Hydraulic Fitting Thread Chart & Size Reference

This chart lists the actual measured thread dimensions for the six major hydraulic fitting standards. Use it to identify unknown fittings by measuring with calipers and matching against the table.

NPT Thread Sizes

NPT (National Pipe Thread) sizes are nominal, not actual. The measured OD is always larger than the nominal size.

1/8 NPT: 0.405 in OD, 27 TPI 1/4 NPT: 0.540 in OD, 18 TPI 3/8 NPT: 0.675 in OD, 18 TPI 1/2 NPT: 0.840 in OD, 14 TPI 3/4 NPT: 1.050 in OD, 14 TPI 1 NPT: 1.315 in OD, 11.5 TPI

All NPT threads are tapered at 1 degree 47 minutes (3/4 inch per foot).

JIC (SAE 37-Degree Flare) Sizes

JIC fittings are identified by dash size, where each dash number equals 1/16 inch of tube OD.

-4 (1/4 in tube): 7/16-20 thread -6 (3/8 in tube): 9/16-18 thread -8 (1/2 in tube): 3/4-16 thread -10 (5/8 in tube): 7/8-14 thread -12 (3/4 in tube): 1-1/16-12 thread -16 (1 in tube): 1-5/16-12 thread

JIC threads are straight (parallel), not tapered. The seal is at the 37-degree flare, not the threads.

BSP Thread Sizes

BSP uses the same nominal sizing as NPT but the measured dimensions differ.

1/8 BSP: 0.383 in OD, 28 TPI 1/4 BSP: 0.518 in OD, 19 TPI 3/8 BSP: 0.656 in OD, 19 TPI 1/2 BSP: 0.825 in OD, 14 TPI 3/4 BSP: 1.041 in OD, 14 TPI 1 BSP: 1.309 in OD, 11 TPI

Note the TPI difference at 1/4 and 3/8: BSP is 19 TPI, NPT is 18 TPI. This is the key identifier.

ORFS Thread Sizes

ORFS fittings use SAE straight thread with an O-ring groove on the face.

-4: 9/16-18 thread -6: 11/16-16 thread -8: 13/16-16 thread -10: 1-14 thread -12: 1-3/16-12 thread -16: 1-7/16-12 thread

The flat face with O-ring groove is the identifying feature. Thread OD is larger than JIC for the same dash size.

Quick Identification Tips

NPT vs BSP at 1/4 inch: NPT is 18 TPI, BSP is 19 TPI. If you have a thread gauge, this is the fastest way to tell them apart.

JIC vs ORFS: Look at the nose. JIC has a 37-degree cone. ORFS has a flat face with an O-ring groove.

Metric (DIN/JIS): If the thread pitch is in whole millimeters (1.0, 1.5, 2.0), it is metric. DIN and JIS differ in thread angle (DIN is 60 degrees, JIS is also 60 degrees but with different sealing methods).

When in doubt, use a fitting identification kit or take the fitting to a hydraulic supply house.