Metric Threads: Dimensions, Classes & Formulas (full Guide) (2024)

List of Charts

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

• ISO standards
• List of Symbols
• Designation Examples
• Possible Diameter/Pitch combinations.
• Basic Dimensions
• Recommended Classes
• Allowance/Deviations (es/EL)
• Tolerances (Td/TD)
• Normal length of engagement

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:

Designations

The description always begins with a capitalM, followed by thenominal thread diameterin millimeters. For example,M8is 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.

Explanation:

• Pitch: Designated by “X P“. For example, M8X 0.75means 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 theCoase Pitch Seriesaccording to ISO-261.
• Number of starts: Designated by“Ph”. For example, M8 XPh2.25P0.75. 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 is3. If Ph is omitted, the thread has a single start.
• Class: The class appearsafter 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 appearsafter the classand is preceded by a “-“. If omitted, the direction is right-hand.If “- LH” appears, it designates a left-hand thread.For example,M8 X 0.75 – 5g6g– LHis a left-hand thread designation.

Standard ISO metric series

The ISO Metric Thread System consists of two series:

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

Formulas for basic dimensions

External Thread

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

See Also

U-Joint Identification | Types of U-Joints ExplainedUnderstanding Universal Joints and Size Charts

internal thread

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

The above formulas provide the exact results. You case use them directly, utilize our ThreadCalculator[ , 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

limits of Thread Dimensions

To manufacture or measure a thread, one has to know the maximum and minimum permissible values of the basic dimensions. These values are calculated according to the thread class (See below). To understand classes, you first need to understand the terms Allowance (Sometimes referred to as deviation) and Tolerances. The below data and explanations are based on ISO 965-1

Definition of terms:

• Allowance (Deviation):The minimum permissible distance between the basic and actual profile.
• Tolerance:The width of the tolerance field of a diameter on the actual thread profile. (Pitch, Major & Minor diameters)

• A small allowance means that the assembly of a male and female thread will be harder, but after assembly, there will be less freedom of movement.
• A large allowance means that the assembly of a male and female thread will be easier, but after assembly, there will be more freedom of movement.
• The allowance size does not influence a thread’s production difficulty or price.
• A wide tolerance is easier and cheaper to produce but yields a larger spread between threads.
• A narrow tolerance is challenging to produce and more expensive but yields a smaller spread between threads.

Metric Thread Classes

The class is written after the pitch. It is preceded by a “–” and consists of two or four characters. For example, for “M8 X 0.75– 5g6g“, the class is 5g6g. The class defines the Deviation (allowance) and tolerance of the thread.

• 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 diameters (The last two characters).
• If the class is omitted, the default class is used.

Metric Thread Class Syntax:

Threading Class Allowed Values:

External Threads:

• D: e, f, g, h
• T (Major Diameter): 4, 6, 8
• T (Pitch Diameter): 3, 4, 5, 6, 7, 8, 9
• Symbols: es, T_d, T_d2

Internal Threads:

• D: G, H
• T (Minor Diameter): 4, 5, 6, 7, 8
• T (Pitch Diameter): 4, 5, 6, 7, 8
• Symbols: EL, T_D1, T_D2

Metric Thread Classes Examples

Class selection

The engineer can select any class for the male and female threads to achieve his design goals. However, ISO 965-1 recommends limiting the choice according to a chart of “Recommended Classes”.

The choice of the recommended class is made according to two parameters:

Tolerance Quality:

• F – Fine:for precision threads, when a minimal variation of fit is needed.
• M – Medium:for general use.
• C – Coarse:for cases where manufacturing difficulties can arise, for example, when threading hot-rolled bars and long blind holes.

Length of Engagement:

• N – Normal: Defined for each thread in the below table.
• S – Short: Shorter than the “Normal” length.
• L – Long: Longer than the “Normal length”.
• Select “N” When the length of engagement is unknown.

Normal length of engagement for metric threads

Type your thread description in the Search Box.

Recommended class selection for metric threads

Blue Background– Default Classes
Green Background– First Choice Classes.
Yello Background – Second Choice Classes.

Metric Thread Allowance (Deviations)

The allowance EL for internal threads and es for external threads is derived from the class. The approximate allowance can be calculated according to the below formulas:

• EL_H = es_h = 0
• EL_G = es_g ≈15 + 11 * P
• es_e ≈50 + 11 * P
• es_f ≈30 + 11 * P
• P is the pitch expressed in microns
• The result es/EL is in microns.

Unfortunately the above formulas are approximations only. The formal values must be obtained from ISO 965-1 or from the below chart

Allowance/Deviation of Metric Threads

• Type your thread description in the Search Box.
• Values are in Microns.
• For millimeters, divide by 1,000. For Inches, divide by 25,4000.

Metric Thread Tolerances

The tolerances T_D for internal threads and T_d for external threads is derived from the class. The approximate tolerances can be calculated according to the below formulas.

• T_d – Tolerance for External Major Diameter
• T_d2 – Tolerance for External Pitch Diameter
• T_D1 – Tolerance for Internal Minor Diameter
• T_D2 – Tolerance for Internal Pitch Diameter
• P & D – Given in mm
• T – Tolerances results are in microns. (for mm divide by 1,000 and for inches by 254,000).
• T(i) – Tolerance for external diameter grade i. For example, Td(6) is the tolerance for class g6, and T_d(8) is for class g8.

Formulas for tolerances of Metric Threads

The above formulas are approximations. For the formal values, use the below chart or standard ISO 965-1

Metric Thread Tolerances Chart

• Type your thread description in the Search Box.
• Values are in Microns.
• For millimeters, divide by 1,000. For Inches, divide by 25,4000.
• The numbers (3 -9) in the top row represent the class grade (e.g., h6, g8, etc.)

Metric Thread limits Calculations

Once the T_d, T_D, es & EL values are known, the final limits can be calculated according to the below formulas.

The Metric thread standard ISO 965-2 provides the limits only for classes 6H & 6g^*. In all other cases, calculations must be made manually according to the below formulas or by our Threading Calculator

* For threads over 1.4 mm and classes 5H & 6h for threads up to and including 1.4 mm

Formulas for Limits of a Metric External Thread

\( \large d_{max}=d-{es} \)
\( \large d_{min}=d_{max}-T_d \)
\( \large d_{2max} = d2 – es \)
\( \large d_{2min}=d_{2max}-T_{d2} \)

Formulas for Limits of a Metric InternalThread

\( \large D_{1min}=D+{EL} \)
\( \large D_{1max}=D_{1min}+T_{D1} \)
\( \large D_{2minx} = D_2 + EL \)
\( \large D_{2max}=D_{2min}+ T_{D2} \)