Whether it’s Celsius versus Fahrenheit or a kilometer versus a mile, there are divisive ways for unit of measurements. Even something as simple as a pen can have specific sizes and profiles to fit their pens while some brands even do refills designed for other brands. Fortunately, pen refills typically abide by ISO 12757-1:2017 to reduce confusion (https://www.iso.org/obp/ui/#iso:std:iso:12757:-1:ed-3:v1:en). While this standard is not required by law -custom and exclusive pen refills aren’t forbidden- it typically helps for practical purposes if you’re making a certain type of refill to ensure your refill dimensions are the same as other makers. 

The ISO 12757 standard has two components: 12757-1 and 12757-2. The former is intended for pen refills for “general use” such as taking school notes. The latter is intended for pen refills for “documentary use” as it covers the legibility of letters/characters and use on archival documents. 

Pen Refills at a Glance 

Below is an image of many ISO pen refill types. These refills typically contain ballpoint ink, but gel and hybrid inks also take these refill profiles as well.

ISO Standard Pen Refill Types

D1 Refill

The D, or more commonly known as D1 is a refill typically seen in multifunction and mini pocket pens due to their slim nature. 

G2 Refill

The G2, also known as the Parker-style G2 is one of the most ubiquitous pen refills sold on the market. A de-facto choice for many pen manufacturers on the market. This refill often goes by “Standard International Ballpoint”. G2 refills are characterized by their sculpted plastic end cap for pen retracting mechanisms. 

A2 Refill

The A2 refill has an incredibly thin writing tip and was popular prior to the 1990s for retractable pens. The A2 is designed for the ISO 12757-2 standard and has been superseded by cheaper proprietary plastic refills. 

B3 Refill 

While the B3 is an ISO standard, it is no longer common. Nonetheless, you may know it as it is the exact refill used in a Bic Cristal 

Ever had to sign a receipt after using a card? You’ve probably used this pen to do so

C1 Refill

A C1 refill is also known as a Cross-style refill. This refill has a plastic screw housing to be used with twist retractable pens. Additionally, this screw housing allows the user to replace the refill without an internal spring so there are no concerns about losing parts of your pen when changing refills.

X20 Refill

The X20 looks a lot like the A2 refill, they are not interchangeable. The X20 have thicker tips when compared to the A2 refill. The X20 is a unique refill standard as it does come in two different shapes: one with a long skinny cylinder with a wide notch in the middle section and a wide cylinder with a plastic endcap. 

X10 Refill

The X10 is often regarded as the little brother to the X20 refill. It features a narrower tip but with the same spring stops. This refill has limited exclusivity

Popular Pen Refills (Non-ISO standard)

Pilot G2

With its smooth and vibrant ink, the Pilot G2 is the best-selling pen in North America. The introduction of the Pilot G2 has caused mass confusion throughout pen refill buyers as it shares the same nomenclature with the G2 ISO standard. 

Euro-format Refill

The Euro-format refills are peculiar. Euro-styled refills aren’t as universal as other pen refill standards although there will be some reciprocity amongst most brands. This is due to some having a stepped shoulder towards the pen tip or/and tip thickness. If you’re risk adverse, stick to your pen’s brand for refills. Otherwise if you’re a thrill-seeking hooligan, there are plethora of refill options that might fit your pen. 

Fisher Space Pen Refill

The Fisher Space Pen Refill is designed for use in outer space due to their sealed and pressurized construction. Be that is it may, due to its patented construction, it is also capable of writing in adverse condition and on many surfaces (e.g. over grease, extreme temperatures, underwater). While the Fisher PR refill itself is not ISO compliant, it does come with a plastic adaptor that allows it to work in pens that use Parker-style G2 refills.