A Japanese Medium Nib Isn't a Western Medium: What the 0.2mm Difference Looks Like on Paper

Order a “Medium” nib from a Japanese maker expecting the same line as a “Medium” from a European one, and the pen that arrives will write noticeably finer than you pictured. This isn't a manufacturing inconsistency or a quality gap — it's a real, well-known mismatch between two separate sizing conventions that happen to use the same words. Here's exactly what that mismatch is worth in millimeters, and what it actually changes about how a line looks and behaves once it's on the page.

The two standards, side by side

Western and Japanese makers each grind five common round sizes — Extra Fine, Fine, Medium, Broad, and Double Broad — but calibrate the width behind each label differently. Typical Western widths run EF at 0.4 mm, F at 0.5 mm, M at 0.7 mm, B at 0.9 mm, and BB at 1.1 mm. Typical Japanese widths for the same five names run EF at 0.28 mm, F at 0.35 mm, M at 0.5 mm, B at 0.7 mm, and BB at 0.9 mm. Line the two tables up and the pattern is immediate: every Japanese size is close to the next size down on the Western table. A Japanese M matches a Western F almost exactly. A Japanese B matches a Western M. The rule of thumb — Japanese sizing runs about one size finer — isn't an approximation invented for convenience; it falls directly out of comparing the two tables.

Converting a size properly, not by rule of thumb

A rule of thumb is a fine starting point, but converting by matching actual widths is more reliable, and it doesn't always round the way you'd expect. Convert a Japanese Medium (0.5 mm) to the Western scale and the closest match is a Western Fine, also 0.5 mm — an exact hit. Run it the other direction and a Western Fine converts back to a Japanese Medium, confirming the pair. But go one step finer and the match gets looser: a Western Extra Fine, at 0.4 mm, is closest to a Japanese Fine at 0.35 mm — the nearest available size, but not an exact match the way the Medium/Fine pair is. And at the broad end, a Japanese Broad (0.7 mm) converts to a Western Medium, not a Western Broad — which means someone expecting a Japanese Broad to write like a bold, Western-style 0.9–1.0 mm line will be a size and a half short of what they pictured. Our nib size converter runs any of these conversions directly; the nib size & dry time reference lays out every size in both standards at once, alongside how each dries on four paper types.

What a 0.15–0.2mm difference actually looks like

Numbers on a chart don't tell you much until you picture them written out. The gap between a Western Medium (0.7 mm) and a Japanese Fine (0.35 mm) is exactly a factor of two — a Japanese Fine line is genuinely half the width of a Western Medium line, not a subtle shift. Even the smaller, more typical gap of 0.15–0.2 mm between adjacent sizes across the two standards is enough that placed side by side, most people can tell the two lines apart without measuring anything, especially in cursive handwriting where the difference compounds across every stroke of a word. It shows up most in two situations: cross-hatching and fine shading, where a narrower nib fits more distinct lines into the same space before they visually merge into solid tone, and small, dense handwriting, where every fraction of a millimeter changes how many words comfortably fit on a ruled line. It shows up least on a single bold signature or a page of widely-spaced notes, where a size difference this small is nearly invisible.

Why the mismatch persists

There's no single international standards body enforcing what “Fine” or “Medium” must measure on a nib — each maker grinds to its own house convention, and those conventions developed along two largely separate tracks. Japanese houses such as Pilot, Sailor, and Platinum settled on one cluster of widths; European houses such as Pelikan, Lamy, and Montblanc settled on a coarser, broader cluster. The result is that a nib's size letter is a reliable, within-brand promise — a Pilot Fine and a Sailor Fine sit close together, a Lamy Medium and a Pelikan Medium sit close together — but crossing from one cluster to the other without converting is exactly where the surprise comes from. Neither convention is more “correct”; they're just calibrated to different defaults.

How ink and paper interact with the mismatch

The standard's width is only the starting point — ink wetness and paper absorbency shift the real line width around it in both standards equally. A wet, free-flowing ink on absorbent paper widens any nib's line, Japanese or Western, while a dry ink on coated paper narrows it. This means a Japanese Medium with a very wet ink on absorbent paper can, in practice, write close to what a Western Fine writes with a dry ink on coated stock — the standards set a baseline, not a fixed outcome. If dry time matters to you as much as width does, how long ink actually takes to dry, by nib, paper, and hand walks through how nib wetness and paper absorbency combine, using the same width categories covered here.

When the mismatch works in your favor

Framed as a problem, the standards gap is also a shortcut. If you've tried a Western Extra Fine and still want a finer line without hunting for a specialist needlepoint grind, a Japanese Fine or Extra Fine gets you there for the price of an ordinary production pen — Japanese makers effectively extend the fine end of the range further than most Western lineups bother to. The reverse is also true: if you want the boldest, wettest line a mainstream pen offers, a Western Broad or Double Broad goes wider than anything labeled equivalently by a Japanese maker, since the Japanese range tops out narrower at every step. Knowing which direction you want to push means you can pick which country's convention to shop from, rather than treating the two as interchangeable and hoping for the best.

Shaped nibs sit outside this comparison

Everything above describes round nibs — the plain, single-width points labeled EF through BB. Stub, italic, and oblique grinds vary their width within a single stroke by design, so a single millimeter figure doesn't describe them the way it describes a round nib, and the Japanese/Western gap is far less consistent across shaped grinds since fewer makers offer them at all. If you're choosing a shaped nib rather than a round one, treat any stated width as a rough guide to the widest part of the stroke rather than a precise, convertible figure.

If this is your first fountain pen

New fountain pen users often buy on a friend's recommendation without realizing that recommendation came from someone using the other convention. If a friend raves about their Japanese Medium as the perfect everyday width, remember that width sits close to a Western Fine — a genuinely good, versatile everyday choice, but not the same experience as buying a Western Medium because the name matched what they said. For a first pen, matching by width rather than by name avoids ending up with a size a full step away from what actually got recommended to you.

A quick before-you-order checklist

• Confirm which country's convention the specific maker uses — don't assume from the pen's country of sale.
• Convert by matching width, not by matching the size name.
• Remember the broad end diverges the most — a Japanese Broad is closer to a Western Medium than to a Western Broad.
• Factor in your ink and paper — a wet ink on absorbent paper can push a fine nib's real line width toward what a medium nib would produce on drier, coated paper.

What this means when you're actually buying

Three practical habits avoid the whole problem. First, when a listing states a maker's country of origin but not an exact millimeter width, treat “Medium” as ambiguous and check which convention that specific maker uses before assuming either standard. Second, when comparing two pens across the Japanese/Western line, convert by width rather than by name — a size-for-size comparison (Japanese M to Western M) is the wrong comparison; a width-matched one (Japanese M to Western F) is the right one. Third, if a maker publishes its own exact width in millimeters, trust that over any general rule of thumb, Japanese/Western or otherwise — house conventions can and do vary even within one country's cluster of makers.

Custom grinding sidesteps both standards

A nib technician who hand-grinds or adjusts a stock nib can move its effective width to nearly anywhere along the spectrum, regardless of which country's convention the pen started in — a Western Broad ground down toward a fine line, or a Japanese Fine opened up slightly, are both routine requests. Custom grinding is the one case where neither the Western nor the Japanese table applies anymore, since the nib's stated size no longer describes what a technician has since reshaped it into. If you've had a nib customized, treat its printed size purely as a historical fact about what it started as, not a description of what it currently writes like.

The takeaway

A Japanese Medium and a Western Medium share a name and very little else. Once you convert by actual width rather than by label, the one-size-finer rule of thumb holds up cleanly across the whole range, with the broad end being the one place it's worth double-checking rather than assuming. Get the width right first, and everything else — how the line looks, how it shades, how fast it dries — follows from a size you actually chose on purpose.