Fountain pens can be refilled by different methods. In the USA, patents were issued for Button, Click, Lever, Matchstick and coin methods of refill. All these methods were used to put a little pressure and deflate the sac or internal plate to allow ink to flow in and out. The vintage pens may require eyedropper fillers. There are self-filling pens that have a sac in the pens. Upon squeezing the sac, the pen gets filled or emptied of ink. The latest method is the use of cartridges or international cartridges; cartridges are known as international cartridges because of the large number of European companies who use them in the fountain pens. There is also the piston filler. This method uses a piston in the pen, which is driven up or down inside the barrel of the pen by twisting a knob at the end of the pen. Today, most pens use piston fillers or cartridges. There are converters that can convert the cartridge refill pen to a piston refill one.

The eyedropper filler ink pens have the maximum capacity to hold ink, with the least capacity attributed to the piston refills pens. The piston sometimes occupies half of the pen