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]}

This article may be too technical for most readers to understand. (December 2020)

The "M" designation for metric screws indicates the 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) will pass through easily to connect to an internally threaded component (e.g. a nut) on the other side. That is, an M6 screw has a nominal outer diameter of 6 millimetres and will therefore be a well-located, co-axial fit in a hole drilled to 6 mm diameter.

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 (D_maj 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 1⁄8 and the innermost 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°:^[3]

${\displaystyle H={\frac {1}{\tan \theta }}\cdot {\frac {P}{2}}={\frac {\sqrt {3}}{2}}\cdot P\approx 0.8660\cdot P}$

or

${\displaystyle P=2\tan \theta \cdot H={\frac {2}{\sqrt {3}}}\cdot H\approx 1.1547\cdot H}$

Because only 5⁄8 of this height is cut, the difference between major and minor diameters is 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_maj and the minor diameter D_min define maximum dimensions of the thread. This means that the external thread must end flat at D_maj, but can be rounded out below the minor diameter D_min. 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_min but may be rounded out beyond D_maj. In practice this means that one can measure the diameter over the threads of a bolt to find the nominal diameter D_maj, and the inner diameter of a nut is D_min.

The minor diameter D_min and effective pitch diameter D_p are derived from the major diameter and pitch as

${\displaystyle {\begin{aligned}D_{\text{min}}&=D_{\text{maj}}-2\cdot {\frac {5}{8}}\cdot H=D_{\text{maj}}-{\frac {5{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-1.082532\cdot P\\[3pt]D_{\text{p}}&=D_{\text{maj}}-2\cdot {\frac {3}{8}}\cdot H=D_{\text{maj}}-{\frac {3{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-0.649519\cdot P\end{aligned}}}$

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

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).^[4]: 17

The length of a machine screw or bolt is indicated by an × and the length expressed in millimetres (e.g. M8-1.25×30 or M8×30).^{[citation needed]}

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.^[4]: 17

ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads.^[5][6] ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.^[7]

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.^[5]

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.^[8] 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.

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 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).^[9]

More information Thread, Spanner (wrench) size (mm) ...

Thread	Spanner (wrench) size (mm)
	Hex nut, and bolt		Socket-head cap screw	Button-head cap screw Counter-sunk flat-head cap screw	Set, or grub, screw
	ISO	DIN
M1	-	2.5	-	-	-
M1.2	-	3	-	-	-
M1.4	-	3	1.25	-	0.7
M1.6	3.2		1.5	0.9	0.7
M2	4		1.5	1.25	0.9
M2.5	5		2	1.5	1.3
M3	5.5		2.5	2	1.5
M3.5	6		-	-	-
M4	7		3	2.5	2
M5	8		4	3	2.5
M6	10		5	4	3
M7	11		-	-	-
M8	13		6	5	4
M10	16	17	8	6	5
M12	18	19	10	8	6
M14	21	22	10	-	-
M16	24		14	10	8
M18	27		14	12	-
M20	30		17	12	10
M22	34	32	17	14	-
M24	36		19	14	12
M27	41		19	17	14
M30	46		22	17	14
M33	50		24	-	-
M36	55		27	-	-
M39	60		-	-	-
M42	65		32	-	-
M45	70		-	-	-
M48	75		36	-	-
M52	80		36	-	-
M56	85		41	-	-
M60	90		-	-	-
M64	95		46	-	-

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