Metric Screw Threads: Charts & Formulas

List of Charts

* Easy access to all the data charts that appear on this page

Overview

The ISO metric screw thread family is a series of general-use, 60° straight threads. It is the most commonly used worldwide thread system. It was one of the first topics standardized by the ISO committee as early as 1947 and is still used today.

ISO Standards

Several ISO standards define together the ISO metric thread system:

ISO StandardTopicWhat’s Include?
ISO 261Possible CombinationsLists in a chart all the possible Diameter/Pitch Combinations.
Defines the Coase Pitch and Fine Pitch series.
ISO 68Basic ProfileDefines the basic profile of the thread including Pitch, Pitch Diameter, Heights, etc.
drawing with the above thread parameters.
Formulas for the above parameters.
ISO 965-1TolerancesThe main document for ISO Metric Threads:
* List and definition of symbols.
* Tolerance classes definition.
* Designation rules
* Allowance Chart – Per class and pitch.
* Normal Length of Engagement Chart – Per diameter and pitch.
* Tolerance fields Charts – Per diameter and pitch.
* Root radius Charts and formulas.
* Recommended Tolerance classes – Charts per tolerance quality and length of engagement.
* Formulas that were used to construct the charts.
ISO 965-2LimitsCharts for the allowed range of pitch, major and minor diameters for classes 6H and 6g.
The values are calculated from the definitions of ISO 965-1.
They have limited use since they provide data only for the default classes.

Designations

The description always begins with a capital M, followed by the nominal thread diameter in millimeters. For example, M8 is a thread with a nominal diameter of 8 mm (0.315″). If no characters follow the M and diameter, it means that all the other parameters are according to the default as defined in ISO 965-1:

ISO Metric Thread Defaults:

  • Pitch – According to the Coarse pitch series.
  • Number of Starts – One.
  • Class – 6g for external threads and 6H for internal threads. (Valid above 1.6 mm / 0.063″)
  • Direction – Right-hand thread.

Metric thread designation Examples

  • Cells with Golden Background – Values are derived from the above defaults.
  • Cells with White Background – Values are derived from the description.
Designation Ø Pitch Internal / External Lead Number of Starts Pitch Diameter Class Major/ Minor Diameter Class Thread Direction
M8 8 1.25 Both 1.25 1 6H/6g 6H/6g Right hand
M8 X 0.75 8 0.75 Both 0.75 1 6H/6g 6H/6g Right hand
M8 X Ph2.25P0.75 8 0.75 Both 2.25 3 6H/6g 6H/6g Right hand
M8 - 4g8e 8 1.25 External 1.25 1 4g 8e Right hand
M8 X 0.75 - 5G 8 0.75 Internal 1.25 1 5G 5G Right hand
M8 - RH 8 1.25 Both 0.75 1 6H/6g 6H/6g Right hand
M8 Ph2.25P0.75 - 4g8e - LH 8 0.75 External 2.25 3 4g 8e Left hand

Explanation:

  • Pitch: Designated by “X P“. For example, M8 X 0.75 means an 8 mm (0.315″) thread with a pitch of 0.75 mm (0.03″ or 34 TPI). If the “X P” is omitted, the pitch is defined by the Coase Pitch Series according to ISO-261.  
  • Number of starts: Designated by “Ph”. For example, M8 X Ph2.25P0.75. In this case, 2.25 mm is the lead, and 0.75 mm is the pitch. Ph must be a multiple of P since the number of starts equals Ph/P. For this example, the number of starts is 3. If Ph is omitted, the thread has a single start.
  • Class: The class appears after the pitch. It is preceded by a “-” and consists of two or four characters. For example, M8 X 0.75 – 5g6g.
    • Small letters represent external threads, while capital letters represent internal threads.
    • Two charters long class mean that the class is valid for both the pitch and major/minor diameters.
    • Four long charters mean a separate class for the pitch diameter (The first two characters) and major/minor diameter (The first two characters).
    •  For detailed explanations about classes, see the class section below.
    • If the class is omitted, the default class is used. (See above table)
  • Thread Direction: The direction appears after the class and is preceded by a “-“. If omitted, the direction is right-hand. If “- LH” appears, it designates a left-hand thread. For exampleM8 X 0.75 – 5g6g – LH is a left-hand thread designation.

Standard ISO metric series

The ISO Metric Thread System consists of two series:

Thread Series From Diameter To Diameter Pitches Combinations per Diameter Number of Combinations
1 mm (0.04") 68 mm (2.7") 1 50
1 mm (0.04") 300 mm (11.8") 1-5 118

Possible Combinations (Diameter/Pitch)

The below charts show all the possible thread size combinations of diameters/pitch as defined in ISO-261

  • MC – Metric Coarse Thread Series
  • MF – Metric Fine Thread Series
  • Click to get the Full Thread Data-Sheet for a specific combination

Possible Combinations for M1 – M24

Possible Combinations for M27 – M300

Basic Thread Dimensions

The basic dimensions are nominal dimensions of a metric thread profile without allowance and tolerances (The thread class defines that). They are based on standard ISO 68-1. The basic dimensions can be used for design. However, for manufacturing and machining, you need the allowable range of each dimension. This data can be found in ISO 951-1 or in the Limits and Dimensions section below. All these parameters are derived from simple formulas based on the thread’s nominal diameter and pitch.

Symbols

List of symbols used in charts and formulas of ISO Metric Threads

Symbol Explanation
Basic Parameters - Diameters and Pitch
D Major (Basic) diameter of internal thread
D1
Minor diameter of internal thread
D2 Pitch diameter of internal thread
d Major (Basic) diameter of external thread
d1 Minor diameter of external thread
d2 Pitch diameter of external thread
d3 Minor diameter for external thread root limits
P Pitch
Ph Lead
Height Parameters
H Height of fundamental triangle
hs Thread Height - External Thread
has Thread addendum - External Thread
hn Thread Height - Internal Thread
hdn Thread dedendum - Internal Thread
Length Parameters
LE Length of Engagment
S / N / L Designation for thread engagement group "Short" / "Medium" / "Long"
Allowance / Tolerance
TD1 Tolerance - D1
TD2 Tolerance - D2
Td Tolerance - d
Td2 Tolerance - d2
EI Allowance - internal thread
es Allowance - External Thread
Root / Crest
R Root radius - External Thread
C Root truncation - External Thread
Fcs Crest width - External Thread
Frs Root width - External Thread
Fcs Crest width - Internal Thread
Fcs Root width - Internal Thread

Formulas for basic dimensions

External Thread

Metric Thread External Basic Profile Sketch
\( \large d=\text{Basic Diameter} \)
\( \large P=\text{Pitch} \)
\( \large H=\frac{\sqrt{3}}{2}\times{P} = 0.866025404\times{P} \)
\( \large h_s=\frac{5}{8}\times{H} \)
\( \large h_{as}=\frac{3}{8}\times{H}\)
\( \large d_2=d-{2}\times{h_{as}} \)
\( \large d_1=d-{2}\times{h_s}\)
\( \large F_{cs}=\frac{P}{8}\)
\( \large F_{rs}=\frac{P}{4}\)

internal thread

Metric Thread Basic Dimensions Internal
\( \large D=\text{Basic Diameter} \)
\( \large P=\text{Pitch} \)
\( \large H=\frac{\sqrt{3}}{2}\times{P} = 0.866025404\times{P} \)
\( \large h_n=\frac{5}{8}\times{H} \)
\( \large h_{dn}=\frac{1}{4}\times{H}\)
\( \large D_1=D-{2}\times{h_n}\)
\( \large D_2=D1+{2}\times{h_{an}}\)
\( \large F_{rn}=\frac{P}{8}\)
\( \large F_{cn}=\frac{P}{4}\)

The above formulas provide the exact results. You case use them directly, utilize our Thread Calculator[ , or browse the below charts.

Metric thread Dimensions Charts

Basic Thread Dimensions – Metric Thread Coarse Pitch Series

Click the Thread Link to get the Full Thread Data for all Classes

* All dimensions are in mm. To view the Inch translation click the Thread Link

Basic Thread Dimensions – Metric Thread Fine Pitch Series

* All dimensions are in mm. To view Inch translation click the Thread Link

Click the Thread Link to get the Full Thread Data for all classes

* All dimensions are in mm. To view Inch translation click the Thread Link

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