Pen refills often come in various line widths or weights. The Pen Refill Guide uses a standard for designating available weights (if known) of various refills in the database. The standards are as follows:
- XF (Extra Fine) – 0.5 mm or thinner
- F (Fine) – 0.4 mm to 0.6 mm (0.5 mm is most common)
- M (Medium) – 0.6 mm to 1 mm (0.7 mm is most common)
- B (Broad or Bold) – 0.9 mm to 1.2 mm (1 mm is most common)
- XB (Extra Broad or Bold) – 1.1 mm to 1.5 mm (1.4 mm is most common)
Most refill makers will offer Fine, Medium, and Broad options in Black ink, with more limited options for Blue, Red, and other colors. If a refill listed in The Pen Refill Guide does not list the available widths in the title, there is only one option or options are not known.
It is also possible that the manufacturer has a different designation than The Pen Refill Guide. For example, a few makers have Fine, Medium, and Broad refills that match up with the Medium, Broad, and Extra Broad width ranges. In addition, it is common for gel refills to have narrower lines than those above. A common Medium gel refill might actually be 0.5 mm in width rather than 0.7 mm.