The Omnipure K-Series K5655-BB inline filter features 1/4" FPT fittings and 1/4" QC fittings on both ends. This filter features an ion exchange process producing deionized water which is water that lacks ions, such as those from sodium, calcium, iron, and copper.
National Pipe Thread (NPT) Size Measured Against a Ruler*
 Actual Diameter
(Inches) 		National Pipe Thread (NPT)
(Fractional)
0.405" 1/8"
0.540" 1/4"
0.675" 3/8"
0.840" 1/2"

*NOTE: A very common misconception is that a 1/4" NPT measures as 1/4 inch on a ruler or tape measurer. However, that is not the case.  NPT will measure larger in diameter.  Above is a sizing reference to help clarify.