How to determine the diameter of a UNC or UNF screw:
The number of threads per inch will determine the diameter of the screw or nut of an American UNC or UNF screw.
Simply count the number of threads per inch (i.e. 25.4 mm) and refer to the table below.

For diameter 1 inch: 8 threads for 1 inch (25.4 mm) = UNC THREAD
|
Ø in inches
|
Ø in mm
|
Number of threads/inch in UNC
|
Number of threads/inch in UNF (fine pitch)
|
|
N°0
|
1.524
|
|
80
|
|
N°1
|
1.854
|
64
|
72
|
|
N°2
|
2.184
|
56
|
64
|
|
N°3
|
2.515
|
48
|
56
|
|
N°4
|
2.845
|
40
|
48
|
|
N°5
|
3.175
|
40
|
44
|
|
N°6
|
3.505
|
32
|
40
|
|
N°8
|
4.166
|
32
|
36
|
|
N°10
|
4.826
|
24
|
32
|
|
N°12
|
5.486
|
24
|
28
|
|
1/4
|
6.35
|
20
|
28
|
|
5/16
|
7.938
|
18
|
24
|
|
3/8
|
9.525
|
16
|
24
|
|
7/16
|
11.113
|
14
|
20
|
|
1/2
|
12.7
|
13
|
20
|
|
9/16
|
14.288
|
12
|
18
|
|
5/8
|
15.875
|
11
|
18
|
|
3/4
|
19.05
|
10
|
16
|
|
7/8
|
22.225
|
9
|
14
|
|
1”
|
25.4
|
8
|
14
|
|
1”1/8
|
28.575
|
7
|
12
|
|
1”1/4
|
31.75
|
7
|
12
|
|
1“1/2
|
38.1
|
6
|
12
|
|
1”3/4
|
44.45
|
5
|
-
|