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, a caliper reading may not represent the external nominal major diameter, so treat results as prompts and verify with a gauge or known mating fastener.

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. Browse the Reference Table (Optional)

   A filterable reference table lists the local database rows by family (Unified, Metric, NPT, BSP, ACME) so you can compare candidates side by side. Matching itself always screens the full local database.

4. Mind External vs Internal Measurements

   The identifier matches against external (major diameter) nominal dimensions. If you measured an internal thread (nut/tapped hole), readings can differ from the nominal major diameter, so treat candidates as prompts and confirm with a gauge or known mating fastener.

5. Review Matches

   The identifier returns ranked candidates with proximity-based confidence scores. Each match shows the thread designation, nominal size, TPI and pitch, thread form, and a tap drill prompt. 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-Family Local Database

Screens against a local database of 115 commonly published nominal sizes covering UNC, UNF, Metric ISO (coarse and fine), NPT, BSP parallel, and ACME. It does not include UNEF, NPS, BSPT, Stub ACME, BA, or proprietary forms.

Confidence Scoring

Each candidate is scored 0-100 based on how closely your measurements fall to the nominal diameter and pitch within fixed tolerance bands. Scoring is measurement proximity only - it is not gauge verification or class-of-fit determination.

Tap Drill Prompts

Most rows include a commonly published tap drill prompt for approximately 75% thread engagement. The correct drill depends on material, tap style, thread class, and depth - verify with the tap manufacturer before drilling.

Designation and Form Data

Results include nominal major diameter, TPI/pitch in both unit systems, thread form (60-degree V, 55-degree Whitworth, 29-degree trapezoidal, tapered pipe), and cross-reference notes for commonly confused sizes. Class of fit and minor/pitch diameters are not computed.

Pipe Thread Awareness

NPT (tapered) and BSP parallel rows are included, with warnings that a single caliper reading cannot confirm taper or distinguish NPT from NPS. Pipe threads are often misidentified because their nominal size doesn't match the measured OD - verify with gauges before pressure use.

Measurement Guidance

Provides guidance for external thread measurements (bolt OD across crests) and cautions for internal threads, where readings differ from nominal major diameter. The identifier matches the entered values against external nominal dimensions.

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