Gasket and AN Fitting Size Converter
Convert between AN fitting sizes, dash sizes, tube OD in inches and millimeters, and hose ID for automotive and aerospace plumbing.
Select a dash size or enter a tube OD — all dimensions calculate automatically.
-8 AN = 1/2" tube OD (12.70 mm) — Return lines
-10 AN = 5/8" tube OD (15.88 mm) — High-flow fuel
Understanding AN Fitting and Dash Sizes
AN (Army-Navy) fittings are a standardized system of fittings and adapters originally developed for military aircraft in the United States during World War II. They are now widely used in automotive racing, aerospace, hydraulic systems, and high-performance plumbing applications. The sizing system uses “dash sizes” that correspond to the tube outside diameter (OD) in sixteenths of an inch.
The Dash Size System:
The dash number indicates the tube OD in sixteenths of an inch:
Tube OD (inches) = Dash size / 16
Dash size = Tube OD (inches) × 16
For example, a -6 (dash six) fitting fits a tube with 6/16 = 3/8 inch OD.
AN Fitting Size Reference Table:
| Dash Size | Tube OD (in) | Tube OD (mm) | Hose ID (in) | Hose ID (mm) | Thread Size |
|---|---|---|---|---|---|
| -2 | 1/8 (0.125) | 3.18 | 0.094 | 2.39 | 5/16-24 |
| -3 | 3/16 (0.188) | 4.76 | 0.156 | 3.96 | 3/8-24 |
| -4 | 1/4 (0.250) | 6.35 | 0.219 | 5.56 | 7/16-20 |
| -5 | 5/16 (0.313) | 7.94 | 0.281 | 7.14 | 1/2-20 |
| -6 | 3/8 (0.375) | 9.53 | 0.344 | 8.73 | 9/16-18 |
| -8 | 1/2 (0.500) | 12.70 | 0.438 | 11.13 | 3/4-16 |
| -10 | 5/8 (0.625) | 15.88 | 0.563 | 14.29 | 7/8-14 |
| -12 | 3/4 (0.750) | 19.05 | 0.688 | 17.48 | 1-1/16-12 |
| -16 | 1 (1.000) | 25.40 | 0.938 | 23.83 | 1-5/16-12 |
| -20 | 1-1/4 (1.250) | 31.75 | 1.188 | 30.18 | 1-5/8-12 |
| -24 | 1-1/2 (1.500) | 38.10 | 1.438 | 36.53 | 1-7/8-12 |
| -32 | 2 (2.000) | 50.80 | 1.875 | 47.63 | 2-1/2-12 |
Hose ID vs Tube OD:
The hose inside diameter (ID) is always slightly smaller than the tube OD for the same dash size. This is because hoses fit OVER fittings, while tubes fit INTO fittings. The hose ID is approximately:
Hose ID (inches) ≈ Tube OD - (1/32 to 1/16 inch)
Common Applications by Dash Size:
| Dash Size | Common Automotive Use |
|---|---|
| -3 | Brake lines, vacuum lines |
| -4 | Brake lines, nitrous lines, small coolant |
| -6 | Fuel feed lines, oil feed lines, power steering |
| -8 | Fuel return, oil drain, larger coolant |
| -10 | Fuel supply (high flow), oil cooler lines |
| -12 | Radiator hoses, large oil lines |
| -16 | Radiator hoses, intercooler piping |
Practical Examples:
- A -6 AN fuel line uses tubing with 3/8 inch (9.53 mm) OD and hose with 0.344 inch (8.73 mm) ID.
- Converting a standard 1/2 inch brake line to AN fittings requires -8 AN components.
- A turbo oil feed line typically uses -4 AN (1/4 inch tube OD, 6.35 mm) fittings and lines.
Tips:
- AN fittings use 37-degree flared seats, while JIC fittings also use 37-degree flares. They are often interchangeable but always verify the standard.
- Never mix AN (37-degree) with SAE 45-degree flare fittings. They look similar but do not seal properly together.
- Aluminum AN fittings are color-coded by size: -4 is blue, -6 is red, -8 is blue, -10 is red, and so on in an alternating pattern.
- Always use the correct wrench sizes for AN fittings to avoid damaging the soft aluminum hex surfaces.