Thread Pitch Identifier — Identify Unknown Threads from Caliper Measurements

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.

Assumptions

• Thread databases include UNC, UNF, Metric ISO, NPT, BSP, and ACME series per ASME B1.1, ISO 261, and ASME B1.20.1
• Matching algorithm uses tolerance bands around nominal major diameter and pitch values, not exact dimension matching
• Confidence scoring weighted by how closely the measured values fall within the standard tolerance class range
• External thread measurements assumed to be taken at the crest (major diameter) with a standard caliper
• TPI measurements assumed accurate to +/-0.5 TPI for coarse threads and +/-1.0 TPI for fine threads
• Tap drill size output based on 75% thread engagement unless otherwise specified by the user

Limitations

• Cannot distinguish left-hand from right-hand threads; hand must be verified visually
• Cannot identify proprietary, specialty, or non-standard thread forms (Whitworth BSF, BA, UNEF extra-fine)
• Worn, damaged, or plated threads may produce measurements that fall outside standard tolerance bands
• NPT vs NPS distinction requires taper verification that a single caliper measurement cannot confirm
• Does not identify thread class of fit (1A/2A/3A) from measurements alone; requires calibrated thread gauges
• Close metric/inch equivalents (e.g., M12x1.75 vs 1/2-13 UNC) may produce ambiguous results without precise pitch measurement

References

• ASME B1.1 - Unified Inch Screw Threads (UN, UNC, UNF thread dimensions and tolerances)
• ISO 261 - ISO General Purpose Metric Screw Threads (basic profile and nominal dimensions)
• ASME B1.20.1 - Pipe Threads, General Purpose (Inch) (NPT dimensions and tolerances)
• ISO 7-1 - Pipe Threads Where Pressure-Tight Joints Are Made on the Threads (BSP tapered)
• ASME B1.5 - Acme Screw Threads (general purpose Acme thread dimensions)
• Machinery's Handbook, 31st Edition - Thread Identification and Measurement Procedures
• ASME B1.2 - Gauges and Gauging for Unified Inch Screw Threads (gauge tolerances)

Frequently Asked Questions