Paper Weight and Bleed Explained: What GSM Really Means

Two notebooks, same shelf, same price. One is 90gsm and handles a wet broad nib without a wobble. The other is 160gsm and feathers the moment the nib touches down. Both numbers are honest. They just do not measure the thing you actually care about: how the sheet behaves when wet ink lands on it.

What GSM actually means

GSM stands for grams per square metre, and that is exactly what it measures: the mass of one square metre of paper. An A4 sheet is one sixteenth of a square metre, so 80gsm A4 weighs about 5g and 160gsm A4 about 10g. That is the whole of it. GSM tells you how much paper you are holding, not how it will take ink.

Thickness is a different measurement, called caliper, usually given in microns. A densely calendered 100gsm sheet can be thinner than a bulky 90gsm one, and the two feel nothing alike. Density matters for ink as well, because dense paper has fewer gaps between fibres for ink to travel along.

Why the American numbers look wrong

US paper is sold by basis weight, the mass of a ream in a sheet size that changes with the paper's category. So an 80lb text and an 80lb cover are not remotely the same thing. As a rough guide, 20lb bond is close to 75gsm, 24lb to 90gsm and 32lb to 120gsm, while 80lb text sits nearer 118gsm and 80lb cover nearer 216gsm. If a listing seems oddly heavy or light for the number shown, this is usually why.

Why a heavier sheet is not automatically better

Weight can come from several places: more fibre, more mineral filler, a heavy coating, or simply denser refining of the same fibre. Only some of those help a fountain pen.

Fillers such as calcium carbonate brighten paper and make it cheaper to produce, but they do nothing for its resistance to ink. A heavily filled 120gsm sheet can feather where a well-sized 80gsm sheet stays crisp. Very dense, heavily calendered paper can also turn brittle, cracking at the fold or refusing to lie flat in a notebook spine.

It helps to separate three complaints that all get blamed on weight:

Feathering — ink spreading sideways along the fibres, giving a fuzzy line. A sizing and fibre problem.
Bleed-through — ink passing right through to the reverse of the sheet. A sizing, absorbency and sometimes coating problem.
Show-through (ghosting) — the writing on the other side being visible without ink soaking through. This one is largely about weight and opacity.

If your real complaint is ghosting, buying heavier paper genuinely helps, or you can slip a spare sheet underneath as a blotter. If your complaint is feathering, weight is the wrong lever entirely, and you will spend more money for the same disappointment.

Coating, sizing and absorbency

Sizing is the paper's resistance to water, and it is the single biggest factor in how a wet nib behaves. Internal sizing is added to the pulp; surface sizing is applied to the finished sheet, often as starch. Both slow the rate at which ink soaks in, so it sits on the surface, dries by evaporation and holds a clean edge. Unsized paper is blotting paper: it drinks.

Coating is another layer, usually clay or calcium carbonate bound with a binder to give a smooth printing surface. Coated papers keep ink on top, which flatters sheen and colour, but they vary enormously. Some swell slightly and stay tacky, so a wet nib smears across the page. Others are porous and dry almost instantly. Inkjet photo papers tend toward the slower, swellable type; ordinary inkjet office paper is usually porous and quick.

There is a trade-off hiding here: more sizing means a crisper line but a longer drying time. That is why some much-praised fountain pen papers feel luxurious and then smudge twenty seconds later when you close the book. If you write quickly, or you are left-handed, test drying time as carefully as you test feathering.

How to test a paper before committing

Manufacturer descriptions are marketing copy. A five-minute test tells you more. Take your wettest pen and your most demanding ink to the shop, or buy one notebook rather than three.

Write a broad, slow line and a hairline with the same pen. Feathering often shows only on the fine nib, where it ruins the line most.
Write on the reverse of the sheet and check for bleed-through, not just a shadow.
Draw a solid block of ink, then time how long it takes to stop transferring to your finger.
Run a highlighter or a wet brush across dried ink to see whether it lifts and smears.
Crease and unfold a corner to check for cracking on coated or very dense stock.
Write near the spine, where notebooks are glued or stitched and harder to use flat.

Matching paper to pen and ink

A dry Japanese fine nib and a wet Italian broad nib want different paper. Broad, italic and flex nibs, and saturated sheening inks, push down more liquid, so they need tighter sizing and better coating. Fine, dry writers are far more forgiving and will cope with an ordinary 80gsm pad.

Ink type matters too. Iron gall and pigment inks bond with the paper and generally behave well on sized stock, though they demand more careful cleaning. Alcohol markers will strike through almost any writing paper and need dedicated bleedproof marker paper or a synthetic sheet. Watercolour is its own discipline, graded from around 190gsm for light washes up to 300gsm and beyond for wet work.

For everyday journaling with a fountain pen, 90 to 120gsm is the comfortable middle: enough opacity to stop ghosting, enough body to feel substantial, without the extra weight doing anything for the ink itself. Above that, you are mostly buying stiffness and presence rather than performance.