Sooner or later every machinist, mechanic, and maintenance tech picks up a bolt, fitting, or stud and has no idea what thread it is. The label is gone, the drawing is missing, and you need to figure out whether it's 1/2-13 UNC, M12x1.75, or 1/2-14 NPT before you can order a replacement or cut a mating part. Get it wrong and you'll strip threads, create leaks, or spend an hour chasing your tail in the hardware drawer.
This guide covers a systematic approach to identifying unknown threads. It starts with the tools you need, walks through the measurement technique, and then provides quick-reference tables for UNC, UNF, metric, NPT, BSP, and ACME threads. It also covers the look-alike pairs that fool even experienced hands — the threads that measure almost the same but are not interchangeable.
Tools You Need for Thread Identification
You can identify most threads with three inexpensive tools: a caliper, a thread pitch gauge, and a thread identification chart. A micrometer is better than a caliper for precision, but a caliper gets you close enough for identification purposes.
Calipers: You need to measure the major diameter of the thread — the outside diameter of external threads or the minor diameter of internal threads. A 6-inch digital caliper with 0.001-inch resolution is adequate. For pipe threads, you'll also measure the nominal pipe size, which is not the same as the thread OD (more on that later).
Thread pitch gauge (screw pitch gauge): This is a set of thin metal leaves, each with a different thread profile cut into it. You hold each leaf against the thread until you find the one that fits perfectly into the thread grooves. Pitch gauges come in inch (TPI) and metric (mm pitch) versions. Buy both. A combined set costs about $10 and is the fastest way to determine thread pitch once you know you're looking at inch or metric.
Thread identification chart: A laminated reference card or poster that lists all common thread sizes with their major diameters and pitches. You measure the diameter, measure the pitch, and cross-reference on the chart. The combination of diameter plus pitch uniquely identifies the thread standard in most cases.
For more precise work, thread ring gauges and thread plug gauges provide go/no-go verification. These are expensive (typically $20-$80 per size) but are the definitive way to confirm thread class and fit. Most shops keep gauges for their most commonly used thread sizes.
Step-by-Step Measurement Technique
Follow this sequence every time you encounter an unknown thread. The order matters because each measurement narrows the possibilities.
Step 1: Measure the major diameter. For external threads (bolts, studs), measure across the thread crests with a caliper. For internal threads (nuts, tapped holes), measure the minor diameter (the smallest diameter inside the threads). Write this number down. If the diameter is close to a round metric number (6mm, 8mm, 10mm, 12mm), it's probably metric. If it's close to a fractional inch (0.250, 0.375, 0.500), it's probably inch.
Step 2: Determine the pitch. Hold thread pitch gauge leaves against the thread one at a time until you find an exact match. The leaf should seat fully into every thread groove with no light visible between the leaf and the thread. If an inch TPI gauge matches, it's an inch thread. If a metric pitch gauge matches, it's metric. Write down the pitch.
Step 3: Check for taper. Lay a straight edge along the thread. If the thread diameter gets smaller toward the end, it's a tapered thread (likely NPT or NPTF for inch, or BSPT for metric/British). If the thread is parallel (same diameter from end to end), it's a straight thread (UNC, UNF, metric, BSP parallel, etc.).
Step 4: Cross-reference diameter and pitch. Use a thread identification chart to find the standard that matches your measured diameter and pitch. Most of the time, this gives you a unique answer. When two standards have similar dimensions (like M12x1.75 and 1/2-13 UNC), additional checks are needed.
Step 5: Verify with a known fastener or gauge. If possible, thread a known bolt or nut onto the unknown thread. If it threads on smoothly with no binding, you've confirmed the identification. If it threads on with resistance or cross-threading, the sizes are close but not a match — keep looking.
Thread Pitch Identifier
Identify unknown threads from caliper measurements. Cross-references UNC, UNF, Metric ISO, NPT, BSP, and ACME thread standards with tolerance matching and confidence scoring.
Thread Standard Overview: The Big Picture
There are dozens of thread standards in the world, but six cover 99% of what you'll encounter in North American shops: UNC (Unified National Coarse), UNF (Unified National Fine), Metric (ISO), NPT (National Pipe Taper), BSP (British Standard Pipe), and ACME (power transmission threads).
UNC (Unified National Coarse): The default thread for general-purpose fasteners in the United States and Canada. When someone says "half-inch bolt" without specifying pitch, they mean 1/2-13 UNC. Coarse threads are stronger per diameter, easier to assemble, and more tolerant of imperfect tapping. They dominate structural, construction, and general industrial use.
UNF (Unified National Fine): Fine-pitch version of the same diameters. A 1/2-20 UNF bolt has the same major diameter as 1/2-13 UNC but more threads per inch. Fine threads have higher tensile strength per diameter (more thread engagement), better resistance to vibration loosening, and allow finer adjustment. Common in aerospace, automotive, and precision applications.
Metric (ISO 261/262): The global standard outside North America. Designated as M(diameter)x(pitch), like M10x1.5. Each diameter has a coarse pitch (default) and one or more fine pitches. Metric coarse is used unless specified otherwise. M10 always means M10x1.5 (coarse); M10x1.25 is explicitly called out as fine.
NPT (National Pipe Taper): Tapered threads used for sealing pipe connections. The taper is 1/16 inch per inch (3/4 degree per side). NPT threads seal by wedging the tapered male into the tapered female, deforming the thread crests. NPT always requires thread sealant (tape or dope) for a leak-free joint.
BSP (British Standard Pipe): Parallel (BSPP) or tapered (BSPT) pipe threads used in Europe, Asia, and many other countries. BSP uses a 55-degree thread angle versus NPT's 60-degree angle. BSP and NPT are NOT interchangeable even when the nominal sizes match.
UNC and UNF Quick Reference
UNC and UNF threads are defined by ASME B1.1. Here are the most common sizes you'll encounter in the shop:
UNC (Coarse) common sizes: #6-32 (0.138" dia), #8-32 (0.164"), #10-24 (0.190"), 1/4-20 (0.250"), 5/16-18 (0.3125"), 3/8-16 (0.375"), 7/16-14 (0.4375"), 1/2-13 (0.500"), 9/16-12 (0.5625"), 5/8-11 (0.625"), 3/4-10 (0.750"), 7/8-9 (0.875"), 1"-8 (1.000").
UNF (Fine) common sizes: #6-40 (0.138" dia), #8-36 (0.164"), #10-32 (0.190"), 1/4-28 (0.250"), 5/16-24 (0.3125"), 3/8-24 (0.375"), 7/16-20 (0.4375"), 1/2-20 (0.500"), 9/16-18 (0.5625"), 5/8-18 (0.625"), 3/4-16 (0.750"), 7/8-14 (0.875"), 1"-12 (1.000").
Notice that at any given diameter, the UNF pitch is always higher (more TPI) than UNC. At 1/2 inch, coarse is 13 TPI and fine is 20 TPI. This is consistent across the entire range. If you measure the diameter and the pitch matches the coarse column, it's UNC. If it matches the fine column, it's UNF. If it doesn't match either, it's probably metric or a specialty thread.
Thread classes (1A/1B, 2A/2B, 3A/3B) define the fit tolerance. Class 2A/2B is the standard fit for general-purpose fasteners. Class 3A/3B is tighter, used in aerospace and precision work. Class 1A/1B is looser, used for easy assembly in dirty environments. The thread class doesn't change the pitch or diameter, only the tolerance band around the nominal dimensions.
For tapped holes, the drill size is critical. A #7 drill (0.201") produces a 75% thread in 1/4-20. A #21 drill (0.159") produces 75% thread in #10-24. Thread percentage above 75% adds very little strength but dramatically increases tapping torque and tap breakage risk. Most shops tap at 60-75% thread engagement.
Thread Pitch Identifier
Identify unknown threads from caliper measurements. Cross-references UNC, UNF, Metric ISO, NPT, BSP, and ACME thread standards with tolerance matching and confidence scoring.
Metric Thread Quick Reference
Metric threads are defined by ISO 261 and ISO 262. The designation format is M(major diameter in mm)x(pitch in mm). When only the M-number is given (e.g., "M10 bolt"), the coarse pitch is implied.
Common metric coarse sizes: M3x0.5 (3mm dia), M4x0.7 (4mm), M5x0.8 (5mm), M6x1.0 (6mm), M8x1.25 (8mm), M10x1.5 (10mm), M12x1.75 (12mm), M14x2.0 (14mm), M16x2.0 (16mm), M20x2.5 (20mm), M24x3.0 (24mm).
Common metric fine sizes: M8x1.0, M10x1.0, M10x1.25, M12x1.25, M12x1.5, M14x1.5, M16x1.5. Fine metric threads are used in automotive (particularly European and Japanese vehicles), precision instruments, and applications where vibration resistance or fine adjustment is needed.
The pitch in metric threads is the distance between thread crests in millimeters. This is conceptually different from TPI (threads per inch) used in inch standards, but the math is straightforward: TPI = 25.4 ÷ pitch(mm). So M10x1.5 has an equivalent of 25.4 ÷ 1.5 = 16.93 TPI, which is close to but not the same as any standard inch TPI. This "close but not right" is how you can distinguish metric from inch when a pitch gauge seems to almost-but-not-quite fit.
Metric bolt head markings indicate grade (strength class). Common grades are 8.8 (medium carbon steel, similar to SAE Grade 5), 10.9 (alloy steel, similar to SAE Grade 8), and 12.9 (alloy steel, socket head cap screws). The first number is the ultimate tensile strength divided by 100 (in MPa), and the second number times the first gives the yield strength percentage.
NPT vs. BSP: The Pipe Thread Trap
NPT and BSP pipe threads are the most commonly confused threads in industrial maintenance. They're both pipe threads, both come in similar nominal sizes, and they can sometimes partially engage each other — just enough to give you false confidence before the leak starts.
Key differences: NPT has a 60-degree thread angle; BSP has a 55-degree thread angle. NPT threads per inch differ from BSP at most sizes: 1/2" NPT is 14 TPI while 1/2" BSP is also 14 TPI, but the diameters differ. 1/4" NPT is 18 TPI while 1/4" BSP is 19 TPI. The thread forms are different enough that cross-threading an NPT fitting into a BSP port (or vice versa) creates a connection that looks engaged but will leak under pressure.
How to tell them apart: The most reliable method is the pitch gauge test combined with a diameter check. NPT 1/2" has a major diameter of approximately 0.840" at the large end. BSP 1/2" (G1/2 or R1/2) has a major diameter of approximately 0.825". The 0.015" difference is measurable with a caliper. At the same time, check the thread angle: an NPT pitch gauge will seat perfectly in NPT threads but will show slight light gaps in BSP threads because of the 5-degree angle difference.
Where you'll encounter BSP: European-made hydraulic equipment, pneumatic components manufactured in Asia, imported pressure gauges, and process instrumentation. If equipment was made in the UK, Germany, Japan, or China, assume BSP until proven otherwise. If it was made in the US, assume NPT.
Adapters: NPT-to-BSP adapters exist and are commonly used in mixed-equipment plants. They use an O-ring or gasket to seal instead of relying on thread interference, since the thread forms can't create a proper taper seal across standards.
Thread Pitch Identifier
Identify unknown threads from caliper measurements. Cross-references UNC, UNF, Metric ISO, NPT, BSP, and ACME thread standards with tolerance matching and confidence scoring.
Common Look-Alike Threads That Fool Everyone
Some thread combinations are close enough in size that they can partially engage, leading machinists and mechanics to think they have a match when they don't. Here are the notorious look-alikes:
M12x1.75 vs. 1/2-13 UNC: M12 has a 12mm (0.472") major diameter; 1/2" UNC has a 0.500" major diameter. The 0.028" difference means they won't cross-thread, but they're close enough that a quick eyeball can't tell them apart. The pitch gauge is the definitive test: 1.75mm pitch (14.5 TPI equivalent) vs. 13 TPI are clearly different.
M8x1.25 vs. 5/16-18 UNC: M8 is 0.315" and 5/16" is 0.3125". Only 0.003" apart. These can actually cross-thread if forced. The pitch is the giveaway: 1.25mm (20.3 TPI equivalent) vs. 18 TPI. Always use a pitch gauge for this pair.
M10x1.5 vs. 3/8-16 UNC: M10 is 0.394" and 3/8" is 0.375". Close but different. The pitches are also close: 1.5mm (16.9 TPI equivalent) vs. 16 TPI. A pitch gauge leaf for 16 TPI will almost fit an M10x1.5 thread but will show slight gaps at the ends. If you're not sure, try both gauge leaves and compare the fit.
NPT vs. NPS (straight pipe): National Pipe Taper and National Pipe Straight have the same TPI at each nominal size, but NPT is tapered and NPS is parallel. An NPS fitting will thread into an NPT port for a few turns before binding. The taper check (straight edge along the thread) instantly distinguishes them.
ACME vs. trapezoidal (metric): ACME threads have a 29-degree included angle; ISO trapezoidal (Tr) threads have a 30-degree angle. The pitches also differ. These are power-transmission threads used on lead screws, jacks, and vises. Cross-threading ACME into a trapezoidal nut causes binding and wear.