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Thread Pitch Identifier — Identify Unknown Threads from Caliper Measurements

Cross-References UNC, UNF, Metric, NPT, BSP, and ACME Thread Standards with Confidence Scoring and Tap Drill Sizes

Free thread identification tool for machinists, mechanics, and maintenance techs. Measure an unknown bolt, stud, or tapped hole with a caliper and enter the major diameter and thread pitch (or TPI). The identifier cross-references your measurements against UNC, UNF, Metric ISO, NPT, BSP, and ACME thread databases to find the best match. Results include a confidence score, full thread designation, tap drill size, minor diameter, pitch diameter, and class of fit information. Handles both external (bolt) and internal (nut/tapped hole) measurements.

Pro Tip: The fastest way to ID a mystery bolt in the field: measure the major diameter with a caliper and count threads per inch with a thread pitch gauge. If the major diameter is a clean metric number (6mm, 8mm, 10mm, 12mm), it's almost certainly metric. If it's a fraction (0.250", 0.375", 0.500"), it's probably UNC or UNF. NPT threads have a taper — if the diameter changes along the length, you're looking at a pipe thread. When in doubt, try screwing on a known nut and see if it catches cleanly.

Look up tap drill sizes once you've identified the thread

Drill & Tap Calculator →

Calculate bolt torque specs for the identified thread

Bolt Torque Calculator →

Read the complete thread identification guide

Thread Identification Guide →

Check bolt circle layouts and hole spacing

Bolt Circle Calculator →
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Thread Pitch Identifier
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How It Works

  1. Measure Major Diameter

    Using a caliper, measure the outside diameter (OD) of the bolt or stud across the crests of the threads. For internal threads (tapped holes), measure the minor diameter at the root of the threads. Enter the measurement in inches or millimeters.

  2. Measure Thread Pitch

    Count the threads per inch (TPI) using a thread pitch gauge or by counting threads over one inch with a scale. For metric threads, measure the distance between adjacent thread crests in millimeters. If you don't have a gauge, the calculator accepts approximate values and uses fuzzy matching.

  3. Select Thread Type (Optional)

    If you already know the thread family (Unified, Metric, NPT, BSP, ACME), select it to narrow results. If you have no idea, leave it on "All" and the identifier will search every standard in the database.

  4. Indicate External or Internal

    Specify whether you measured an external thread (bolt/stud) or an internal thread (nut/tapped hole). Internal measurements are typically smaller than external for the same thread, and the calculator adjusts matching tolerances accordingly.

  5. Review Matches

    The identifier returns ranked matches with confidence scores. Each match shows the full thread designation, nominal size, TPI or pitch, tap drill size, minor diameter, pitch diameter, and the thread standard. High-confidence matches (>95%) are highlighted green. Multiple close matches indicate your measurement may be between standards — verify with a thread gauge or known nut.

Built For

  • Maintenance mechanics identifying unknown bolts and studs on imported machinery with mixed metric and imperial fasteners
  • Machinists determining thread specs for replacement parts when drawings are unavailable
  • Plumbers identifying pipe threads (NPT vs BSP) on valves and fittings from unknown sources
  • Auto mechanics matching thread sizes on aftermarket parts to OEM fasteners
  • Fabrication shops verifying thread specs before tapping or chasing threads on customer parts
  • Hobbyist machinists and restorers identifying obsolete or uncommon thread standards on antique equipment

Features & Capabilities

Multi-Standard Database

Cross-references UNC, UNF, UNEF, Metric ISO (coarse and fine), NPT, NPS, BSP (parallel and taper), ACME, and Stub ACME thread standards. Over 400 thread designations indexed with full dimensional data.

Confidence Scoring

Each match is scored from 0-100% based on how closely your measurements match the standard dimensions. A 98% match on major diameter with an exact TPI match returns high confidence. Fuzzy matching catches worn threads and imprecise measurements.

Tap Drill Sizes

Every identified thread includes the recommended tap drill size for approximately 75% thread engagement (the standard for most applications). Shows both fractional inch and number drill sizes for Unified threads, and millimeter sizes for metric threads.

Full Thread Geometry

Results include major diameter, minor diameter, pitch diameter, thread height, thread angle, and class of fit (1A/1B through 3A/3B for Unified, 6g/6H for Metric). Complete data for machining or inspection.

Pipe Thread Detection

Distinguishes between NPT (tapered, for sealing) and NPS (straight, for mechanical joints), and between BSP parallel (BSPP) and BSP taper (BSPT). Pipe threads are often misidentified because their nominal size doesn't match the measured OD.

External & Internal Mode

Adjusts matching algorithms based on whether you measured an external thread (bolt OD) or internal thread (tapped hole). Internal measurements are naturally smaller and the identifier compensates for this when matching against standards.

Frequently Asked Questions

Real-world threads are manufactured to tolerances, not exact nominal dimensions. A 1/2-13 UNC bolt has a nominal major diameter of 0.5000" but the actual diameter can range from 0.4985" to 0.4876" depending on the class of fit. Worn threads, plating thickness, and caliper measurement technique all add variation. The identifier uses tolerance bands, not exact values, which is why it returns a confidence percentage rather than a simple yes/no match.
NPT threads are tapered at 1 degree 47 minutes (3/4 inch per foot). If you measure the OD at two points along the thread length and get different readings, it's tapered (NPT). NPS (straight pipe) threads have the same OD everywhere. The thread pitch (TPI) is identical for NPT and NPS at the same nominal size. NPT is used for pressure-sealing connections; NPS is used for mechanical joints like conduit couplings.
UNC (Unified National Coarse) has fewer threads per inch and is the standard general-purpose thread in North America. UNF (Unified National Fine) has more threads per inch for the same diameter, providing higher tensile strength and better vibration resistance but less tolerance for cross-threading. For example, a 1/2" bolt comes in 1/2-13 UNC (13 TPI) or 1/2-20 UNF (20 TPI). UNC is used for most structural and general applications; UNF is common in automotive, aerospace, and precision instruments.
The identifier matches thread size and pitch, which are the same for left-hand and right-hand threads of the same designation. To determine hand, look at the thread helix: hold the bolt pointing away from you and if the threads spiral clockwise (up to the right), it's right-hand. Left-hand threads are uncommon and are typically found on left-side bicycle pedals, gas fittings (acetylene), and some turnbuckle ends. If you suspect a left-hand thread, this tool identifies the size and pitch while you verify the hand visually.
Some thread standards have very similar dimensions. For example, M12x1.75 metric and 1/2-13 UNC are close enough in major diameter that caliper measurements alone may not distinguish them. The TPI/pitch measurement usually breaks the tie: 1/2-13 has 13 TPI (1.954mm pitch) while M12x1.75 has 1.75mm pitch (14.29 TPI). When you see multiple close matches, verify the pitch measurement more carefully or try threading a known nut onto the fastener.
Disclaimer: Thread identification results are estimates based on caliper measurements and should be verified with thread gauges (go/no-go rings or plugs) before use in critical applications. Misidentified threads can cause joint failure, leaks, or fastener pullout. Always confirm thread identification on pressure vessels, structural connections, and safety-critical assemblies with proper inspection tools. ToolGrit is not responsible for identification errors or resulting assembly failures.

Learn More

Machinist

How to Identify an Unknown Thread: UNC, UNF, Metric, NPT, BSP, and ACME Thread Reference

Step-by-step guide to identifying unknown threads using calipers and a thread pitch gauge. Covers all major thread standards with comparison tables and common look-alike thread pairs.

Read Guide →

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