M18 - Basic Thread Dimensions

Property mm Inch
Nominal Diameter 18 0.7087
Pitch (Distance) 2.5 0.0984
Pitch Diameter 16.376 0.6447
Minor Diameter 15.294 0.6021
Thread Height 1.353 0.0533
Addendum 0.812 0.032
Crest (External)
Root (Internal)		 0.313 0.0123
Tap Drill Size
(75% depth)		 15.56 0.613
Lead Angle
(Single Start)		 2.78°
Thread Family Metric Coarse Pitch (MC)
Relevant Standards ISO 68-1, ISO 9651-1, ISO 965-2, ISO 261,DIN 13
Equivalent Inch Thread11/16 - 12 12-UN

For more threading data scroll down, or use the Thread Calculator

Table of Content

M18 Possible Thread Classes

External Threads

Choose tolerance class: 4g, 4h, 6e, 6g, 6h, 8e, 8g, 5g4g, 5h4h, 5g6g, 7g6g, 7h6h, 7e6e, 9e8e, 9g8g

Metric ISO External Thread - Tolerance sketch

Internal Threads

Choose tolerance class: 4H, 5H, 5G, 6H, 6G, 7H, 7G, 8G

Metric ISO Internal Thread - Tolerance sketch

M18 4g (External)

* Permisable values for thread Class 4g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.746 - 17.958 0.6987 - 0.707
d2 Pitch Diameter 16.228 - 16.334 0.6389 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.534 0.0123 - 0.021
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 4h (External)

* Permisable values for thread Class 4h

# Description Value (mm) Value (Inch)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 17.788 - 18 0.7003 - 0.7087
d2 Pitch Diameter 16.27 - 16.376 0.6406 - 0.6447
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.534 0.0123 - 0.021
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 6e (External)

* Permisable values for thread Class 6e

# Description Value (mm) Value (Inch)
es Allowance 0.08 (Ref) 0.0031 (Ref)
d Major Diameter 17.585 - 17.92 0.6923 - 0.7055
d2 Pitch Diameter 16.126 - 16.296 0.6349 - 0.6416
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.585 0.0123 - 0.023
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 6g (External)

* Permisable values for thread Class 6g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.623 - 17.958 0.6938 - 0.707
d2 Pitch Diameter 16.164 - 16.334 0.6364 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.585 0.0123 - 0.023
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 6h (External)

* Permisable values for thread Class 6h

# Description Value (mm) Value (Inch)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 17.665 - 18 0.6955 - 0.7087
d2 Pitch Diameter 16.206 - 16.376 0.638 - 0.6447
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.585 0.0123 - 0.023
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 8e (External)

* Permisable values for thread Class 8e

# Description Value (mm) Value (Inch)
es Allowance 0.08 (Ref) 0.0031 (Ref)
d Major Diameter 17.39 - 17.92 0.6846 - 0.7055
d2 Pitch Diameter 16.031 - 16.296 0.6311 - 0.6416
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.651 0.0123 - 0.0256
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 8g (External)

* Permisable values for thread Class 8g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.428 - 17.958 0.6861 - 0.707
d2 Pitch Diameter 16.069 - 16.334 0.6326 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.651 0.0123 - 0.0256
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 5g4g (External)

* Permisable values for thread Class 5g4g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.746 - 17.958 0.6987 - 0.707
d2 Pitch Diameter 16.202 - 16.334 0.6379 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.556 0.0123 - 0.0219
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 5h4h (External)

* Permisable values for thread Class 5h4h

# Description Value (mm) Value (Inch)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 17.788 - 18 0.7003 - 0.7087
d2 Pitch Diameter 16.244 - 16.376 0.6395 - 0.6447
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.556 0.0123 - 0.0219
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 5g6g (External)

* Permisable values for thread Class 5g6g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.623 - 17.958 0.6938 - 0.707
d2 Pitch Diameter 16.202 - 16.334 0.6379 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.556 0.0123 - 0.0219
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 7g6g (External)

* Permisable values for thread Class 7g6g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.623 - 17.958 0.6938 - 0.707
d2 Pitch Diameter 16.122 - 16.334 0.6347 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.615 0.0123 - 0.0242
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 7h6h (External)

* Permisable values for thread Class 7h6h

# Description Value (mm) Value (Inch)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 17.665 - 18 0.6955 - 0.7087
d2 Pitch Diameter 16.164 - 16.376 0.6364 - 0.6447
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.615 0.0123 - 0.0242
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 7e6e (External)

* Permisable values for thread Class 7e6e

# Description Value (mm) Value (Inch)
es Allowance 0.08 (Ref) 0.0031 (Ref)
d Major Diameter 17.585 - 17.92 0.6923 - 0.7055
d2 Pitch Diameter 16.084 - 16.296 0.6332 - 0.6416
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.615 0.0123 - 0.0242
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 9e8e (External)

* Permisable values for thread Class 9e8e

# Description Value (mm) Value (Inch)
es Allowance 0.08 (Ref) 0.0031 (Ref)
d Major Diameter 17.39 - 17.92 0.6846 - 0.7055
d2 Pitch Diameter 15.961 - 16.296 0.6284 - 0.6416
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.695 0.0123 - 0.0274
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 9g8g (External)

* Permisable values for thread Class 9g8g

# Description Value (mm) Value (Inch)
es Allowance 0.042 (Ref) 0.0017 (Ref)
d Major Diameter 17.428 - 17.958 0.6861 - 0.707
d2 Pitch Diameter 15.999 - 16.334 0.6299 - 0.6431
R Root Radius 0.313 (Min) 0.0123 (Min)
C Root Truncation 0.313 - 0.695 0.0123 - 0.0274
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 4H (Internal)

* Permisable values for thread Class 4H

# Description Value (mm) Value (Inch)
EL Allowance 0 (Ref) 0 (Ref)
D1 Minor Diameter 15.294 - 15.574 0.6021 - 0.6131
D2 Pitch Diameter 16.376 - 16.516 0.6447 - 0.6502
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

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M18 5H (Internal)

* Permisable values for thread Class 5H

# Description Value (mm) Value (Inch)
EL Allowance 0 (Ref) 0 (Ref)
D1 Minor Diameter 15.294 - 15.649 0.6021 - 0.6161
D2 Pitch Diameter 16.376 - 16.556 0.6447 - 0.6518
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 5G (Internal)

* Permisable values for thread Class 5G

# Description Value (mm) Value (Inch)
EL Allowance 0.042 (Ref) 0.0017 (Ref)
D1 Minor Diameter 15.336 - 15.691 0.6038 - 0.6178
D2 Pitch Diameter 16.418 - 16.598 0.6464 - 0.6535
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 6H (Internal)

* Permisable values for thread Class 6H

# Description Value (mm) Value (Inch)
EL Allowance 0 (Ref) 0 (Ref)
D1 Minor Diameter 15.294 - 15.744 0.6021 - 0.6198
D2 Pitch Diameter 16.376 - 16.6 0.6447 - 0.6535
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 6G (Internal)

* Permisable values for thread Class 6G

# Description Value (mm) Value (Inch)
EL Allowance 0.042 (Ref) 0.0017 (Ref)
D1 Minor Diameter 15.336 - 15.786 0.6038 - 0.6215
D2 Pitch Diameter 16.418 - 16.642 0.6464 - 0.6552
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 7H (Internal)

* Permisable values for thread Class 7H

# Description Value (mm) Value (Inch)
EL Allowance 0 (Ref) 0 (Ref)
D1 Minor Diameter 15.294 - 15.854 0.6021 - 0.6242
D2 Pitch Diameter 16.376 - 16.656 0.6447 - 0.6557
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 7G (Internal)

* Permisable values for thread Class 7G

# Description Value (mm) Value (Inch)
EL Allowance 0.042 (Ref) 0.0017 (Ref)
D1 Minor Diameter 15.336 - 15.896 0.6038 - 0.6258
D2 Pitch Diameter 16.418 - 16.698 0.6464 - 0.6574
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

M18 8G (Internal)

* Permisable values for thread Class 8G

# Description Value (mm) Value (Inch)
EL Allowance 0.042 (Ref) 0.0017 (Ref)
D1 Minor Diameter 15.336 - 16.046 0.6038 - 0.6317
D2 Pitch Diameter 16.418 - 16.773 0.6464 - 0.6604
D Major Diameter 18 (Min) 0.7087 (Min)
LE Normal Length of Engagement 10 - 30 0.3937 - 1.1811

* Back to Thread Class Selection

Tap Drill Sizes for M18

  • If not specified, assume a thread depth of 75%.
  • You can learn more about thread depth in tapping here.
  • To get the tap drill size in other depths, use our Tap Drill Size Calculator .
Thread Depth Tap Drill Size (Inch) Tap Drill Size (mm)
55% 0.638 16.21
60% 0.632 16.05
65% 0.626 15.89
70% 0.619 15.73
75% 0.613 15.56
80% 0.606 15.40
85% 0.600 15.24

Over Wire Measurements for M18


* OverWire is a measuring method to obatin the pitch diameter of threads. It is mostly used when a manufacturer does not have a Go/Nogo gauge for a thread. You can learn more about it here

mm Inch
Permissible Wire Diameter Range 1.263 - 2.526 0.0497 - 0.0994
Ideal Wire Diameter 1.443 0.0568
Possible Wires
From Standard Sets		 * 1.443
* 1.732
* 2.021
* 2.309
 * 0.0642
* 0.0722
* 0.077
* 0.0825
* 0.0962

Once you knoe your wire diameter, you need to find out what is the premissable Measurement for your pitch diameter (Depending on the class). Use our Over Wire Calculator to find out!


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