The Only Pen You Need: A Practical Guide to Modular Pens

From School Desk to Work Desk

Back in high school, I was the kid who lingered at the stationery counter, hypnotized by the rainbow of gel pens and mechanical pencils. Fast forward a decade or two, and that habit didn't exactly fade—it just evolved. I still buy new pens, but I also have a stash from years ago that I'm slowly working through. That's when I realized the difference between student stationery and adult stationery. Modular pens, which felt useless during my school years, have become the one thing I keep in my bag and on my desk.

What Exactly Is a Modular Pen?

A modular pen, also called a multi-pen or multifunction pen, crams multiple ink cartridges into a single barrel. You get several colors or tools—like a pencil, stylus, or highlighter—without carrying a handful of separate pens. If you were around before 2000, you probably remember the craze: multi-color ballpoint pens with eight or ten colors in one body. Some artistic kids even used them to color entire drawings. The concept hasn't changed much, but modern versions add more modules and better ink.

Why They Make Sense for Grown-Ups

For students, modular pens are a money pit. The cartridges are slim to fit the barrel, so they run out fast. And schools often ban oil-based ballpoints because they don't work with scanning machines. If you wrote with gel cartridges all day, you'd burn through a small fortune in refills. But once you're working—or even in college—your writing volume drops. Suddenly, having the right color at your fingertips beats having a pen that lasts a month. That's where modular pens shine.

Picking Your First Modular Pen

The golden rule: check refill availability first. You want a pen that supports the colors and functions you need, then worry about ink quality and price. The big brands each have their own systems, so let's break them down.

Budget Options: The Basics

Most generic modular pens from Chinese brands like Aihao, Deli, or Kaco are dirt cheap and get the job done. You'll find them online and in stores like Miniso or KKV. They usually stick to standard red, black, blue, and green gel or ballpoint refills. The catch? Build quality can be hit or miss. Some models have threads that loosen on their own, which is annoying. If you can, try them in a physical store before buying.

LAMY: Design First

LAMY, now owned by Mitsubishi Pencil (uni), is all about aesthetics. Their st, logo, cp1, and 2000 series all have modular options, with two, three, or four colors. The refills are standard D1 ballpoints—black, red, blue, green—with a highlighter option that's just orange. The st and cp1 use a twist mechanism; the 2000 and logo use a gravity system where you point the color you want upward and click. That's not great for fast switching, but it looks sleek and works fine for occasional use.

Pilot: The Coleto System

Pilot's Coleto is the star of their Hi-Tec-C line. The name comes from the satisfying click when you pop the top to swap refills. Coleto bodies come in 2 to 5 slots and are often released in collabs with anime and IPs. The refills are where it gets wild: 15 ink colors on the Japanese site, but JetPens lists 33 including pastel, metallic, and fluorescent shades, each in 0.3mm, 0.4mm, or 0.5mm. There are also mechanical pencil and eraser modules. If you're a collector, beware—limited editions swap out the button shapes entirely.

Pentel: Quality Ink, Simple Shells

Pentel doesn't fuss over pen bodies, but their ink is top-notch. Their modular lines include the oil-based Vicuña, the quiet Calme, and the i+ series, which is fully modular. The i+ lets you mix Energel gel, Sliccies colored gel, and mechanical pencil modules. Sliccies comes in 15 colors, 0.3mm to 0.5mm. The cool part: the refill's stopper matches the ink color. MUJI used to rebrand these as all-clear pens, which looked great. Pentel's own bodies are only half transparent, so you can't see the color as easily. Also, the i+ has a little cap slot on top—a relic from headphone jack days, but you can stick a decorative plug in it.

For artists, Pentel's Multi 8 is a rarity: an 8-color mechanical pencil that uses 2.0mm lead. It's a niche tool for drafting and sketching, but if you need it, it's basically the only option.

uni (Mitsubishi Pencil): The Heavyweight

uni is the modular pen king. Every line has a multi-pen variant, from budget to premium. The two big ones are Style Fit (gel) and Jetstream (oil-based). Style Fit lets you build your own pen with 3 to 5 modules, and there's even a single-module version—great for testing colors. Refills range from 0.28mm to 0.5mm, with 16 standard colors plus mechanical pencil. Jetstream, on the other hand, is famous for its smooth, controlled oil-based ink. Hobonichi planners include a custom Jetstream multi-pen every year, which says a lot.

Refill Compatibility: The Dirty Secret

Here's where things get tricky. Modular pen refills mostly fall into two camps. The first is the ISO D1 standard: 2.3mm thick, 67mm long, usually metal. LAMY's M21 and M55 are D1. The second is a Japanese standard: 3mm thick, with lengths varying around 88 or 98mm, and they may or may not have a little ear for the spring. Many Chinese brands use this 3mm diameter too, so cheap replacements are easy to find.

But each brand tweaks it. Pilot Coleto refills are 93mm without the button—they also fit Juice Up multi-pens, and you can trim longer refills. Pentel Sliccies and uni Style Fit use 98mm; Pilot Acroball refills fit those as well. Zebra's modular pens are a mess—some use 3mm, some 3.5mm, some D1—so unless you love their ink or the Sharbo design, skip them. uni Jetstream refills are 88mm, which is the most common size for Chinese pens, so you can grab cheap generic refills easily.

Practical Tips for Refill Shopping

Before buying any refill, check the diameter and length. If you're just starting out, I'd suggest Pilot Coleto for the most color choices, or uni Jetstream for better ink and cheaper refills. Both are solid entry points.

Modular pens aren't for everyone. If you write a ton every day, the tiny cartridges will frustrate you. But for most adults, they're a smart way to cut down on clutter. One pen, all the colors you need, and a little bit of joy every time you click.

Happy writing.