Thread Pitch Identifier
The Thread Pitch Identifier matches measured outer diameter and pitch/TPI to NPT, BSP, UNC/UNF, and Metric thread standards. It features an interactive SVG thread profile comparison (60° vs 55° angles), tap drill recommendations, and a cross-reference table across all standards — free, no signup required.
Thread Measurement
Thread Profile Comparison
NPT/Metric 60°
BSP 55°
UN/Metric 60°
Standard Comparison
| Standard | Thread Angle | Tapered | Sealing | Common Use |
|---|---|---|---|---|
| NPT | 60° | Yes | Thread taper (tape/sealant required) | North America plumbing, gas, hydraulic |
| BSP (BSPT) | 55° | Yes | Thread taper | International plumbing, pneumatic |
| BSP (BSPP) | 55° | No | Bonded washer / O-ring | International hydraulic, instrumentation |
| UNC | 60° | No | Mechanical fastening | General purpose bolts (coarse) |
| UNF | 60° | No | Mechanical fastening | Precision bolts, automotive (fine) |
| Metric (ISO) | 60° | No | Mechanical fastening | Worldwide standard fasteners |
Thread Reference Table
| Designation | Major OD (mm) | Major OD (in) | Pitch (mm) | TPI | Tap Drill (mm) | Tap Drill |
|---|---|---|---|---|---|---|
| 1/8" NPT | 10.29 | 0.405 | 0.941 | 27.0 | 8.7 | 11/32 |
| 1/4" NPT | 13.72 | 0.540 | 1.411 | 18.0 | 11.1 | 7/16 |
| 3/8" NPT | 17.14 | 0.675 | 1.411 | 18.0 | 14.3 | 9/16 |
| 1/2" NPT | 21.34 | 0.840 | 1.814 | 14.0 | 17.9 | 45/64 |
| 3/4" NPT | 26.67 | 1.050 | 1.814 | 14.0 | 23.0 | 29/32 |
| 1" NPT | 33.40 | 1.315 | 2.209 | 11.5 | 29.4 | 1-5/32 |
| 1-1/4" NPT | 42.16 | 1.660 | 2.209 | 11.5 | 38.1 | 1-1/2 |
| 1-1/2" NPT | 48.26 | 1.900 | 2.209 | 11.5 | 44.2 | 1-47/64 |
| 2" NPT | 60.33 | 2.375 | 2.209 | 11.5 | 56.1 | 2-7/32 |
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What is Thread Pitch Identification?
Thread pitch identification is the process of determining the thread standard (NPT, BSP, UNC/UNF, or Metric) of an unknown fastener or fitting by measuring its outer diameter and thread pitch. NPT threads are tapered at 1°47' with a 60° thread angle and seal on the threads themselves. BSP threads have a 55° angle and can be parallel (BSPP) or tapered (BSPT). UNC/UNF are straight threads with 60° angle defined by threads per inch. Metric threads use a 60° angle with pitch measured in millimeters. Confusing NPT and BSP is a common cause of cross-threading and leaks.
How to Use This Identifier
- Measure the outer diameter of the thread crests using calipers (mm or inches)
- Count threads per inch (TPI) using a thread pitch gauge, or measure pitch in mm between thread crests
- Select whether you measured pitch in mm or TPI
- Click Identify to see all matching thread standards and their specifications
- Compare the SVG thread profiles to understand the differences between matching standards
Frequently Asked Questions
How do I tell NPT from BSP threads apart?
The key differences are: (1) Thread angle — NPT is 60°, BSP is 55°. (2) Taper — NPT is always tapered at 1°47'; BSP can be parallel (BSPP) or tapered (BSPT). (3) TPI values differ at the same nominal size. For example, 1/2" NPT has 14 TPI while 1/2" BSP has 14 TPI as well, but the major diameters differ (0.840" vs 0.825").
Can I use NPT fittings with BSP threads?
No, NPT and BSP threads are not interchangeable even when they appear similar. The different thread angles (60° vs 55°) and taper dimensions mean they will cross-thread and leak. Use the correct adapter fitting or replace one side with the matching standard. PTFE tape or pipe dope cannot compensate for the mismatch.
What is the difference between UNC and UNF?
UNC (Unified National Coarse) has fewer threads per inch and is the standard for general purpose fastening — easier to assemble and more resistant to cross-threading. UNF (Unified National Fine) has more TPI and provides stronger holding power in the same bolt diameter, better vibration resistance, and finer adjustment. Example: 1/4-20 UNC vs 1/4-28 UNF.