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.

AN fittings and JIC fittings are interchangeable, depending on how critical your application is. AN fittings are MIL spec fittings designed for the aerospace industry, thus the materials used in them are higher tolerance, and the materials are stronger under various advanced conditions.

JIC fittings are the commercial equivalent to MIL Spec (AN) components.

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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