uni-ball One: Pigment Gel Ink, Sizes & Refills

The uni-ball One 0.7 mm Medium, 139 mm long retracted (drawn at 1 mm = 5 px).

Product facts

Product: uni-ball One retractable gel pen (US uniball ONE, 0.7 mm Medium and 0.5 mm Micro; Japan UMN-S, also 0.38 mm)
Brand: uni-ball
Brand origin: Japan
Made in: Japan^*
Material: Plastic barrel, rubber grip, movable open wire clip
Released: February 2020^*
Price range: $under $5 per pen (multi-packs divided by the count)

Notes on these facts

Made by Mitsubishi Pencil Co., Ltd.. Made in Japan: per a US business-supply listing for item 70363 (0.7 mm blue, dozen). Japanese retailer Cainz lists the 0.5 mm black pack (JAN 4902778261354) as made in Japan (生産国日本), and Escribo (Poland) lists Japan for the 0.5 mm pen. Mitsubishi Pencil's Japanese pages don't state it. A Swiss retailer lists China for one 0.7 mm multi-pack and Japan for a refill set, so origin may vary by pack; check the packaging. Source. Price: example listing. Released: Japan release date per Mitsubishi Pencil's January 21, 2020 press release (Yoseka Stationery agrees); we didn't find a US launch date.
Price bands are typical US retail, checked October 2026. Conversions to US dollars are approximate, at October 2026 rates (US$1 = ¥158 = ₩1,330 = NT$32.5 = CN¥6.7 = SEK 9.9 = DKK 6.7 = ₺49.6 = C$1.41 = A$1.43; €1 = US$1.12; £1 = US$1.32; CHF 1 = US$1.21).

Key takeaways

JetPens lists the standard One at 13.9 cm (5.5 in) long and 10.3 mm across at the grip, and both JetPens and Escribo give 10 g.
US packs come in 0.7 mm Medium and 0.5 mm Micro with a white barrel; Japan adds 0.38 mm.
uni-ball's US copy says Super Ink protects against water, fading and fraud.
Mitsubishi Pencil launched the One in Japan on February 26, 2020, at ¥120 (about US$0.80) plus tax; its site now lists standard colors at ¥165 (about US$1.00) and UMR refills at ¥110 (about US$0.70) including tax.
A US business-supply listing (0.7 mm blue dozen, 70363), Japan's Cainz and Poland's Escribo all give Japan as the country of origin.

What is the uni-ball One?

The uni-ball One is a retractable gel pen from Mitsubishi Pencil, built around a pigment ink that stays on top of the paper. Mitsubishi Pencil’s January 2020 press release put it on sale in Japan on February 26, 2020, as the UMN-S-38 (0.38 mm, 20 colors) and UMN-S-05 (0.5 mm, 10 colors), with a black barrel for black ink and an off-white barrel for every color. In the US it’s sold as uniball ONE, in a white barrel, at Walmart, Kroger and office suppliers.

The One won a 2020 Good Design Award in Japan, per Mitsubishi Pencil. Its Japanese site now lists the standard colors at ¥165 (about US$1.00) including tax, up from ¥120 (about US$0.80) plus tax at launch, and cites a study with Ritsumeikan University suggesting darker handwriting is easier to remember.

It sits alongside the older uni-ball Signo 207 in uni-ball’s US range.

uni-ball One specs

Spec	Value
Maker	Mitsubishi Pencil Co. (uni-ball), Japan
Type	Retractable gel pen, pigment ink
Tip sizes	US: 0.5 mm Micro, 0.7 mm Medium · Japan: 0.38 and 0.5 mm (0.7 mm in some sets)
Length	13.9 cm (5.5 in)
Diameter	10.3 mm grip (JetPens, Escribo); 10.5 mm at the widest (JetPens)
Weight	10 g (0.34 oz), per JetPens and Escribo
US items	70362 (0.7 black, dozen) · 70363 (0.7 blue, dozen) · 70307, 70309, 70381 (color packs)
Japanese models	UMN-S-38 (0.38 mm) · UMN-S-05 (0.5 mm); black barrel, black ink 0.5 mm: UMNS05BK.24, JAN 4902778261316
Japanese refills	UMR-05S (0.5 mm) · UMR-38S (0.38 mm)
Price in Japan	¥165 (about US$1.00) including tax for standard colors (2026 list); ¥120 (about US$0.80) plus tax at the 2020 launch
Made in	Japan (US, Japanese and Polish retailer listings)

Size and weight are from JetPens and Escribo. US item numbers come from business-supply listings; Japanese model codes, prices and refill codes come from Mitsubishi Pencil’s Japanese site.

Why does uni-ball One ink look darker?

uni-ball formulates the ink with larger pigment particles that stay on the paper’s surface instead of soaking in. JetPens explains that because the paper doesn’t absorb and wash out the color, dark inks look darker and bright colors more vivid.

Mitsubishi Pencil’s Japanese page also says the ink dries fast without feathering or bleeding through, because its liquid part soaks in quickly while the pigment stays on top. These are maker claims; we haven’t measured dry time or darkness yet.

Which uni-ball One size should you choose?

Choose 0.7 mm Medium for everyday notes and the boldest line, 0.5 mm Micro for small handwriting, and 0.38 mm (an import) for planners.

To-scale drawing of the three uni-ball One ball sizes at 1 mm = 100 px: 0.38 mm, 0.5 mm and 0.7 mm — The 0.7 mm Medium ball is about 1.8 times the diameter of the 0.38 mm.

Mitsubishi Pencil’s launch release confirms that the 0.38 mm comes in more colors (20) than the 0.5 mm (10) in Japan. Those are Japanese colors, so don’t assume each one is sold in US packs. US packs include black, blue and assorted color sets.

What refills does the uni-ball One use?

In Japan, the One takes UMR-05S (0.5 mm) and UMR-38S (0.38 mm) refills. Mitsubishi Pencil lists both at ¥110 (about US$0.70) including tax. We didn’t find a US-packaged uniball ONE refill item number, so US buyers usually order the Japanese refills from importers.

What are the One F and One P?

They are the same pigment ink in different bodies. The One F has what Mitsubishi Pencil calls a stabilizer mechanism: a precision metal tip that lowers the center of gravity for steadier writing. The One P (P for pocket) is a short version that the maker says is shaped to stay easy to hold.

The Gentleman Stationer named the One F a favorite in 2024, while noting early reports of skipping in the first release.

How does the uni-ball One compare with other gel pens?

It’s the pick for the darkest, most saturated line; the G2 and Sarasa win on availability and sizes.

Pen	Ink	Tip sizes	Notes
uni-ball One	Pigment gel, large particles	0.5, 0.7 mm (US); 0.38 mm (Japan)	Darker color claim; wire clip shows ink color
Pilot G2	Gel	0.38, 0.5, 0.7, 1.0 mm	US best-seller claim; widest color range
Zebra Sarasa Clip	Water-based pigment gel	0.3 to 1.0 mm	Movable binder clip
Pentel EnerGel	Liquid gel	0.3 to 1.0 mm	Usually the quick-drying pick

Our best gel pens roundup picks one pen for each job.

Where is the uni-ball One made?

A US business-supply listing for the 0.7 mm blue dozen (70363) gives Japan as the country of origin, and so do Cainz in Japan and Escribo in Poland. Mitsubishi Pencil’s own pages don’t say. A Swiss retailer lists China for one 0.7 mm multi-pack and Japan for a refill set, so origin may vary by pack. Mitsubishi Pencil, which owns the uni-ball brand, is based in Tokyo.

Frequently asked questions

What makes the uni-ball One different from other gel pens?

Its ink. uni-ball formulates the One's pigment gel ink with larger particles that stay on the paper's surface instead of soaking in, so colors look darker and more vivid. That's the maker's claim; we haven't measured it.

What sizes does the uni-ball One come in?

In the US, uniball ONE packs come in 0.7 mm Medium and 0.5 mm Micro. In Japan, the One is sold in 0.38 and 0.5 mm, with 0.7 mm in some sets.

What refill does the uni-ball One use?

Japanese refills are UMR-05S for 0.5 mm and UMR-38S for 0.38 mm, each in several colors, listed by Mitsubishi Pencil at ¥110 (about US$0.70). We didn't find a US-packaged uniball ONE refill item number.

Is the uni-ball One waterproof?

uni-ball's US copy says its Super Ink protects against water, fading and fraud. It doesn't publish a test standard in the sources we found.

What is the difference between the uni-ball One, One F and One P?

They use the same pigment ink in different bodies. The One F has what Mitsubishi Pencil calls a stabilizer mechanism, a precision metal tip that lowers its center of gravity; the One P is a short, pocket-sized version.

How long is the uni-ball One?

JetPens lists the standard One at 13.9 cm (5.5 in) long and 10.3 mm in diameter. JetPens gives 10.5 mm at the widest point and 10 g, and Escribo agrees on length, grip and weight.

Where is the uni-ball One made?

A US business-supply listing, Japan's Cainz and Poland's Escribo give Japan as the country of origin. One Swiss retailer lists China for a 0.7 mm multi-pack, so check the pack you buy.

Does the uni-ball One skip?

Some early users reported skipping. The Gentleman Stationer noted in 2024 that the initial release had reports of skipping, though its author now calls the One F a favorite.

Sources (24)

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