Date Published:Apr. 23. 2026

Screw Size Chart: Comprehensive Guide to Metric & Inch Dimensions

Complete reference guide for Unified (UNC/UNF) and ISO Metric threads. Identify nominal sizes, TPI/pitch, and diameter specifications.

Unified Thread Size Chart (UNC / UNF)

Thread Size Basic Major
Diameter
(inch)		 Series
Designation		 Threads
Per Inch
(TPI)		 Minor
Diameter
(inch)		 Thread
Pitch
(inch)		 Pitch
Diameter
(inch)
#0 0.06 UNF 80 0.0447 0.0125 0.0496
#1 0.073 UNC 64 0.0538 0.015625 0.0603
UNF 72 0.056 0.013888 0.0615
#2 0.086 UNC 56 0.0641 0.017857 0.0717
UNF 64 0.0668 0.015625 0.0733
#3 0.099 UNC 48 0.0734 0.020833 0.0825
UNF 56 0.0771 0.017857 0.0845
#4 0.112 UNC 40 0.0813 0.025 0.0925
UNF 48 0.0864 0.020833 0.0954
#5 0.125 UNC 40 0.0943 0.025 0.1054
UNF 44 0.0971 0.022727 0.107
#6 0.138 UNC 32 0.0997 0.03125 0.1141
UNF 40 0.1073 0.025 0.1184
#8 0.164 UNC 32 0.1257 0.03125 0.1399
UNF 36 0.1299 0.027778 0.1424
#10 0.19 UNC 24 0.1389 0.041667 0.1586
UNF 32 0.1517 0.03125 0.1658
#12 0.216 UNC 24 0.1649 0.041667 0.189
UNF 28 0.1722 0.035714 0.193
UNEF 32 0.1777 0.03125 0.1957
1/4" 0.25 UNC 20 0.1887 0.05 0.2127
UNF 28 0.2062 0.035714 0.2225
UNEF 32 0.2117 0.03125 0.2287
5/16" 0.3125 UNC 18 0.2443 0.055556 0.2712
UNF 24 0.2614 0.041667 0.2806
UNEF 32 0.2742 0.03125 0.2912
3/8" 0.375 UNC 16 0.2983 0.0625 0.3287
UNF 24 0.3239 0.041667 0.343
UNEF 32 0.3367 0.03125 0.3537
7/16" 0.4375 UNC 14 0.3499 0.071428 0.385
UNF 20 0.3762 0.05 0.3995
UNEF 28 0.3937 0.035714 0.4132
1/2" 0.5 UNC 13 0.4056 0.076923 0.4435
UNF 20 0.4387 0.05 0.4619
UNF 28 0.4562 0.035714 0.4757
9/16" 0.5625 UNC 12 0.4603 0.083333 0.5016
UNF 18 0.4943 0.055556 0.5205
UNEF 24 0.5514 0.041667 0.5303
5/8" 0.625 UNC 11 0.5135 0.090909 0.5589
UNF 18 0.5568 0.055556 0.5828
UNEF 24 0.5739 0.041667 0.5967
3/4" 0.75 UNC 10 0.6273 0.1 0.6733
UNF 16 0.6733 0.0625 0.7029
UNEF 20 0.6887 0.05 0.7162
7/8" 0.875 UNC 9 0.7387 0.111111 0.7946
UNF 14 0.7874 0.071428 0.8216
UNEF 20 0.8137 0.05 0.8412
1" 1 UNC 8 0.8466 0.125 0.91
UNF 12 0.8978 0.083333 0.9382
UNEF 20 0.9387 0.05 0.9661
1 1/8" 1.125 UNC 7 0.9497 0.142857 1.0228
UNF 12 1.0228 0.083333 1.0631
1 1/4" 1.25 UNC 7 1.0725 0.142857 1.1476
UNF 12 1.146 0.083333 1.1879
1 3/8" 1.375 UNC 6 1.1681 0.166667 1.2563
UNF 12 1.2709 0.083333 1.3127
1 1/2" 1.5 UNC 6 1.2931 0.166667 1.3812
UNF 12 1.3959 0.083333 1.4376
1 3/4" 1.75 UNC 5 1.5019 0.2 1.6085
2" 2 UNC 4 1/2 1.9751 0.222222 1.8528
2 1/4" 2.25 UNC 4 1/2 2.2251 0.222222 2.1028
2 1/2" 2.5 UNC 4 2.4731 0.25 2.3345
2 3/4" 2.75 UNC 4 2.723 0.25 2.5844
3" 3 UNC 4 2.973 0.25 2.8344
3 1/4" 3.25 UNC 4 3.2229 0.25 3.0843
3 1/2" 3.5 UNC 4 3.4729 0.25 3.3343
3 3/4" 3.75 UNC 4 3.7228 0.25 3.5842
4" 4 UNC 4 3.9728 0.25 3.8342

Source: ASME B1.1-2003

 

Metric Thread Size Chart (ISO)

Unit : mm
Nominal diameter＊＝
Male screw outside diameter d,
female screw root diameter D		 Pitch
P		 Height of
engagement
H₁		 Effective
diameters
D₂ /ｄ₂		 Female screw
inside diameter D₁
Male screw root
diameterｄ₁
Field 1 Field 2 Field 3
Choice 1 Choice 2 Choice 3
1 - - 0.25 0.135 0.838 0.729
0.2 0.108 0.870 0.783
- 1.1 - 0.25 0.135 0.938 0.829
0.2 0.108 0.970 0.883
1.2 - - 0.25 0.135 1.038 0.929
0.2 0.108 1.070 0.983
- 1.4 - 0.3 0.162 1.205 1.075
0.2 0.108 1.270 1.183
1.6 - - 0.35 0.189 1.373 1.221
0.2 0.108 1.470 1.383
- 1.8 - 0.35 0.189 1.573 1.421
0.2 0.108 1.670 1.583
2 - - 0.4 0.217 1.740 1.567
0.25 0.135 1.838 1.729
- 2.2 - 0.45 0.244 1.908 1.713
0.25 0.135 2.038 1.929
2.5 - - 0.45 0.244 2.208 2.013
0.35 0.189 2.273 2.121
3 - - 0.5 0.271 2.675 2.459
0.35 0.189 2.773 2.621
- 3.5 - 0.6 0.325 3.110 2.850
0.35 0.189 3.273 3.121
4 - - 0.7 0.379 3.545 3.242
0.5 0.271 3.675 3.459
- 4.5 - 0.75 0.406 4.013 3.688
0.5 0.271 4.175 3.959
5 - - 0.8 0.433 4.480 4.134
0.5 0.271 4.675 4.459
- - 5.5 0.5 0.271 5.175 4.959
6 - - 1 0.541 5.350 4.917
0.75 0.406 5.513 5.188
- 7 - 1 0.541 6.350 5.917
0.75 0.406 6.513 6.188
8 - - 1.25 0.677 7.188 6.647
1 0.541 7.350 6.917
0.75 0.406 7.513 7.188
- - 9 1.25 0.677 8.188 7.647
1 0.541 8.350 7.917
0.75 0.406 8.513 8.188
10 - - 1.5 0.812 9.026 8.376
1.25 0.677 9.188 8.647
1 0.541 9.350 8.917
0.75 0.406 9.513 9.188
- - 11 1.5 0.812 10.026 9.376
1 0.541 10.350 9.917
0.75 0.406 10.513 10.188
12 - - 1.75 0.947 10.863 10.106
1.5 0.812 11.026 10.376
1.25 0.677 11.188 10.647
1 0.541 11.350 10.917
- 14 - 2 1.083 12.701 11.835
1.5 0.812 13.026 12.376
1.25 0.677 13.188 12.647
1 0.541 13.350 12.917
- - 15 1.5 0.812 14.026 13.376
1 0.541 14.350 13.917
16 - - 2 1.083 14.701 13.835
1.5 0.812 15.026 14.376
1 0.541 15.350 14.917
- - 17 1.5 0.812 16.026 15.376
1 0.541 16.350 15.917
- 18 - 2.5 1.353 16.376 15.294
2 1.083 16.701 15.835
1.5 0.812 17.026 16.376
1 0.541 17.350 16.917
20 - - 2.5 1.353 18.376 17.294
2 1.083 18.701 17.835
1.5 0.812 19.026 18.376
1 0.541 19.350 18.917
- 22 - 2.5 1.353 20.376 19.294
2 1.083 20.701 19.835
1.5 0.812 21.026 20.376
1 0.541 21.350 20.917
24 - - 3 1.624 22.051 20.752
2 1.083 22.701 21.835
1.5 0.812 23.026 22.376
1 0.541 23.350 22.917
- - 25 2 1.083 23.701 22.835
1.5 0.812 24.026 23.376
1 0.541 24.350 23.917
- - 26 1.5 0.812 25.026 24.376
- 27 - 3 1.624 25.051 23.752
2 1.083 25.701 24.835
1.5 0.812 26.026 25.376
1 0.541 26.350 25.917
- - 28 2 1.083 26.701 25.835
1.5 0.812 27.026 26.376
1 0.541 27.350 26.917
30 - - 3.5 1.894 27.727 26.211
3 1.624 28.051 26.752
2 1.083 28.701 27.835
1.5 0.812 29.026 28.376
1 0.541 29.350 28.917
- - 32 2 1.083 30.701 29.835
1.5 0.812 31.026 30.376
- 33 - 3.5 1.894 30.727 29.211
3 1.624 31.051 29.752
2 1.083 31.701 30.835
1.5 0.812 32.026 31.376
- - 35 1.5 0.812 34.026 33.376
36 - - 4 2.165 33.402 31.670
3 1.624 34.051 32.752
2 1.083 34.701 33.835
1.5 0.812 35.026 34.376
- - 38 1.5 0.812 37.026 36.376
- 39 - 4 2.165 36.402 34.670
3 1.624 37.051 35.752
2 1.083 37.701 36.835
1.5 0.812 38.026 37.376
- - 40 3 1.624 38.051 36.752
2 1.083 38.701 37.835
1.5 0.812 39.026 38.376
42 - - 4.5 2.436 39.077 37.129
4 2.165 39.402 37.670
3 1.624 40.051 38.752
2 1.083 40.701 39.835
1.5 0.812 41.026 40.376
- 45 - 4.5 2.436 42.077 40.129
4 2.165 42.402 40.670
3 1.624 43.051 41.752
2 1.083 43.701 42.835
1.5 0.812 44.026 43.376
48 - - 5 2.706 44.752 42.587
4 2.165 45.402 43.670
3 1.624 46.051 44.752
2 1.083 46.701 45.835
1.5 0.812 47.026 46.376
- - 50 3 1.624 48.051 46.752
2 1.083 48.701 47.835
1.5 0.812 49.026 48.376
- 52 - 5 2.706 48.752 46.587
4 2.165 49.402 47.670
3 1.624 50.051 48.752
2 1.083 50.701 49.835
1.5 0.812 51.026 50.376
- - 55 4 2.165 52.402 50.670
3 1.624 53.051 51.752
2 1.083 53.701 52.835
1.5 0.812 54.026 53.376

＊ Excerpt from JIS B 0205-2•4:2001 / JIS B 0205:1997 (old standards) / JIS B 0207:1982 (old standards)
＊ Prioritize Field 1 and, if needed, choose Field 2 and Field 3 in that order.

 

How to Measure Screw Sizes

Key Thread Terminology


Pitch: The distance between corresponding points on adjacent thread forms, measured parallel to the axis.


Pitch Diameter: The diameter of an imaginary cylinder where the width of the thread ridge and the width of the thread groove are equal (as shown by P/2 in the diagram).


Thread Angle: The included angle between the thread flanks. For Metric (Coarse/Fine) and Unified (UNC/UNF) threads, this angle is 60°.


Minor Diameter (External Thread): The diameter of an imaginary cylinder that touches the roots of an external thread.


Major Diameter (External Thread): The diameter of an imaginary cylinder that touches the crests of an external thread.

Major Diameter: The largest diameter of a thread, measured from crest to crest.


Minor Diameter: The smallest diameter of a thread, measured from root to root.


Major Diameter (Internal Thread): The diameter of an imaginary cylinder that touches the roots of an internal thread.


Minor Diameter (Internal Thread): The diameter of an imaginary cylinder that touches the crests of an internal thread.

 

Step-by-Step Measurement

Step 1: Determine the system (Inch or Metric)

Inch threads are measured in inches, while metric threads are measured in millimeters. They are not interchangeable.

Measure the major diameter with calipers:

 ・A round inch value (e.g., 0.250") indicates Unified (UNC/UNF).

 ・A round millimeter value (e.g., 6 mm) indicates metric.

Step 2: Measure the pitch

Use a pitch gauge:

 ・Inch threads use TPI (threads per inch).

 ・Metric threads use pitch in millimeters.


Although inch threads are often coarser than metric threads, fine-thread standards (UNF) also exist. Always confirm with proper measurement. For more accurate identification, we recommend confirming the thread using a matching nut or a thread gauge.


Related Products by Screw Size

Select Screw Size (Inch Size)

 

Select Screw Size (Metric Size)

 

FAQ

What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) threads have fewer threads per inch and are commonly used for general fastening.
UNF (Unified National Fine) threads have more threads per inch and are typically used where higher strength or vibration resistance is required.

 

How do I decode a thread "Callout"?
A thread callout is a compact way to describe a screw’s diameter, pitch (or TPI), and thread type.
Here is how to read the examples shown in the diagram:
Metric example: M5 × 0.5
    M: Symbol indicating a metric thread
    5:  Nominal diameter (5 mm major diameter)
    0.5: Thread pitch (0.5 mm between thread crests)
In metric threads, the pitch is expressed directly in millimeters.
Unified example: 3/8-16 UNC
    3/8: Nominal diameter (3/8 inch major diameter)
    16:  Threads Per Inch (TPI)
    UNC: Unified National Coarse (thread series/type)
In Unified threads, the pitch is expressed as TPI (threads per inch) rather than millimeters.

 

When does a screw switch from a "Number" to a "Fraction" in Unified threads?
Screws smaller than 1/4" are identified by a gauge number (#0 to #12).
Once a screw reaches 0.250" in diameter, the chart switches to fractions (1/4", 5/16", etc.).

 

 

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