Pen Refills Guide | UnsharpenPen Refill ISO Standard Many pen refills adhere to ISO standard 12757 and the later revision ISO 12757-1:2017 which is set by the International Organization for Standardization. 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
