A thread name packs the size, the pitch, the series, the fit and sometimes the hand into a few characters. Type any designation to see it taken apart, or read the families below: how each one is written, what every part means, and every series we have a page for.
The letters name the family: M metric, UN Unified inch, ACME, BUTT, NPT and the other pipe series, NH hose.
A diameter in millimetres (M10), in inches (1/4), a screw number (#10) or a pipe size (1/2 NPT).
Millimetres after the diameter for metric threads, threads per inch for inch threads. Without it, the coarse pitch is meant.
The last code, such as 6g, 2A or 2G, says how closely the thread is made, and for most families whether it is a bolt or a nut.
The metric thread used for general fasteners all over the world. The letter M, the diameter and the pitch name the thread; a tolerance class says how closely it is made.
Standards: ISO 68-1 and ISO 965
| Written | Part | What it says |
|---|---|---|
| M | Profile letter | Says it is the ISO metric thread with a 60° profile. |
| 10 | Nominal diameter | The nominal major diameter in millimetres. |
| 1.5 | Pitch | The pitch in millimetres, the distance from one thread to the next. If it is left out the coarse pitch of that diameter is meant, so M10 is the same as M10 × 1.5. |
| 6g | Tolerance class | A number and a letter. The number is the tolerance grade: how wide the tolerance zone is, smaller means tighter. The letter is the tolerance position: where the zone sits against basic size. Lower-case letters are for external threads and capitals for internal threads. |
| 5g6g | Two classes | When the pitch diameter and the crest diameter have different grades, the first class is for the pitch diameter and the second for the crest. A single class, as in 6g, applies to both. |
| 6H/6g | A fit | A class for the nut and a class for the bolt written together, the internal thread first. It names the whole fit, not one part. |
| S, N, L | Length of engagement | A short, normal or long group. Normal is understood when nothing is written. The class is chosen for the group. |
| LH | Hand | Left-hand thread. Right-hand is understood when nothing is written. |
| Ph3 P1.5 | Lead and pitch | A multi-start thread gives the lead (Ph) and the pitch (P). The number of starts is the lead divided by the pitch. |
| e, f, g | External positions that leave an allowance, large for e and small for g. The allowance leaves room for a coating and keeps the thread from binding. |
| h | External position with no allowance: the zone ends at basic size. |
| G | Internal position with a small allowance. |
| H | Internal position with no allowance: the zone starts at basic size. The usual position for nuts. |
| 6g and 6H | The usual medium classes for a bolt and a nut. Grade 6 is the medium grade. |
| Designation | What it is | Example on this site |
|---|---|---|
| M | ISO metric thread, coarse and fine pitches. One page for every diameter and pitch. | M10 × 1.5 |
Words used here, explained in the terminology guide: pitch, tolerance class, tolerance grade, tolerance position, lead, major diameter.
The inch thread of North America. The designation gives the size, the number of threads per inch, the series and a class that says fit and member.
Standards: ASME B1.1 (UN and UNR forms) and B1.15 (UNJ form)
| Written | Part | What it says |
|---|---|---|
| 1/4 | Nominal size | The nominal diameter in inches, as a fraction or a decimal such as 0.2500. The smallest sizes use screw numbers such as #10, which are gauge numbers and not diameters. |
| 20 | Threads per inch | How many threads there are in one inch. The pitch is 1 inch divided by it. |
| UNC | Series | The pitch series: C coarse, F fine, EF extra fine. A series with no letter after UN, simply UN, means a constant pitch used over many diameters. |
| UNR, UNJ | Thread form | A letter after UN changes the form of the root. R is a rounded root on the external thread. J is a controlled rounded root, used where fatigue strength matters. So UNRC is UNR form in the coarse series and UNJF is UNJ form in the fine series. |
| 2A | Class | The number is the class of fit (1, 2 or 3). The letter is the member: A for an external thread, B for an internal thread. |
| UNS | Special | A thread with a diameter, pitch or length of engagement that is not in the standard series. |
| LH | Hand | Left-hand thread. Right-hand is understood when nothing is written. |
| 1A, 1B | The loosest fit: generous clearance, for quick assembly even with dirty or lightly damaged threads. Class 1A has an allowance. |
| 2A, 2B | The standard class for most commercial fasteners. Class 2A has an allowance, which leaves room for a coating. |
| 3A, 3B | Close tolerances for precise fits. Class 3A has no allowance. |
| A and B | A is the external thread (bolt), B is the internal thread (nut). A class 2A bolt goes with a class 2B nut. |
| UNJ | The UNJ form is normally specified in the tightest classes, 3A and 3B. |
| Designation | What it is | Example on this site |
|---|---|---|
| UNC | Coarse-pitch member of the Unified family: the usual choice for general fasteners. | 1/4-20 UNC |
| UNF | Fine-pitch member: more threads for the same diameter, used where finer adjustment or more strength in thin walls is needed. | 1/4-28 UNF |
| UNEF | Extra-fine pitch, for thin walls and fine adjustment. | 1/4-32 UNEF |
| UN | Constant pitch: one pitch used over many diameters, for example for large sizes and for special products. | 1/2-16 UN |
| UNR / UNRC, UNRF, UNREF | The same series with a rounded root on the external thread. Not separate pages: the Thread form button on the page of the base series. | – |
| UNJ / UNJC, UNJF, UNJEF | The same series with the controlled root radius of aerospace and fatigue-loaded parts. Also the Thread form button. | – |
| UNS | Sizes outside the standard series. The page calculates them with the Other size tool. | – |
Words used here, explained in the terminology guide: class of thread, threads per inch, thread series, allowance, height of thread, root radius.
ASME also publishes metric threads. M is written as in ISO, MJ is the metric thread with a rounded root, UNM is the miniature series.
Standards: ASME B1.13M, B1.21M and B1.10M
| Written | Part | What it says |
|---|---|---|
| M | M profile | Written exactly like the ISO metric thread. The ASME standard B1.13M carries its own tables, shown on the ASME M profile pages. |
| MJ | MJ profile | The metric thread with a rounded, controlled root radius. It is the metric counterpart of the UNJ form and is used in aerospace and for parts under cyclic load. |
| 4h6h | MJ classes | Written like an ISO class: grade and position, pitch diameter first and crest diameter second. 4h6h is the tight external class with no allowance, and 4H5H the matching internal class. |
| UNM | Miniature series | The Unified miniature thread for very small diameters, written in millimetres with the series name UNM. |
| Designation | What it is | Example on this site |
|---|---|---|
| M (ASME B1.13M) | Metric M profile in the ASME standard. | M10 × 0.75 |
| MJ | Metric thread with a rounded root and tight classes. | MJ10 × 0.75 |
| UNM | Unified miniature series. | 0.70 UNM |
Words used here, explained in the terminology guide: root radius, tolerance class, flat form, rounded form.
Trapezoidal threads for lead screws and other threads that move a load. The designation gives the size, the threads per inch, the series and a class.
Standards: ASME B1.5 (Acme) and B1.8 (Stub Acme)
| Written | Part | What it says |
|---|---|---|
| 1 3/4 | Nominal size | Nominal major diameter in inches. |
| 4 | Threads per inch | The thread count in one inch. |
| ACME / STUB ACME | Series | Acme is the full trapezoidal thread. Stub Acme has shorter teeth for places where a shallow thread is needed. |
| 2G | Class | The number sets the fit, with higher numbers closer. The letter is G for general purpose, with clearance at the major diameter, or C for centralizing, where the major diameter fit centres the screw in its nut. |
| M1, M2 | Modified stub forms | Two modified Stub Acme forms of B1.8, listed as separate series. |
| 2G, 3G, 4G | General purpose, with clearance on the major diameter. 4G is the closest. |
| 2C, 3C, 4C | Centralizing, where the major diameter fit keeps the screw centred. 4C is the closest. |
| Designation | What it is | Example on this site |
|---|---|---|
| ACME | General purpose and centralizing Acme threads. | 1/4-16 ACME |
| STUB ACME | Stub Acme thread with shorter teeth. | 1/4-16 STUB ACME |
| STUB ACME M1 | Modified stub form M1. | 1/4-16 STUB ACME M1 |
| STUB ACME M2 | Modified stub form M2. | 1/4-16 STUB ACME M2 |
Words used here, explained in the terminology guide: lead, multiple start thread, pitch, major clearance.
A thread with one steep flank that takes the load and one sloping flank, for loads that act in one direction. Pull and push types are separate designations.
Standards: ASME B1.9
| Written | Part | What it says |
|---|---|---|
| 2.5 | Nominal size | Nominal major diameter in inches. |
| 8 | Threads per inch | Thread count in one inch. |
| BUTT / PUSH-BUTT | Series | BUTT is the pull type, PUSH-BUTT the push type. The dimensions are the same; the direction of the load flank is turned over. |
| 2, 3 | Class | Class 2 is the standard grade and class 3 the precision grade. |
| FL | Flat root | The root can be rounded (the usual form) or flat, written FL. |
| Designation | What it is | Example on this site |
|---|---|---|
| BUTT | Buttress thread, pull type. | 6-8 BUTT |
| PUSH-BUTT | Buttress thread, push type. | 6-8 PUSH-BUTT |
Words used here, explained in the terminology guide: load flank, clearance flank, flank angle.
Threads for pipes and fittings. A taper thread tightens by wedging; a straight thread holds a joint that seals some other way. The size is a pipe size, not a thread diameter.
Standards: ASME B1.20.1 and B1.20.3
| Written | Part | What it says |
|---|---|---|
| 1/2 | Nominal pipe size | The size name of the pipe. The thread diameter and the outside diameter of the pipe are larger than the number says. |
| 14 | Threads per inch | Set by the pipe size, so it need not be remembered. |
| NPT | Series | The letters say what the thread is: N for national, P for pipe, T for taper, S for straight. The last letters say the use: C coupling, M mechanical, L locknut, R railing, F Dryseal fuel or Dryseal. |
| Class 1, 2 | Dryseal class | For Dryseal threads class 1 does not require inspecting the truncation of crests and roots, class 2 does, for the tightest sealing joints. |
| LH | Hand | Left-hand thread, rare on pipes. |
| NPT (external) ↔ NPT or NPSC (internal) | A tapered external thread goes into a tapered nut or into a straight coupling thread (NPSC). |
| NPTF ↔ NPTF, NPSF | Dryseal taper threads seal on the threads themselves. NPSF is the straight internal thread that goes with NPTF. |
| PTF-SAE SHORT ↔ NPSI | The SAE short Dryseal thread and its internal straight thread. |
| NPSM, NPSL | Straight threads for mechanical joints, loose (NPSM) or with a locknut (NPSL). They do not seal on the threads. |
| Designation | What it is | Example on this site |
|---|---|---|
| NPT | Taper pipe thread for general use. | 1/2-14 NPT |
| NPTR | Taper pipe thread for railing joints. | 1/2-14 NPTR |
| NPSC | Straight pipe thread inside couplings. | 1/2-14 NPSC |
| NPSM | Straight thread for free-fitting mechanical joints. | 1/2-14 NPSM |
| NPSL | Straight thread for loose joints with a locknut. | 1/2-14 NPSL |
| NPTF | Dryseal taper pipe thread. | 1/2-14 NPTF |
| PTF-SAE SHORT | Dryseal taper thread, SAE short series. | 1/2-14 PTF-SAE SHORT |
| NPSF | Dryseal fuel thread, internal and straight. | 1/2-14 NPSF |
| NPSI | Dryseal intermediate thread, internal and straight. | 1/2-14 NPSI |
| PTF-SPL SHORT | Dryseal taper thread, special short series. | 1/2-14 PTF-SPL SHORT |
| PTF-SPL EXTRA SHORT | Dryseal taper thread, special extra-short series. | 1/2-14 PTF-SPL EXTRA SHORT |
| F-PTF | Dryseal taper thread of the fine series. | 1/2-18 F-PTF |
| SPL-PTF | Special-purpose Dryseal taper thread. | 1/2-27 SPL-PTF |
Words used here, explained in the terminology guide: taper thread, straight thread, wrench make-up, standoff, gauge plane, vanish thread.
Straight threads of hose couplings, from garden hose to fire hose. Not pipe threads: they are not interchangeable with NPT.
Standards: ASME B1.20.7
| Written | Part | What it says |
|---|---|---|
| 3/4 | Nominal size | The nominal size of the coupling in inches. |
| 11.5 | Threads per inch | The thread count in one inch. Half-values are normal for hose threads. |
| NH, NHR, NPSH | Series | NH is the full-form hose thread, NHR the version for garden hose, NPSH the straight thread for joining pipe and hose fittings. The fire hose series are listed separately. |
| Designation | What it is | Example on this site |
|---|---|---|
| NH | Hose coupling thread of full form. | 3/4-11.5 NH |
| NHR | Hose coupling thread for garden hose. | 3/4-11.5 NHR |
| NPSH | Straight hose coupling thread for joining. | 1/2-14 NPSH |
| NH (SPL) | Special hose coupling thread. | 4-6 NH |
| NH fire hose | Fire hose coupling threads of the appendix. | 3.5-6 NH |
Words used here, explained in the terminology guide: straight thread, pitch, thread series.
Series that have a page here or are listed in the terminology standard, and the old designations that the Unified series replaced.
Standards: ASA B1.11, ISO 7-1, ISO 228-1, ISO 2901 to 2904, B1.1 appendix
| Written | Part | What it says |
|---|---|---|
| AMO | Microscope objective | The thread of microscope objectives, written with the size and the threads per inch. |
| G, R, Rc, Rp | ISO pipe threads | G is a straight thread where the joint does not seal on the threads. R is a taper external thread that seals on the threads, Rc the taper internal thread and Rp the straight internal thread that go with R. All have the 55° Whitworth form. |
| Tr | Trapezoidal | The ISO metric trapezoidal thread for lead screws, written Tr, the diameter and the pitch in millimetres, for example Tr 20 × 4. |
| S | Miniature | The ISO miniature thread for very small diameters. |
| NC, NF, NEF | Old American National | Coarse, fine and extra-fine series of older drawings. They were replaced by UNC, UNF and UNEF; the threads are close but the old classes are not the new ones. |
| Designation | What it is | Example on this site |
|---|---|---|
| AMO | Microscope objective thread. | 0.800-36 AMO |
Words used here, explained in the terminology guide: thread series, taper thread.
| Written | Means | Family |
|---|---|---|
| M | ISO metric thread (and the ASME M profile). | ISO metric threads (M) |
| MJ | Metric thread with a rounded, controlled root radius. | Metric threads in the ASME standards (M, MJ, UNM) |
| UNM | Unified miniature thread. | Metric threads in the ASME standards (M, MJ, UNM) |
| 6g / 6H | Usual medium ISO classes for a bolt (g) and a nut (H). | ISO metric threads (M) |
| e, f, g / G | Metric tolerance positions with an allowance. | ISO metric threads (M) |
| h / H | Metric tolerance positions with no allowance. | ISO metric threads (M) |
| S, N, L | Short, normal or long length of engagement group. | ISO metric threads (M) |
| LH | Left-hand thread. Right-hand is the default. | ISO metric threads (M) |
| UNC | Unified coarse. | Unified inch threads (UN) |
| UNF | Unified fine. | Unified inch threads (UN) |
| UNEF | Unified extra fine. | Unified inch threads (UN) |
| UN | Unified constant pitch. | Unified inch threads (UN) |
| UNS | Unified special size. | Unified inch threads (UN) |
| UNR… | Unified with a rounded root on the external thread. | Unified inch threads (UN) |
| UNJ… | Unified with the controlled root radius, for fatigue. | Unified inch threads (UN) |
| 1A, 2A, 3A | Unified classes for an external thread, loosest to tightest. | Unified inch threads (UN) |
| 1B, 2B, 3B | Unified classes for an internal thread, loosest to tightest. | Unified inch threads (UN) |
| #10 | A screw number (gauge number), a small size. | Unified inch threads (UN) |
| ACME | Acme trapezoidal thread. | Acme and Stub Acme threads |
| STUB ACME | Acme with shorter teeth. | Acme and Stub Acme threads |
| 2G, 3G, 4G | Acme general purpose classes. | Acme and Stub Acme threads |
| 2C, 3C, 4C | Acme centralizing classes. | Acme and Stub Acme threads |
| BUTT | Buttress thread, pull type. | Buttress threads (BUTT) |
| PUSH-BUTT | Buttress thread, push type. | Buttress threads (BUTT) |
| NPT | Taper pipe thread for general use. | Pipe threads (NPT, NPTF and the straight series) |
| NPTF | Dryseal taper pipe thread. | Pipe threads (NPT, NPTF and the straight series) |
| NPSC | Straight pipe thread in couplings. | Pipe threads (NPT, NPTF and the straight series) |
| NPSM | Straight pipe thread, free-fitting mechanical joints. | Pipe threads (NPT, NPTF and the straight series) |
| NPSL | Straight pipe thread with a locknut. | Pipe threads (NPT, NPTF and the straight series) |
| NPTR | Taper pipe thread for railing joints. | Pipe threads (NPT, NPTF and the straight series) |
| NH, NHR, NPSH | Hose coupling threads. | Hose coupling threads (NH, NHR, NPSH) |
| AMO | Microscope objective thread. | Other designations you may meet |
| G, R, Rc, Rp | ISO pipe threads, 55° Whitworth form. | Other designations you may meet |
| Tr | ISO metric trapezoidal thread. | Other designations you may meet |
| NC, NF, NEF | Old American National series, replaced by UNC, UNF and UNEF. | Other designations you may meet |
| Standard | Series | Example |
|---|---|---|
| ISO 68-1 · Metric | Metric V-Thread (60°) | M10 × 1.5 |
| ASME B1.1 · Unified inch | UNC (coarse) | 1/4-20 UNC |
| ASME B1.1 · Unified inch | UNF (fine) | 1/4-28 UNF |
| ASME B1.1 · Unified inch | UNEF (extra fine) | 1/4-32 UNEF |
| ASME B1.1 · Unified inch | UN (constant pitch) | 1/2-16 UN |
| ASME B1.5 · Acme | Acme (general purpose) | 1/4-16 ACME |
| ASME B1.8 · Stub Acme | Stub Acme | 1/4-16 STUB ACME |
| ASME B1.8 · Stub Acme | Modified stub Acme M1 | 1/4-16 STUB ACME M1 |
| ASME B1.8 · Stub Acme | Modified stub Acme M2 | 1/4-16 STUB ACME M2 |
| ASME B1.9 · Buttress | Buttress (pull type) | 6-8 BUTT |
| ASME B1.9 · Buttress | Buttress (push type) | 6-8 PUSH-BUTT |
| ASME B1.10M · Unified miniature | UNM miniature | 0.70 UNM |
| ASA B1.11 · Microscope objective | Microscope objective | 0.800-36 AMO |
| ASME B1.13M · Metric M profile | M profile | M10 × 0.75 |
| ASME B1.20.1 · Pipe, general purpose | NPT (taper) | 1/2-14 NPT |
| ASME B1.20.1 · Pipe, general purpose | NPTR (taper, railing) | 1/2-14 NPTR |
| ASME B1.20.1 · Pipe, general purpose | NPSC (straight, coupling) | 1/2-14 NPSC |
| ASME B1.20.1 · Pipe, general purpose | NPSM (straight, mechanical) | 1/2-14 NPSM |
| ASME B1.20.1 · Pipe, general purpose | NPSL (straight, locknut) | 1/2-14 NPSL |
| ASME B1.20.3 · Dryseal pipe | NPTF | 1/2-14 NPTF |
| ASME B1.20.3 · Dryseal pipe | PTF-SAE SHORT | 1/2-14 PTF-SAE SHORT |
| ASME B1.20.3 · Dryseal pipe | NPSF (fuel, internal straight) | 1/2-14 NPSF |
| ASME B1.20.3 · Dryseal pipe | NPSI (intermediate, internal straight) | 1/2-14 NPSI |
| ASME B1.20.3 · Dryseal pipe | PTF-SPL SHORT | 1/2-14 PTF-SPL SHORT |
| ASME B1.20.3 · Dryseal pipe | PTF-SPL EXTRA SHORT | 1/2-14 PTF-SPL EXTRA SHORT |
| ASME B1.20.3 · Dryseal pipe | F-PTF (fine) | 1/2-18 F-PTF |
| ASME B1.20.3 · Dryseal pipe | SPL-PTF (OD tubing) | 1/2-27 SPL-PTF |
| ASME B1.20.7 · Hose coupling | NH (hose coupling) | 3/4-11.5 NH |
| ASME B1.20.7 · Hose coupling | NHR (reduced) | 3/4-11.5 NHR |
| ASME B1.20.7 · Hose coupling | NPSH (pipe-size hose) | 1/2-14 NPSH |
| ASME B1.20.7 · Hose coupling | NH (SPL) | 4-6 NH |
| ASME B1.20.7 · Hose coupling | NH fire hose | 3.5-6 NH |
| ASME B1.21M · Metric MJ profile | MJ profile | MJ10 × 0.75 |
This guide explains how designations are written. It does not give dimensions: open a thread page for the sizes and limits, or check a pair in the Fit Check. Always confirm a designation against the drawing and the standard it names.