ISO metric screw thread

Last updated

The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. [1] They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.[ citation needed ]


The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres (e.g. an M6 screw has a nominal outer diameter of 6 millimetres).

Basic profile

Basic profile of all ISO metric screw threads ISO and UTS Thread Dimensions.svg
Basic profile of all ISO metric screw threads

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. [2] Each thread is characterized by its major diameter, D (Dmaj in the diagram), and its pitch, P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The thread depth is 0.54125 × pitch. The outermost 18 and the innermost 14 of the height H of the V-shape are cut off from the profile.

The relationship between the height H and the pitch P is found using the following equation where θ is half the included angle of the thread, in this case 30° : [3]


In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions; therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.

The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as


A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by the hyphen sign, - (e.g., M8-1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8). The length of a machine screw or bolt is indicated by a following x and the length expressed in millimetres (e.g., M8-1.25x30 or M8x30). Tolerance classes defined in ISO 965-1 can be appended to these designations, if required (e.g., M500– 6g in external threads).

External threads are designated by lowercase letter, g or h. Internal threads are designated by upper case letters, G or H.

Preferred sizes

ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. The 1st choice column follows (approximately) the R10 series from ISO 3, while the 2nd choice column are rounded off values from the R20 series from ISO 3. [4] [5]

ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261. [6]

ISO 262 selected sizes for screws, bolts and nuts
Nominal diameter
D (mm)
P (mm)
Nominal diameter
D (mm)
P (mm)
10.250.21621.5 or 1.5 or 1.5
1.60.350.2222.52 or 1.5
5.5 DIN13
& ISO [5]
81.251 or 0.75565.54
101.51.25 or 1605.54
121.751.5 or 1.256464

The “coarse” pitch is the commonly used default pitch for a given diameter. In addition, one or two smaller “fine” pitches are defined, for use in applications where the height of the normal “coarse” pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms “coarse” and “fine” have (in this context) no relation to the manufacturing quality of the thread.

In addition to coarse and fine threads, there is another division of extra fine, or “superfine” threads, with a very fine pitch thread. Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations. [7] Fine and superfine threads also have a greater minor diameter than coarse threads, which means the bolt or stud has a greater cross-sectional area (and therefore greater load-carrying capability) for the same nominal diameter.

Spanner (wrench) sizes

Below are some common spanner (wrench) sizes for metric screw threads. Hex head widths (width across flats, spanner size) are for DIN 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur for reasons of weight and cost reduction.

Spanner (wrench) size (mm)
Unified Screw Threads.png
Hex cap screw-no markings.svg
Hex nut, and bolt
Socket head cap screw.png
Socket-head cap screw
Button head screw.png Screw.PNG
  • Button-head cap screw
  • Counter-sunk
    flat-head cap screw
Grub Screw.svg
Set, or grub, screw




See also

Related Research Articles

Taps and dies are tools used to create screw threads, which is called threading. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair. A die is used to cut or form the male portion of the mating pair. The process of cutting or forming threads using a tap is called tapping, whereas the process using a die is called threading.

The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

Washer (hardware) Thin plate with a hole, normally used to distribute the load of a threaded fastener

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring, wear pad, preload indicating device, locking device, and to reduce vibration. Washers often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.

Bolted joint

Bolted joints are one of the most common elements in construction and machine design. They consist of fasteners that capture and join other parts, and are secured with the mating of screw threads.

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

A go/no-go gauge refers to an inspection tool used to check a workpiece against its allowed tolerances via a go/no-go test. Its name is derived from two tests: the check involves the workpiece having to pass one test (go) and fail the other (no-go).

British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, based on the Whitworth thread devised and specified by Joseph Whitworth in 1841, which was the world's first national screw thread standard, BSW thread angle 55°

British Association screw threads, or BA screw threads, are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.


A dowel is a cylindrical rod, usually made from wood, plastic, or metal. In its original manufactured form, a dowel is called a dowel rod. Dowel rods are often cut into short lengths called dowel pins. Dowels are commonly used as structural reinforcements in cabinet making and in numerous other applications, including:

Trapezoidal thread form Screw thread profiles with trapezoidal outlines

Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews. They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a vise or the leadscrew of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads.

British Standard Pipe (BSP) technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has been adopted as standard in plumbing and pipe fitting, except in North America, where NPT and related threads are used.

The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present.

Screw Type of fastener characterized by a thread wrapped around a cylinder core

A screw and a bolt are similar types of fastener typically made of metal, and characterized by a helical ridge, known as a male thread. Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread in the matching part.

United States Standard thread, also known as Sellers Standard thread, Franklin Institute thread and American Standard thread, is a standard for inch based threaded fasteners and washers.

ISO 898 is an international standard that defines mechanical and physical properties for metric fasteners. This standard is the origin for other standards that define properties for similar metric fasteners, such as SAE J1199 and ASTM F568M. It is divided into five (nonconsecutive) parts:

ISO 965 is an International Organization for Standardization (ISO) standard for metric screw thread tolerances. It specifies the basic profile for ISO general purpose metric screw threads (M) conforming to ISO 261.

The Swiss-designed Thury screw thread is a metric thread standard that was developed in the late nineteenth century for screws used in scientific and horological instruments. The thread is named after Marc Thury, an engineer and professor at the University of Geneva who contributed heavily to the standard's development.

The Löwenherz thread is a largely obsolete metric thread form designed in the late nineteenth century and frequently applied in precision instruments. It is named after Dr. Leopold Löwenherz, who was the director of the metrology institute Physikalisch-Technische Bundesanstalt in Berlin.

Preferred metric sizes are a set of international standards and de facto standards that are designed to make using the metric system easier and simpler, especially in engineering and construction practices. One of the methods used to arrive at these preferred sizes is the use of preferred numbers and convenient numbers such as the Renard series, the 1-2-5 series to limit the number of different sizes of components needed.


  1. ISO/TC/ 1 Business Plan, 2007-03-05, Version 1.3. Table 3: The market share of each screw thread, p. 7.
  2. ISO 68-1:1998 ISO general purpose screw threads – Basic profile – Part 1: Metric screw threads. International Organization for Standardization.
  3. Oberg et al. 2000 , p. 1706
  4. ISO 261:1998 ISO general purpose metric screw threads – General plan. International Organization for Standardization. 17 Dec 1998.
  5. 1 2 ISO & DIN13
  6. ISO 262:1998 ISO general purpose metric screw threads – Selected sizes for screws, bolts and nuts. International Organization for Standardization. 17 Dec 1998.
  7. "Final report" (PDF).
  8. ISO 965 in the Catalogue on the Official ISO website