Pen refills adhere to ISO standard 12757 and the later revision ISO 12757-1:2017. This isn’t something that’s mandatory — custom and proprietary refills certainly aren’t illegal — but for practical purposes if you are making a certain refill type, it makes sense that yours is the same dimensions as that of all the other makers.
The standard has two main components: one for general use and one for “documentary use,” basically a stricter standard. This latter application is ISO 12757-2, and it would cover the legibility of letters as well as writing for archival documents.
Most refills in this guide were defined by ISO standards 12757-1 and 12757-2. These documents are not freely available, nor are they cheaply obtained.
Pen Refills Compared
Here is an image of some of the many ISO standard pen refills (mostly ballpoints, but gel and hybrid refills could take these shapes as well):
The leftmost column of the image is the diameter in millimeters, the circled item is the name, then there is the length in millimeters, and finally ou an see an image of refill with the contours. A true technical drawing of each refill would have many more measurements, as there are very exact specifications for the entire refill, but this will give you some idea of what each refill looks like. Additionally, this should be a handy visual guide to standard pen refills if you happen to have a refill you are unable to identify.
ISO Standard Pen Refill Types
The D format, commonly known as the D1, is a small refill usually seen in multipens. Unlike some of the other standards in this list, the D1 remains a very popular refill to this day.
The ISO 12757-2-A2, commonly known as the A2 format, is a slim, metal cylinder with a very thin writing tip. It was mainly used in retractable ballpoint pens before the 1990s. This refill is rarely seen any more as it’s largely been replaced with proprietary refills and plastic parts.
Example Refill: Schneider 75 M ($16.05)
Specs: 106.8mm long, spring shoulder 33.4mm from the tip of the pen, 3.2mm diameter
A C1, or Cross style refill, is a screw-type refill that is almost always used with twist retractable pens. The refill (almost always) has a metal body which is a long, thin cylinder. On the top of the cylinder is a plastic piece which can be screwed into the pen body.
The metal cylinder is usually replaceable as the two parts are separate. The advantage to this is that the metal component can be taken from a compatible refill that does not use the plastic part on top.
Example Refill: Cross Ballpoint refills ($5.10)
Specs: 117 mm long
The ISO G2 refill, usually known as a Parker-style G2 refill, is one of the most common and versatile pen refills ever sold. It’s still popular today with pens in all price ranges. This refill size is sometimes known as the “Standard International Ballpoint” refill, but it’s no more standard than anything else in this section.
The X10 ballpoint refill is a variant of the more popular X20 refill (explained below), but it has a narrower tip. It’s also thin metal cylinder with the same spring-stopping wings as the X20.
Example Pens: Aurora Hastil, Aurora Thesi
This is another ISO standard refill that takes shape in a long cylinder. The X20 looks a lot like the A2 refill but it is sometimes seen in modern pens. The X20 has a thicker writing tip than the A2 so the two are not interchangeable. The X20 is usually made of metal, but it does not have to be.
It’s important to note that the X20 refill can take multiple shapes, some of which are long, skinny metal cylinders while others has a plastic piece at the top and are wider in the middle section. An example of this is the Schneider Express 740.
Example Refill: Schneider Office 765 ($12.50) and Schneider Express 775
Example Pen: Schneider Fave ($14.75)
Specs: 107mm long, 27mm from tip to middle of spring wings
Rollerball refills are briefly touched on in the “RB” refill above, noting a standard length of 110 mm and a diameter of 6.3 mm. And this refill is actually commonly adhered to, but the naming for varies wildly based on the company and where you are located.
For example, these refills are often called “European Rollerball” refill when it’s full-sized and a larger width, as seen in something like the Rotring Jazz.
Types of Rollerball Refill
Rollerball refills might loosely adhere to the RB standard, but there is still an unfortunate amount of variation from one to the next. In the image above you can see a “Pilot G2” style Zebra Sarasa gel refill next to a Schmidt 8120. The Schmidt 8120 refill is a full-sized capless, “European” rollerball refill.
You can clearly see that the two are about the same length, but their contours are different. The Schmidt has pronounced steps made out of black plastic at the top and bottom of the main cylinder. There is also a recessed spring holder in the lower piece of plastic. You can’t tell from the image, but the Schmidt refill is about 6.81 mm in diameter where the Zebra is 6.09 mm. This means the two are not completely compatible.
The refill at the bottom of the image is a rebranded Schmidt “P” rollerball refill. This is a P8126 to be exact, which is a short version of the Schmidt 8126 (without the “P”). This refill is 97.6 mm long versus the 110 mm of a full rollerball refill that is aimed at the RB standard set by ISO.
Pilot G2 Refill vs Parker-Style G2
Look at this image:
That’s a Zebra Sarasa Clip, which is a Japanese rollerball. Its refill fits the exact dimensions set in the ISO standard for the RB size! Interestingly, Japanese rollerball refills are often referred as being “Pilot G2” or just “G2” sized, due to the wild popularity of the Pilot G2 gel pen. This has caused endless amount of confusion with pen refill buyers over the years, but it’s understandable why it happened: the G2 is an ISO ballpoint standard, but also the name of a mega-popular pen. More people know about the pen than the ISO 12757 standard, so the “G2” is often thought of as a rollerball refill.
This is why people who review pens will often call the ISO standard the “Parker-style G2” while calling the rollerball size the “Pilot G2.”