, ISO 536:2019, Paper and board — Determination of grammage.

Cut a square metre out of a sheet, weigh it, and that is the number. Ordinary office paper is 80gsm. A paperback interior is usually somewhere between 70 and 90. A paperback cover is around 240 to 300. Greetings card is 300 and up.

The useful thing about grammage is that it is honest and universal. It does not depend on sheet size, grade or country. Every mill in Europe measures it the same way, and two papers of the same grammage genuinely contain the same amount of material per unit area.

The unhelpful thing is that people reach for it as a proxy for quality and thickness, and it is neither. A thin, dense, cheap paper and a lofty, opaque, expensive one can both be 80gsm.

You may occasionally see grammage called substance. Several European mills still head the column that way — Stora Enso and Fedrigoni both print "Substance (g/m²)" on current datasheets. Be careful with the word, though, because American usage is different: there "substance 20" means 20lb bond, a weight in pounds. Same word, two systems, no relation.

Caliper: the thickness

Caliper is the measured thickness of a single sheet. In Britain and Europe it is quoted in microns — a micron being one thousandth of a millimetre, sometimes written µm.

A typical book paper is around 100 to 160 microns, which is 0.1 to 0.16 mm. That sounds trivially small until you stack three hundred of them.

Caliper is defined in ISO 534:2011, Paper and board — Determination of thickness, density and specific volume. The standard specifies two ways of measuring it, which is worth knowing because they do not always agree. Single sheet thickness is one sheet measured under a defined load. Bulking thickness measures a stack and divides by the number of sheets. A stack is slightly compressed by its own arrangement, so bulking thickness tends to read a shade lower — and for a book, which is a stack, bulking thickness is the more honest predictor.

Americans quote board thickness in points instead, where one point is one thousandth of an inch. That converts to 25.4 microns exactly. Points are the North American convention for board; no European mill datasheet in general circulation uses them.

Bulk and volume basis: the missing number

Bulk is thickness relative to weight. It answers the question "how much thickness does this paper give me per gram", and it is quoted in cubic centimetres per gram.

Book papers typically run from about 1.2 for a dense, low-bulk offset up to about 2.3 for a deliberately lofty one. Most novels are printed on something between 1.5 and 2.0.

ISO 534 calls it apparent specific volume, which is precise and nobody says it. The mills themselves head the column "Bulk (cm³/g)" and cite ISO 534 as the test method, so the trade word and the standard's word describe the same measurement.

Volume basis is the same number, ten times bigger

British merchants and printers usually multiply bulk by ten and call the result the volume basis, or just "vol". A paper with a bulk of 1.75 is quoted as 17.5 vol.

There is no deep reason for this. It removes a decimal place from everyday conversation. But it is the single commonest source of confusion when reading a mill datasheet alongside a printer's quotation, because the mill says 1.75 and the printer says 17.5 and they are the same paper.

If a figure ever looks out by a factor of ten, this is why.

The arithmetic, which is one multiplication

The relationship at the centre of all of this is:

Caliper in microns = grammage × bulk

Or, if you have been given the volume basis rather than the bulk:

Caliper in microns = grammage × volume ÷ 10

That is genuinely all of it. An 80gsm paper at bulk 1.75 is 80 × 1.75 = 140 microns. The same paper quoted as 17.5 vol is 80 × 17.5 ÷ 10 = 140 microns.

You can check this against published mill data and it holds. UPM's Book Creamy 2.0 lists 50, 60, 70 and 80 g/m² at bulk 2.0, giving 100, 120, 140 and 160 microns — every row exactly the product. Stora Enso's Lux Cream table does the same across four bulks and five grammages.

Working backwards

The same relationship rearranges when you know the thickness and want the bulk, which happens whenever a printer quotes you a caliper without mentioning bulk:

• Bulk = caliper ÷ grammage
• Volume basis = 10 × caliper ÷ grammage

Arctic Paper's Amber Graphic, for instance, publishes 80gsm at 99 microns but never states a bulk. Dividing gives 1.24 — a low-bulk sheet, which is consistent with it being sold as a smooth graphic paper rather than a bulky book paper.

From one sheet to a whole book

A book is a stack of sheets, and every sheet carries two pages. So the thickness of the text block is:

Text block (mm) = pages × grammage × volume ÷ 20,000

The divisor is not a magic number. It is three ordinary conversions collapsed together: divide pages by 2 to get sheets, divide by 10 because volume is ten times bulk, and divide by 1,000 to turn microns into millimetres. Two times ten times a thousand.

A 320-page novel on 80gsm 17.5 vol therefore comes to 320 × 80 × 17.5 ÷ 20,000 = 22.4 mm.

How to read a mill datasheet

Mill datasheets look forbidding and contain about six things worth knowing. Once you can read one you can specify paper properly rather than accepting whatever comes as standard.

• Grammage or substance, in g/m². The weight. Usually the first column.
• Bulk, in cm³/g, tested to ISO 534. The number this guide exists to explain. Often a whole family of products differing only in this.
• Thickness or caliper, in microns. Usually the product of the previous two.
• Opacity, as a percentage, tested to ISO 2471. How much light the sheet stops. See below.
• Brightness or whiteness, as a percentage. How much light it reflects. A cream book paper sits around 76 to 83; a white one around 96.
• Shade. Cream or white, and the reason novels are usually cream — lower contrast is easier on the eye over hundreds of pages.

Arctic Paper's Munken range is a useful one to read, because the products are explicitly organised around bulk. Munken Print Cream and Munken Print White are each offered at bulk 1.5, 1.8 and 2.0. At 80gsm those give 120, 144 and 160 microns. Munken Premium Cream goes further, at 1.3, 1.5, 1.75 and 1.95, giving 104, 120, 140 and 156 microns at the same weight.

Note that the ranges are not symmetrical. Munken Premium White publishes only the 1.75 bulk. It is always worth checking the actual sheet rather than assuming that a cream paper's stablemate offers the same options.

The American system, and why it makes no sense

If you use Amazon KDP, IngramSpark or any American printing advice, you will meet paper described in pounds: 50lb text, 80lb cover, 20lb bond. The system is genuinely confusing, and it is worth understanding rather than avoiding, because it is what most self-publishing advice online assumes.

Basis weight is the weight in pounds of 500 sheets of that paper — but at the paper's own basic size, and each grade has a different basic size.

Because the basic sizes differ, the same pound number means different amounts of paper depending on the grade. The consequences are absurd and completely routine in the trade.

80lb text is 118gsm. 80lb cover is 216gsm. Nearly twice the weight, same number.

And in the other direction, 20lb bond and 50lb text are the same paper — 75 and 74gsm respectively. One says twenty, the other says fifty.

The most striking illustration comes from Neenah's own equivalence tables. A single sheet of 216gsm cover stock can legitimately be called any of these:

• 80lb cover
• 146lb text
• 58lb bond
• 133lb tag
• 120lb index

Five numbers, one piece of paper, all correct.

Converting to gsm

You can derive the conversion from the definitions. Text stock at 25 × 38 inches, 500 sheets, is 306.45 square metres; a pound is 453.59 grams; so one pound of basis weight is 1.48 g/m². Cover stock at 20 × 26 inches works out at 2.70 g/m² per pound.

The figures below come from Neenah's published tables rather than from that arithmetic, which is the safer source:

The practical rule for a British author: when an American source says a number of pounds, find out the grade before you assume anything at all.

Pages per inch, the American shortcut

American print-on-demand companies avoid the whole question by publishing pages per inch, or PPI: how many pages of their stock stack to an inch. Because a sheet is two pages, PPI is roughly twice the number of sheets per inch.

It converts cleanly:

• Caliper in microns = 50,800 ÷ PPI
• PPI = 50,800 ÷ caliper in microns
• Thickness per page in mm = 25.4 ÷ PPI

The 50,800 is 25,400 microns to the inch, doubled because a sheet carries two pages.

So IngramSpark's 50lb crème at 444 PPI is a leaf of 114 microns, which at 80gsm implies a bulk of about 1.43. Their 50lb white at 512 PPI is 99 microns, and at 74gsm that is a bulk of about 1.34. Both are on the low-bulk side of typical book paper, which is worth knowing if you are choosing between a print-on-demand company and a British printer and wondering why the same manuscript comes out noticeably thinner from one of them.

PPI is convenient and completely non-transferable. It describes one paper on one company's presses. The moment you change supplier, the number is meaningless, which is exactly why mills publish grammage and bulk instead.

Opacity, and why bulky paper shows through less

Opacity is the percentage of visible light the sheet stops. It is what determines whether you can see the previous page through the one you are reading. Printing papers generally fall between 80 and 98 per cent, and book papers usually sit in the high eighties to mid nineties.

The intuition most people have is that a bulkier paper, being fluffier, must show through more. The opposite is true, and the reason is worth understanding.

Opacity comes from light being scattered as it crosses boundaries between fibre and air inside the sheet. More boundaries mean more scattering and less light getting through. A bulky sheet at a given weight contains more air and therefore more of those boundaries. Press the same sheet denser and you squeeze the air out, remove the boundaries, and light passes more easily.

Holmen, who make book paper, put it plainly: because the paper contains more air and is thicker, you get better opacity than a paper of the same grammage with lower bulk.

So a high-bulk paper gives you three things at once — a thicker book, a lighter book for its thickness, and better show-through — which is why book papers are bulky and why the cheapest option is rarely the best one. The trade-off is strength: a lofty sheet has less fibre-to-fibre bonding and is structurally weaker.

Shade matters too. A white sheet is typically one or two percentage points more opaque than the cream equivalent at the same weight and bulk.

Grain direction

When paper is made, the fibres arrive suspended in water on a moving wire and tend to line up with the direction of travel. That alignment is locked in when the sheet dries, and it is called the grain.

Paper folds cleanly along the grain and reluctantly across it. It also expands and contracts far more across the grain than along it when the moisture content changes.

Merchants mark it in one of three ways: an arrow, an underline beneath whichever dimension runs with the grain, or the letters LG for long grain — grain along the longer edge — and SG for short grain. British merchants sell stock described exactly this way, so "SRA3 short grain" is an ordinary line on an ordinary invoice.

For a book, the grain must run parallel to the spine, head to tail. The spine then acts as a hinge along the fibres: the pages turn easily, the folds tolerate repeated opening, and the whole block responds to changes in humidity as one thing.

Bind against the grain and the failure modes are specific and permanent: waving along the fore-edge, cracking at the spine folds, and boards that warp with the seasons. None of it can be corrected afterwards. Boards are grained too, and their grain must run head to tail as well.

This is one reason a printer may quietly steer you away from an unusual trim size. Grain constrains how pages can be laid out on the parent sheet, and some sizes waste a great deal of paper once the grain has to run the right way.

Paper and weather

Paper is hygroscopic: it takes up moisture from damp air and gives it back to dry air, continuously, and its dimensions change as it does.

Sappi's technical guidance on climate quantifies it. A change of ten percentage points in relative humidity moves the sheet by roughly 0.1 to 0.2 per cent across its width. On a metre-wide sheet that is one to two millimetres. And the movement is markedly directional: typical swelling is 0.1 to 0.3 per cent along the grain against 0.3 to 0.7 per cent across it, which is the measurable form of the grain effect described above.

This is why printers condition paper before running it — leaving it wrapped in the pressroom for something like 24 to 36 hours so it reaches equilibrium with the air it will be printed in. European practice prepares paper for sheet-fed offset at around 50 per cent relative humidity. Sappi notes that differences of up to five percentage points cause no trouble, but at eight to ten the situation deteriorates quickly into wavy or tight edges.

For an author the practical consequences are modest but real. A book bound in one climate and measured in another is not quite the same object. It is one of several reasons why a calculated spine is a good prediction rather than a guarantee.

Why published figures do not always multiply out

Everything above rests on caliper being grammage times bulk. It nearly always is. It is worth knowing where it is not, because encountering a table that does not add up otherwise makes you doubt the whole method.

Published bulk figures are nominal targets, not measurements of the reel your book will be printed on. Datasheets say so themselves, in the small print about values being targets subject to change.

Two examples make the point. Arctic Paper's Munken Print Cream at bulk 1.8 lists 124 microns for 70gsm and 209 for 115gsm, where the arithmetic gives 126 and 207. The equivalent Munken Print White sheet lists exactly 126 and 207. Two datasheets from the same mill for near-identical products, disagreeing by a couple of microns.

Holmen goes further. Its BOOK grades publish calipers consistently three to four per cent below grammage times bulk — 60gsm at bulk 2.0 is listed at 116 microns rather than 120. The explanation is that the bulk figure there functions as a product name: the grade is literally called BOOK 2.0. Holmen's own explanatory material uses exact arithmetic in its worked examples, so it is the naming rather than the physics that differs.

Neither undermines the method. It simply means a spine calculation is accurate to a fraction of a millimetre rather than exactly, and that a long book accumulates that fraction over more leaves than a short one. If a job matters, ask for a bulking dummy: a blank copy made from the real paper and bound as the book will be bound. It costs very little and it is the only figure with no assumptions in it at all.

Choosing paper for a book

Putting all of it together, here is what to actually ask for.

Start with how you want the book to feel

A 60,000-word novel is a short book. Set at a comfortable size it might run to 250 pages. On an 80gsm paper at 12.5 vol that is a spine of 12.5 mm — thin enough to look slight next to comparable titles. On the same weight of paper at 20 vol it is 20 mm, and it looks like a proper novel. Neither is dishonest; the words are the same. But the second is what most publishers would choose, and the paper costs no more per tonne.

Then check the practical limits

Bulk works in the other direction too. A long novel on a bulky paper can exceed what a perfect binder will accept — commonly somewhere between 50 and 60 mm of spine — at which point the printer will suggest a lower-bulk paper rather than a shorter book. This is the standard reason a thousand-page book is printed on thin, dense stock.

Ask the four questions

• What grammage, and what bulk or volume? Both, always. A quote with only the first is incomplete.
• Cream or white? Cream for fiction and continuous reading; white for anything with photographs or fine line work.
• What opacity? Below about 90 per cent, show-through starts to be noticeable on a well-inked page.
• Which way does the grain run? Parallel to the spine. A book printer will do this without being asked; it is worth confirming with anyone else.

And then ask for the thickness in writing

Once the paper is chosen, ask the printer to confirm the spine width they will build the cover to. You can check it against the arithmetic here, and a disagreement is worth resolving before the covers are printed rather than after.

Common mistakes

• Treating grammage as thickness. The mistake this guide exists to prevent. Weight and thickness are separate properties and a heavier paper is often thinner.
• Asking for heavier paper to get a thicker book. It may work slightly, and it makes the book heavier to post, more expensive, and stiffer to hold. Ask for higher bulk instead.
• Mixing bulk and volume basis. They differ by a factor of ten. A spine calculated with 1.75 where 17.5 was meant comes out ten times too thin, which at least is obvious. The reverse is equally possible and equally wrong.
• Assuming pound numbers are comparable. 80lb cover is nearly twice the weight of 80lb text. Always establish the grade.
• Using one company's pages-per-inch figure for another company's book. PPI describes a specific stock. It does not travel.
• Confusing pages with sheets. Every sheet carries two pages. A 320-page book has 160 sheets. Getting this wrong doubles or halves the spine.
• Ignoring grain on a short-run or hand-bound job. A commercial book printer handles it automatically. A general printer may not, and the damage shows up months later as warping.
• Believing a calculation over a dummy. The arithmetic is a good prediction. A bulking dummy is the answer.

Related tools

References

These standards, mills and merchants publish the figures behind this page.

• ISO 536:2019 — Paper and board: determination of grammage. The standard defining gsm, adopted in Britain as BS EN ISO 536.
• ISO 534:2011 — Paper and board: determination of thickness, density and specific volume. Defines caliper, and the specific volume the trade calls bulk.
• Arctic Paper — Munken book papers — datasheets giving grammage, bulk, caliper and opacity for each grade; the source of the Munken figures quoted above.
• Stora Enso — Lux book papers — the four-bulk table used at the top of this guide.
• UPM — Book Creamy — book paper published at fixed bulks, with caliper matching the arithmetic on every row.
• Holmen — Paper bulk explained — a paper mill's own explanation of bulk and its effect on opacity.
• Neenah Paper — Basis weights — the American basic sizes and the equivalence tables behind the pound-to-gsm figures.
• Sappi — Climate and Paper — humidity, dimensional movement and paper conditioning, with figures.
• Pimlico Bookbinding — Understanding paper grain — grain marking conventions and what goes wrong when a book is bound against the grain.
• University of Brighton — Grain direction — the bookbinding rule that grain runs head to tail, and why.
• Denmaur — What is paper caliper and why does it matter — a British paper merchant on caliper, grammage and bulk.

Mill specifications change, and published bulk figures are nominal targets rather than guarantees. Check the current datasheet with your printer or merchant before committing a specification.