Fountain Pen Paper Thickness: Is 100 GSM Enough?

If you've spent any time shopping for notebooks, you've probably seen GSM printed somewhere on the packaging and had no real sense of what it means for how the pen will actually feel on the page. It's one of the most quoted numbers in paper, and one of the least explained.

GSM stands for grams per square metre, and it's a measure of paper weight, not thickness. Two sheets can share the same GSM and still feel completely different under a nib, because weight doesn't tell you how it's calendered, coated, or fibred: it only tells you how much a square metre of paper weighs.

Thickness (sometimes it can be called caliper) is a separate physical measurement, and the two don't always move together. A denser, more compressed 80 gsm sheet can feel thicker than a looser, fluffier 100 gsm one.

That distinction matters more for fountain pens than for almost any other kind of writing.

What actually determines fountain pen performance

Fountain pen paper thickness gets asked about so often because people are really asking a different question: will this bleed through, and will it hold up to a wet nib? The truth is that GSM is only one input: choosing a notebook only by the GSM number is like choosing a car only by its colour. It might work, though!

A few things matter more:

• Opacity — how much the paper blocks light and ink from showing through, independent of weight

• Ink load — how much ink a nib actually lays down, which depends on nib width and the ink's own flow and paper compatibility

• Surface treatment — sizing and coating control how quickly a sheet absorbs ink, and absorbent paper behaves very differently from coated paper at the same weight

• Nib wetness — a wet-writing nib will push ink further into any sheet than a drier one, regardless of GSM

As we found when comparing five different notebooks for fountain pen friendly paper, two sheets of almost matching weight behaved like entirely different papers once ink touched the page.

80, 100 and 120 GSM: what the difference actually buys you

As a general guideline among fountain pen users, paper under 80 gsm is more likely to let ink bleed through, particularly with wetter nibs or more liquid inks. Paper at 80 gsm and above is generally considered "less prone" to bleed-through, because surface treatment and ink choice still decide the outcome case by case.

Moving from 100 to 120 gsm doesn't reliably buy you much extra protection. Once you're past the roughly-80 gsm threshold, weight stops being the main variable, and coating, sizing, and how absorbent the sheet is matter more than another 20 grams. A heavier sheet can still bleed if it's poorly sized; a lighter one can perform well if it's treated for it.

A closer look: five papers, five densities

To see how far these differences actually go, I tested five papers across a range of weights, all with the same pen and ink.

75 GSM Midori paper: no bleed, strong ghosting

This is the thinnest paper in the selection, and it's different in composition: this particular collection uses 80% cellulose and 20% cotton fibres. The cotton content makes for a smoother, more porous sheet that resists tearing and yellowing over time. With this ink, there was no bleed-through, though at this GSM, I'd expect a wetter nib or more liquid ink to behave differently.

80 GSM Leuchtturm1917 paper: no bleed, ghosting visible

One of the staples for fountain pen people, Leuchtturm1917 offers a nice glide and soaks up ink evenly. It's a well-balanced paper for fine and medium nibs, writing without bleeding or feathering. That said, I find this level of ghosting a little less comfortable than the papers shown below.

90 GSM Clairefontaine paper: no bleed, minimal ghosting

This paper is coated, and it shows: the surface feels extra smooth, and the ink sits on top rather than sinking into the fibres. That coating is also why the line comes out almost 40% thinner than on the uncoated papers here: less ink penetrates, so less shows through the back, which is why ghosting is so minimal, yet it might get smudgy.

100 GSM Scriveiner paper: no bleed, almost no ghosting

This one's a preview: it's the paper going into something new we've been working on, launching in a couple of weeks, and I couldn't resist testing it early. Unlike the previous test, it's uncoated, a little thicker, and the fibres feel different underhand. Ink is absorbed quickly, but with no feathering and no meaningful ghosting: just a clean, evenly saturated line. Of everything I tested, this is the one I've reached for most, for longhand, journaling, and quick notes alike, because that even ink laydown makes the page genuinely easy to read back.

120 GSM Art Creation paper: no bleed, no ghosting

This notebook, like the Midori, is a sketchbook: built to handle more than just fountain pens, from brush pens to pigment-based tools. Unlike the cotton-infused Midori paper, though, its strength comes simply from a higher density of cellulose fibres. I use it for sketching and drawing, but it rarely leaves my desk; next to the others here, it's noticeably heavier to carry around.

So, is 100 GSM enough for fountain pens?

For most everyday fountain pens, inks, and nib widths, 100 gsm sits comfortably above the point where bleed-through and ghosting become a problem. But the honest short answer is: it doesn’t depend solely on the magic number. Moreover, its coating, sizing, fiber types, and how wet your particular pen runs (depending on the fountain pen nib sizes you use), they all shape how the writing actually feels.