View source for ISO metric screw thread

{{short description|Hardware threading standard}}
{{Use British English|date=September 2022}}
{{Use dmy dates|date=September 2022}}
{{jargon|date=December 2020}}

The '''ISO metric screw thread''' is the most commonly used type of general-purpose [[screw thread]] worldwide.<ref>[http://isotc.iso.org/livelink/livelink/fetch/2000/2122/687806/ISO_TC_001__Screws_.pdf?nodeid=4938938&vernum=0 ISO/TC/ 1 Business Plan], 2007-03-05, Version 1.3. Table 3: The market share of each screw thread, p. 7.</ref> They were one of the first [[international standard]]s agreed when the [[International Organization for Standardization]] (ISO) was set up in 1947.{{Citation needed|date=July 2009}}

The "M" designation for metric [[Screw|screws]] indicates the [[Real versus nominal value|nominal]] outer [[diameter]] of the screw thread, in [[millimetres]]. This is also referred to as the "major" diameter in the information below. It indicates the diameter of smooth-walled hole that a male thread (e.g. on a [[Bolt (fastener)|bolt]]) will pass through easily to connect to an internally threaded component (e.g. a [[Nut (hardware)|nut]]) on the other side. For example, an M6 screw has a nominal outer diameter of 6&nbsp;millimetres and will therefore be a well-located, co-axial fit in a hole drilled to 6&nbsp;mm diameter.

==Basic profile==
[[Image:ISO and UTS Thread Dimensions.svg|thumb|right|400px|Basic profile of all ISO metric screw threads, where the male part has the external thread]]

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1.<ref>{{cite book | title=ISO 68-1:1998 ISO general purpose screw threads – Basic profile – Part 1: Metric screw threads | publisher=International Organization for Standardization | url=http://www.iso.org/iso/catalogue_detail.htm?csnumber=3707}}</ref> Each thread is characterized by its [[Screw thread#Diameters|major diameter]], ''D'' (''D''<sub>maj</sub> in the diagram), and its [[Thread pitch|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 {{frac|1|8}} and the innermost {{frac|1|4}} 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°:<ref>{{harvnb|Oberg|Jones|Horton|Ryffel|2000|p=1706}}</ref>

:<math>H  = \frac {1}{\tan\theta} \cdot \frac {P}{2} = \frac{\sqrt 3}{2}\cdot
P \approx 0.8660 \cdot P</math>

or

:<math>P  = 2\tan\theta\cdot H = \frac {2}{\sqrt 3} \cdot H \approx 1.1547 \cdot H</math>

Because only {{frac|5|8}} of this height is cut, the difference between major and minor diameters is {{frac|5|4}} × 0.8660 × ''P'' = 1.0825 × ''P'', so the [[tap drill]] size can be approximated by subtracting the thread pitch from the major diameter.

In an external (male) thread (e.g. on a bolt), the major diameter ''D''<sub>maj</sub> and the minor diameter ''D''<sub>min</sub> define ''maximum'' dimensions of the thread. This means that the external thread must end flat at ''D''<sub>maj</sub>, but can be rounded out below the minor diameter ''D''<sub>min</sub>. 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 ''D''<sub>min</sub> but may be rounded out beyond ''D''<sub>maj</sub>. In practice this means that one can measure the diameter over the threads of a bolt to find the nominal diameter ''D''<sub>maj</sub>, and the inner diameter of a nut is ''D''<sub>min</sub>.

The minor diameter ''D''<sub>min</sub> and effective pitch diameter ''D''<sub>p</sub> are derived from the major diameter and pitch as

:<math>\begin{align}
  D_\text{min} &= D_\text{maj} - 2\cdot\frac58\cdot H = D_\text{maj} - \frac{ 5 {\sqrt 3}}{8}\cdot P \approx D_\text{maj} - 1.082532 \cdot P \\[3pt]
    D_\text{p} &= D_\text{maj} - 2\cdot\frac38\cdot H = D_\text{maj} - \frac{ 3 {\sqrt 3}}{8}\cdot P \approx D_\text{maj} - 0.649519 \cdot P
\end{align}</math>

Tables of the derived dimensions for screw diameters and pitches defined in ISO 261 are given in [[ISO 724]].

==Designation==
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 a dash or sometimes the multiplication sign, ''×'' (e.g. M8-1.25 or 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).{{r|iso965-1|p=17}}

The length of a [[Screw#Machine screw|machine screw]] or bolt is indicated by an ''×'' and the length expressed in millimetres (e.g. M8-1.25×30 or M8×30).{{cn|date=May 2021}}

Tolerance classes defined in [[ISO 965|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.{{r|iso965-1|p=17}}

=={{anchor|ISO 261|ISO 262}} Preferred sizes==
{{See also|Preferred metric sizes#ISO_261 and 262 fastener diameters}}
'''ISO 261''' specifies a detailed list of preferred combinations of outer diameter ''D'' and pitch ''P'' for ISO metric screw threads.<ref name="iso261">{{cite book | title=ISO 261:1998 ISO general purpose metric screw threads – General plan | publisher=International Organization for Standardization | date=17 Dec 1998 | url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=4165}}</ref><ref>[http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf ISO & DIN13]</ref> '''ISO 262''' specifies a shorter list of thread dimensions – a subset of ISO 261.<ref>{{cite book | title=ISO 262:1998 ISO general purpose metric screw threads – Selected sizes for screws, bolts and nuts | publisher=International Organization for Standardization | date=17 Dec 1998 | url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=4167}}</ref>

{| class="wikitable"
|+ ISO 262 selected sizes for screws, bolts and nuts
|-
! rowspan=2 | Nominal <br/>diameter, <br />''D'' (mm)
! rowspan=2 | Series
! colspan=2 | [[Pitch (screw)|Pitch]], <br />''P'' (mm) 
| rowspan="21" scope="col" style="width:2em; border-top:0px; border-bottom:0px;" |
! rowspan=2 | Nominal <br/>diameter, <br />''D'' (mm)
! rowspan=2 | Series
! colspan=2 | [[Pitch (screw)|Pitch]], <br />''P'' (mm)
|-
! Coarse !! Fine
! Coarse !! Fine
|-
|  1  || R10 || 0.25 || 0.2
| colspan=4 {{n/a|︙}}
|-
| 1.2 || R10 || 0.25 || 0.2
| 16  || R10 || 2    || 1.5
|-
| 1.4 || R20 || 0.3  || 0.2
| 18  || R20 || 2.5  || 2 or 1.5
|-
| 1.6 || R10 || 0.35 || 0.2
| 20  || R10 || 2.5  || 2 or 1.5
|-
| 1.8 || R20 || 0.35 || 0.2
| 22  || R20 || 2.5  || 2 or 1.5
|-
|  2  || R10 || 0.4  || 0.25
| 24  || R10 || 3    || 2
|-
| 2.5 || R10 || 0.45 || 0.35
| 27  || R20 || 3    || 2
|-
|  3  || R10 || 0.5  || 0.35
| 30  || R10 || 3.5  || 2
|-
| 3.5 || R20 || 0.6  || 0.35
| 33  || R20 || 3.5  || 2
|-
|  4  || R10 || 0.7  || 0.5
| 36  || R10 || 4    || 3
|-
|  5  || R10 || 0.8  || 0.5
| 39  || R20 || 4    || 3
|-
|  6  || R10 || 1    || 0.75
| 42  || R10 || 4.5  || 3
|-
|  7  || R20 || 1    || 0.75
| 45  || R20 || 4.5  || 3
|-
|  8  || R10 || 1.25 || 1 or 0.75
| 48  || R10 || 5    || 3
|-
| 10  || R10 || 1.5  || 1.25 or 1
| 52  || R20 || 5    || 4
|-
| 12  || R10 || 1.75 || 1.5 or 1.25
| 56  || R10 || 5.5  || 4
|-
| 14  || R20 || 2    || 1.5
| 60  || R20 || 5.5  || 4
|-
| colspan=4 {{n/a|︙}}
| style="border-bottom:1px;" | 64  || R10 || 6    || 4
|}
{{noteslist}}

The thread values are derived from rounded [[Renard series]].  They are defined in ISO 3, with "1st choice" sizes being from the Rˈˈ10 series and "2nd choice" and "3rd choice" sizes being the remaining values from the Rˈˈ20 series.<ref name="iso261"/>

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.<ref>{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950018571.pdf |title=Final report |website=ntrs.nasa.gov |access-date=2017-07-07 |archive-date=2017-03-14 |archive-url=https://web.archive.org/web/20170314024016/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950018571.pdf |url-status=dead }}</ref> Fine and superfine threads also have a greater [[Screw_thread#Minor_diameter|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.

== Obsolete pre ISO DIN sizes ==
{| class="wikitable"
|+ Obsolete DIN sizes<br/>for historical purposes<br/>DIN 13, Din 13-1 <ref>[https://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf Gewindetabellen 15, page 519]</ref> 
|-
! rowspan=2 | Nominal <br/>diameter, <br />''D'' (mm)
! rowspan=2 | Series
! colspan=2 | [[Pitch (screw)|Pitch]], <br />''P'' (mm) 
| rowspan="21" scope="col" style="width:2em; border-top:0px; border-bottom:0px;" |
! rowspan=2 | Nominal <br/>diameter, <br />''D'' (mm)
! rowspan=2 | Series
! colspan=2 | [[Pitch (screw)|Pitch]], <br />''P'' (mm)
|-
! Coarse !! Fine
! Coarse !! Fine
|-
|  1.  || -|| 0.35 ||-
| colspan=4 {{n/a|︙}}
|-
| 2.3 || - || 0.4 ||-
| 2.6 ||- || 0.45 || -
|-
|-
| colspan=4 {{n/a|︙}}
| style="border-bottom:1px;" | 5.5  ||-|| 6    || *
|}

==Spanner (wrench) sizes==
Below are some common spanner ([[wrench]]) sizes for metric screw threads. Hexagonal (generally abbreviated to "hex") head widths (width across flats, spanner size) are for [[Deutsches_Institut_für_Normung|DIN]] 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur to reduce weight or cost, including the small series flange bolts defined in ISO 4162 which typically have hexagonal head sizes corresponding to the smaller 1st choice thread size (eg. M6 small series flange bolts have 8mm hexagonal heads, as would normally be found on M5 bolts).<ref name=iso4162>{{cite web |url=https://www.iso.org/standard/56451.html |title=ISO 4162:2012 |publisher=[[International Organization for Standardization]] |access-date=2022-12-23 }}</ref>

{| class=wikitable
! rowspan=3 | [[File:Unified Screw Threads.png|70x70px]]<br/>Thread
! colspan="5" style="text-align: center;" | Spanner (wrench) size (mm)
|-
! colspan=2 | [[File:Hex cap screw-no markings.svg|60x60px]]<br/>Hex nut, <br/>and bolt
! rowspan=2 | [[File:Socket head cap screw.png|140x50px]]<br/>Socket-head cap screw
! rowspan=2 | [[File:Button head screw.png|60x60px]][[File:Screw.PNG|60x60px]]{{ubli
 | style=text-align:left;
 | Button-head cap screw
 | Counter-sunk <br/>flat-head cap screw
 }}
! rowspan=2 | [[File:Grub Screw.svg|60x60px]]<br/>Set, <br/>or grub, <br/>screw
|-
! scope="col" | {{abbr|ISO|ISO 4014-1999}}
! scope="col" | {{abbr|DIN|DIN 934-1987}}
|- style="text-align:center;"
| M1
| -
| 2.5
| -
| -
| -
|- style="text-align:center;"
| M1.2
| -
| 3
| -
| -
| -
|- style="text-align:center;"
| M1.4
| -
| 3
| 1.25
| -
| 0.7
|- style="text-align:center;"
| M1.6
| colspan=2 | 3.2
| 1.5
| 0.9
| 0.7
|- style="text-align:center;"
| M2
| colspan=2 | 4
| 1.5
| 1.25
| 0.9
|- style="text-align:center;"
| M2.5
| colspan=2 | 5
| 2
| 1.5
| 1.3
|- style="text-align:center;"
| M3
| colspan=2 | 5.5
| 2.5
| 2
| 1.5
|- style="text-align:center;"
| M3.5
| colspan=2 | 6
| -
| -
| -
|- style="text-align:center;"
| M4
| colspan=2 | 7
| 3
| 2.5
| 2
|- style="text-align:center;"
| M5
| colspan=2 | 8
| 4
| 3
| 2.5
|- style="text-align:center;"
| M6
| colspan=2 | 10
| 5
| 4
| 3
|- style="text-align:center;"
| M7
| colspan=2 | 11
| -
| -
| -
|- style="text-align:center;"
| M8
| colspan=2 | 13
| 6
| 5
| 4
|- style="text-align:center;"
| M10
| 16
| 17
| 8
| 6
| 5
|- style="text-align:center;"
| M12
| 18
| 19
| 10
| 8
| 6
|- style="text-align:center;"
| M14
| 21
| 22
| 10
| -
| -
|- style="text-align:center;"
| M16
| colspan=2 | 24
| 14
| 10
| 8
|- style="text-align:center;"
| M18
| colspan=2 | 27
| 14
| 12
| -
|- style="text-align:center;"
| M20
| colspan=2 | 30
| 17
| 12
| 10
|- style="text-align:center;"
| M22
| 34
| 32
| 17
| 14
| -
|- style="text-align:center;"
| M24
| colspan=2 | 36
| 19
| 14
| 12
|- style="text-align:center;"
| M27
| colspan=2 | 41
| 19
| 17
| 14
|- style="text-align:center;"
| M30
| colspan=2 | 46
| 22
| 17
| 14
|- style="text-align:center;"
| M33
| colspan=2 | 50
| 24
| -
| -
|- style="text-align:center;"
| M36
| colspan=2 | 55
| 27
| -
| -
|- style="text-align:center;"
| M39
| colspan=2 | 60
| -
| -
| -
|- style="text-align:center;"
| M42
| colspan=2 | 65
| 32
| -
| -
|- style="text-align:center;"
| M45
| colspan=2 | 70
| -
| -
| -
|- style="text-align:center;"
| M48
| colspan=2 | 75
| 36
| -
| -
|- style="text-align:center;"
| M52
| colspan=2 | 80
| 36
| -
| -
|- style="text-align:center;"
| M56
| colspan=2 | 85
| 41
| -
| -
|- style="text-align:center;"
| M60
| colspan=2 | 90
| -
| -
| -
|- style="text-align:center;"
| M64
| colspan=2 | 95
| 46
| -
| -
|}{{Citation needed|date=August 2024|reason=detailed table needs reference}}

==Standards==

===International===
* ISO 68-1: ISO general purpose screw threads — Basic profile — Metric screw threads.
* ISO 261: ISO general purpose metric screw threads — General plan.
* ISO 262: ISO general purpose metric screw threads — Selected sizes for [[Screw#Differentiation between bolt and screw|screws, bolts]] and [[Nut (hardware)#Standard metric hex nuts sizes|nuts]].
* [[ISO 965]]: ISO general purpose metric screw threads — [[Tolerances]]<ref name="iso965-1">
{{cite book
 | title=ISO 965-1:2013 ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data
 | publisher=International Organization for Standardization
 | date=15 Sep 2013
 | url=https://www.iso.org/standard/57778.html
}}</ref>
** ISO 965-1: Principles and basic data
** ISO 965-2: Limits of sizes for general purpose external and internal screw threads.
** ISO 965-3: Deviations for constructional screw threads
** ISO 965-4: Limits of sizes for hot-dip galvanized external screw threads to mate with internal screw threads tapped with tolerance position H or G after galvanizing
** ISO 965-5: Limits of sizes for internal screw threads to mate with hot-dip galvanized external screw threads with maximum size of tolerance position h before galvanizing

===National===
* BS 3643: ISO metric screw threads
* [https://webstore.ansi.org/RecordDetail.aspx?sku=ASME+B4.2-1978+(R2009) ANSI/ASME B1.13M]: [https://web.archive.org/web/20071231051947/http://catalog.asme.org/Codes/PrintBook/B113M_2005_Metric_Screw.cfm Metric Screw Threads: M Profile]
* [https://webstore.ansi.org/RecordDetail.aspx?sku=ASME+B4.2-1978+(R2009) ANSI/ASME B4.2-1978 (R2009)]: Preferred Metric Limits and Fits
* [http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf DIN13, page 519] 

==== Derived standards ====
Japan has a [[Japanese Industrial Standards|JIS]] metric screw thread standard that largely follows the ISO, but with some differences in pitch and head sizes.

==See also==
{{div col|colwidth=30em}}
* [[ASTM A325#Mechanical properties, metric|ASTM A325M]]
* [[ASTM F568M]]
* [[British Association screw threads]] (BA)
* [[British Standard Cycle]] (BSC)
* [[British standard fine thread]] (BSF)
* [[British standard pipe thread]] (BSP)
* [[British Standard Whitworth]] (BSW) – a British thread standard with 55° profile.
* [[Buttress thread]]
* [[Engineering tolerance]]
* [[Garden hose thread]]
* [[List of drill and tap sizes]]
* [[National pipe thread]] (NPT)
* [[National Standard Thread|National thread]]
* [[Nominal size]]
* ''[[Panzergewinde]]''
* [[Filter thread|Photographic Filter thread]]
* [[Preferred metric sizes]]
* [[Screw thread]]
* [[Square thread form]]
* [[Thread angle]]
* [[Trapezoidal thread forms]]
* [[United States Standard thread]]
* [[Unified Thread Standard]] (UTS, UNC, UNF, UNEF and UNS) – a US/Canadian/British thread standard that uses the same 60° profile as metric threads, but an inch-based set of diameter/pitch combinations.
{{div col end}}

==References==
{{reflist}}

===Bibliography===
* {{Citation | last1 = Oberg | first1 = Erik | last2 = Jones | first2 = Franklin D. | last3 = Horton | first3 = Holbrook L. | last4 = Ryffel | first4 = Henry H. | title = [[Machinery's Handbook]] | place = New York | publisher = Industrial Press Inc. | year = 2000 | edition = 26th | isbn = 0-8311-2635-3 | postscript =.}}

==External links==
* [http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf Diagrams and tables of many screwthread series. In German]
* [http://www.sinofastener.com/uppics/201010161010087778.pdf DIN 931: M1,6 to M39 Hexagon head bolts (Product grades A and B)] {{Webarchive|url=https://web.archive.org/web/20221210050204/http://www.sinofastener.com/uppics/201010161010087778.pdf |date=10 December 2022 }}(1987)
* [[iarchive:gov.in.is.9519.2005|IS 9519: Fasteners - Hexagon products - Width across flats]], Indian standard (2013)
* [https://www.threadspecification.com/m/ Metric screw thread profile, dimensions and tolerances]
* [https://www.metricbolt.com/metric-bolt-sizes/ Metric bolt sizes, threads and materials]


{{ISO standards}}

[[Category:Thread standards]]
[[Category:Screws]]
[[Category:ISO 68]]