How To Check Tube & Fitting Size

Learn How To Check Tube & Fitting Size. Be sure to check and order the correct size needed for your system. All tubes are measured by the OD (outer diameter) of the tube. Fittings have 2 sizes to check. One is the tube size and the other is the fittings thread size. Most residential POU RO reverse Osmosis systems use either 1/4" or 3/8" tube and NPT thread sizes.