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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 SIZE |
Hose/Tube O.D. Inches |
Closest SAE Thread Size |
| -2 | 1/8" | 5/16-24 |
| -3 | 3/16" | 3/8-24 |
| -4 | 1/4" | 7/16-20 |
| -5 | 5/16" | 1/2-20 |
| -6 | 3/8" | 9/16-18 |
| -8 | 1/2" | 3/4-16 |
| -10 | 5/8" | 7/8-14 |
| -12 | 3/4" | 1-1/16-12 |
| -16 | 1" | 1-5/16-12 |
| -20 | 1-1/4" | 1-5/8-12 |
| -24 | 1-1/2" | 1-7/8-12 |
| -28 | 1-3/4" | 2-1/4-12 |
| -32 | 2" | 2-1/2-12 |
| NOTE: A sealer is NOT required for "AN" type fittings. | ||
"NPT" Thread Sizes (National Pipe Taper)
NPT sizes 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).
| Pipe Thread Size |
Threads Per Inch |
App. Interior Diameter |
Closest AN Size |
| 1/16" | 27 | 1/16" | 2 |
| 1/8" | 27 | 1/8" | 4 |
| 1/4" | 18 | 1/4" | 6 |
| 3/8" | 18 | 3/8" | 8 |
| 1/2" | 14 | 1/2" | 10 |
| 3/4" | 14 | 3/4" | 12 |
| 1" | 11-1/2 | 1" | 16 |
| 1-1/4" | 11-1/2 | 1-1/4" | 20 |
| 1-1/2" | 11-1/2 | 1-1/2" | 24 |
| 2 | 11-1/2 | 2" | 32 |
| NOTE: A sealer is required for "NPT" type fittings. | |||
