通徑在各國、各行業標準中都各有解讀。公稱通徑是一個圓整數它和實際通徑不*相同,它以不同形式的閥門和不同公稱壓力、不同規格尺寸來劃分為縮徑、不縮徑及全通徑、流通直徑。我國按GB/T12237、GB/T12238、GB/T7746來執行。 
         美國的通徑劃分。按美國國家標準ANSI.B16.1；美國工程師協會ASME.B16.34；美國石油協會AP16D；標準以不同的壓力規定不縮徑的zui小流通直徑。 
         英國的通徑體系劃分以BS5351規定了球閥、通孔閥和不同的壓力等級劃分zui小流通直徑。
         日本的通徑體系劃分以JIS.2001規定了通用閥門、船用、室外下水道通用，以不同的壓力等級劃分流通直徑，以我國標準為例。 
  
There are different understandings for DN in different countries. It is an integer which is a little different from practical diameter.It is divided into reduced bore, non-reduced bore,full bore and flow diameter by different valve, nominal Preassure, spec sizes. It is carried out by GB/T12238、GB/T7746 in China.
American standard.ANSI.B16.1;ASME.B16.34;AP16D.
Inch system is applied to BS5351.
Japanese diameter system conform to JIS.2001.