AN and NPT Fitting Sizing

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Originally posted on the now defunct:

AN and NPT Fittings Chart​

"AN" Thread Sizes​

AN sizes, originally developed for use by the U.S. Armed Forces ("A" for Army and "N" for Navy), describe the outside diameter (O.D.) of tubing in 1/16-inch increments. For example, an AN 2 fitting will fit a tube with an O.D. of 2/16", or 1/8", while an AN 8 fitting will fit a tube with an O.D. of 8/16", or 1/2". Because the actual thickness of tube walls can vary from brand to brand, the inside diameter of a tube is not used as a reference. You will also find the dash (-) symbol or the word "dash" itself used in conjunction with AN sizes. A "dash six" fitting translates to AN-6.

Each AN fitting has an established thread sizing. The following chart shows the relationship between AN size, tube O.D., and SAE thread size:

-AN SizeMetal Tube O.D. InchesClosest SAE Thread Size

"NPT" Thread Sizes​

NPT sizes (National Pipe Taper) are the most commonly used fitting sizes for general plumbing, piping, and tubing use; not quite as popular as AN for automotive use, but still very common. While AN fittings depend on the outside diameter of a tube for sizing, NPT fittings depend on the interior diameter (I.D.) of the fitting itself. The following chart shows the each size's thread-per-inch count, the I.D. of the fitting, and the AN fitting size with the closest-matching I.D.(inside dimension).

NPT SizeThreads Per InchApp. Interior DiameterClosest AN Size
1"11 1/21"16
1-1/4"11 1/21-1/4"20
1-1/2"11 1/21-1/2"24
2"11 1/22"32

NOTE: A thread sealer (such as Teflon Tape or Pipe Dope) is not required when "AN" type fittings are used, but is required for "NPT".


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