Pick up two notebooks, both labelled 90gsm, and one may take a broad fountain pen without complaint while the other lets a single line strike through to the next page. Paper weight tells you something useful, but it is only part of the story. Coating, sizing and absorbency decide how ink behaves on the surface — and whether your writing stays where you left it.

This guide unpacks what those terms mean, why two sheets of identical weight can behave like completely different materials, and how to judge a notebook before you fill a hundred pages with a pen it cannot handle.

What GSM Actually Means

GSM stands for grams per square metre. It is the weight of one square metre of paper, not its thickness. A 100gsm sheet is heavier than an 80gsm sheet of the same type, but different pulps and finishes mean a 90gsm paper can feel more substantial than a 100gsm one.

Most notebook pages sit between 70gsm and 120gsm. Below that, paper feels flimsy and show-through is common. Above it, pages feel substantial and are often used for sketchbooks or journals. Covers are usually 250gsm or more, but that tells you nothing about the pages inside.

Use GSM as a first filter. Pencil and ballpoint often cope well with 70–80gsm. Gel pens and fountain pens usually need 90gsm or higher as a starting point. Then look at the surface.

Here is the part that trips people up: GSM says nothing about fibre density, pulp type or how the sheet was finished. A sheet of 80gsm copier paper and an 80gsm coated notebook page weigh exactly the same, yet the copier paper will drink a fountain pen line and spread it into a fuzzy mess, while the coated sheet holds a line sharp enough to read across a room. Same number on the label, entirely different experience at the nib.

Coating and Sizing: What Keeps Ink on the Surface

Coating is a layer applied to the paper surface, often clay or calcium carbonate with a binder. Sizing is a treatment added to the pulp or painted onto the surface to make paper less absorbent. Both control how quickly ink sinks in.

Sizing comes in two broad forms. Internal sizing is mixed into the pulp before the sheet is formed, making the fibres themselves resist water. Surface sizing is applied afterwards, sealing the top of the sheet. Papers that resist ink strongly are often described as hard-sized; lightly sized papers accept ink readily and let it spread. Most quality notebook papers use a combination of the two.

Uncoated, lightly sized paper drinks ink. Lines dry quickly, but they can feather along the fibres and show through to the reverse. Heavily coated or well-sized paper holds ink on top. Lines stay crisp, inks show shading and sheen, and dry times stretch.

Not all coating is glossy. Matte and silk coatings are common in notebooks. A coated 90gsm paper can handle fountain pen ink better than an uncoated 120gsm sheet. Some Japanese papers are only around 52gsm yet take wet nibs beautifully because they are coated or heavily sized. Weight alone would never tell you that.

The trade-off is also about feel. Coated paper is smoother and often cooler to the touch, and the ink sits on the surface rather than marrying with the fibre. Some writers love that crispness; others prefer the slight tooth of an uncoated sheet, even if it means a little more show-through. Neither is better — they are different tools for different hands.

Absorbency, Feathering and Dry Time

Absorbency is the paper's willingness to pull liquid into its fibres. High absorbency gives fast drying and a matte finish, but it also encourages feathering, wider lines, and ink bleeding along the fibres. Feathering is what happens when ink travels sideways through the paper instead of staying where the nib placed it. The result is a line with a soft, hairy edge rather than a clean one, and it is most obvious with wet nibs, broad nibs and free-flowing inks.

Low absorbency does the opposite. Ink stays where it lands, lines stay narrow and crisp, and shading or sheen becomes visible. The cost is time. A wet line on a heavily coated sheet may take thirty seconds or more to become touch-dry, which is a genuine problem for left-handed writers and for anyone who closes a notebook in a hurry.

There is also the question of what happens afterwards. On absorbent paper, ink bonds into the fibre and the page becomes difficult to smudge once dry. On coated paper, ink sits on the surface for longer and stays vulnerable until it has fully cured. If you highlight, erase or rest your hand on the page, that difference matters.

The Dry-Time Trade-Off

Fast drying and crisp lines are usually opposites. You can have either on a given sheet, but rarely both at once — and it is the paper, not the pen, that decides which.

There is no setting that gives you everything. The practical move is to decide what you can live with. If you write quickly and close the book, prioritise absorbency and accept a little feathering. If you care about line quality and ink character, prioritise coating and learn to wait.

Show-Through vs Bleed-Through

People use these terms interchangeably, but they describe two different failures, and the difference decides whether a notebook is usable on both sides.

• Show-through, or ghosting: you can see the writing from the reverse side as a faint shadow. The ink has not left the page; the paper is simply translucent enough that dark marks show through. Annoying, but harmless.
• Bleed-through, or strike-through: the ink has physically soaked through the sheet and deposited pigment on the page beneath. The mark is permanent and the next page is ruined.

Show-through is largely a function of opacity, which is driven by fibre density, fillers and coating rather than weight. Bleed-through is a function of absorbency and how much ink the sheet can hold at one point. That is why a thin, well-sized 52gsm paper can show every word from the reverse but never bleed, while a chunky uncoated 100gsm sheet may let a single pooled dot pass straight through to the next page.

For anyone who writes on both sides of the page — most of us — show-through is a minor irritation and bleed-through is a deal-breaker.

How to Test Paper Before You Commit

You do not need a laboratory. Five minutes and your wettest pen will tell you most of what you need to know.

1. Choose the pen and ink you use most, or the wettest combination you own.
2. Write a few lines at normal speed, then add a deliberate dot where you hold the nib still for two seconds. Standing ink is the real test.
3. Wait ten seconds, then examine the reverse of the sheet in good light.
4. Turn to the next page and check whether anything has actually transferred.
5. Rest your hand across the writing and see whether it smears.

A sample pack or a single inexpensive notebook is a far cheaper mistake than a shelf full of paper that fights your pen every day.

Matching Paper to Your Pen

• Pencil and ballpoint: almost anything works, including 70–80gsm. These lay down dry marks and cannot bleed.
• Fineliners and rollerballs: 80–90gsm is usually enough, though rollerballs run wet and can feather on cheap stock.
• Gel pens: 90gsm plus a little sizing, or the line spreads and shadows through the page.
• Fountain pens: 90gsm as a floor, but prioritise coating and sizing over weight. A well-made 90gsm coated paper beats a heavy uncoated one almost every time.
• Broad or flex nibs: look for 100gsm or above, or a coated sheet, and accept longer dry times.

The Bottom Line

GSM is a useful starting point, but it measures mass, not performance. Two 90gsm papers can behave like chalk and glass because coating, sizing and absorbency — not weight — decide what ink does on contact.

Judge a notebook by how your pen behaves on it: whether lines stay crisp, whether the reverse stays clean, and whether the writing is dry by the time your hand arrives. Test a page before committing to the whole book, and treat the GSM number as the opening line of a conversation rather than the final word.

Photo: Steve A Johnson / Pexels