The Pilot G2 is a rollerball style refill designed to fit in Pilot G2 retractable pens. While not technically a standardized size, many refills are commonly referred to as being Pilot G2-sized or Pilot G2 compatible.

Please note: Pilot’s G2 is completely different from the ISO standard ballpoint refill, which is generally known as the Parker-style G2. It’s unfortunately that they are both referred to as G2, but they are clearly different and not interchangeable if viewed outside of a pen body.