A 1/4 in bolt is 1/4-20 UNC or 1/4-28 UNF, and an M10 bolt is 1.5 mm coarse pitch, with 1.25 mm and 1 mm as the fine pitches used on bolts.
Thread pitch chart: UNC, UNF and UNEF threads per inch, ISO metric coarse and fine pitch
Unified inch threads (ASME B1.1), #0 to 2 in
Size
Major diameter (in)
Major diameter (mm)
UNC (TPI)
UNF (TPI)
UNEF (TPI)
#0
0.06
1.524
–
80
–
#1
0.073
1.854
64
72
–
#2
0.086
2.184
56
64
–
#3
0.099
2.515
48
56
–
#4
0.112
2.845
40
48
–
#5
0.125
3.175
40
44
–
#6
0.138
3.505
32
40
–
#8
0.164
4.166
32
36
–
#10
0.19
4.826
24
32
–
#12
0.216
5.486
24
28
32
1/4
0.25
6.35
20
28
32
5/16
0.3125
7.938
18
24
32
3/8
0.375
9.525
16
24
32
7/16
0.4375
11.112
14
20
28
1/2
0.5
12.7
13
20
28
9/16
0.5625
14.287
12
18
24
5/8
0.625
15.875
11
18
24
11/16
0.6875
17.462
–
–
24
3/4
0.75
19.05
10
16
20
13/16
0.8125
20.637
–
–
20
7/8
0.875
22.225
9
14
20
15/16
0.9375
23.812
–
–
20
1
1
25.4
8
12
20
1-1/16
1.0625
26.987
–
–
18
1-1/8
1.125
28.575
7
12
18
1-3/16
1.1875
30.162
–
–
18
1-1/4
1.25
31.75
7
12
18
1-5/16
1.3125
33.337
–
–
18
1-3/8
1.375
34.925
6
12
18
1-7/16
1.4375
36.512
–
–
18
1-1/2
1.5
38.1
6
12
18
1-9/16
1.5625
39.688
–
–
18
1-5/8
1.625
41.275
–
–
18
1-11/16
1.6875
42.862
–
–
18
1-3/4
1.75
44.45
5
–
–
2
2
50.8
4.5
–
–
ISO metric threads (ISO 261 / ISO 262), M1 to M36
Size
Major diameter (in)
Major diameter (mm)
Coarse pitch (mm)
Fine pitches (ISO 261)
Fine pitches for bolts (ISO 262)
ISO 261 choice
M1
0.0394
1
0.25
0.2
–
1
M1.1
0.0433
1.1
0.25
0.2
–
2
M1.2
0.0472
1.2
0.25
0.2
–
1
M1.4
0.0551
1.4
0.3
0.2
–
2
M1.6
0.063
1.6
0.35
0.2
–
1
M1.8
0.0709
1.8
0.35
0.2
–
2
M2
0.0787
2
0.4
0.25
–
1
M2.2
0.0866
2.2
0.45
0.25
–
2
M2.5
0.0984
2.5
0.45
0.35
–
1
M3
0.1181
3
0.5
0.35
–
1
M3.5
0.1378
3.5
0.6
0.35
–
2
M4
0.1575
4
0.7
0.5
–
1
M4.5
0.1772
4.5
0.75
0.5
–
2
M5
0.1969
5
0.8
0.5
–
1
M6
0.2362
6
1
0.75
–
1
M7
0.2756
7
1
0.75
–
2
M8
0.315
8
1.25
1, 0.75
1
1
M10
0.3937
10
1.5
1.25, 1, 0.75
1.25, 1
1
M12
0.4724
12
1.75
1.5, 1.25, 1
1.5, 1.25
1
M14
0.5512
14
2
1.5, 1.25*, 1
1.5
2
M16
0.6299
16
2
1.5, 1
1.5
1
M18
0.7087
18
2.5
2, 1.5, 1
2, 1.5
2
M20
0.7874
20
2.5
2, 1.5, 1
2, 1.5
1
M22
0.8661
22
2.5
2, 1.5, 1
2, 1.5
2
M24
0.9449
24
3
2, 1.5, 1
2
1
M27
1.063
27
3
2, 1.5, 1
2
2
M30
1.1811
30
3.5
(3), 2, 1.5, 1
2
1
M33
1.2992
33
3.5
(3), 2, 1.5
2
2
M36
1.4173
36
4
3, 2, 1.5
3
1
UNC = Unified coarse, UNF = Unified fine, UNEF = Unified extra-fine (ASME B1.1 as published in NBS H28). Inch threads are designated size-TPI series-class, e.g. 1/4-20 UNC-2A (external) or 2B (internal).
Metric threads are designated M diameter x pitch, e.g. M10 x 1.25. ISO 965-1 allows the pitch to be left off for coarse threads, so M10 means the coarse pitch.
ISO 261 lists diameters by choice: use 1st choice where possible, then 2nd. Pitches in parentheses, e.g. (3) for M30, are to be avoided as far as possible. *M14 x 1.25 is only for engine spark plugs (ISO 261 footnote).
fine_iso262 is the shorter list of fine pitches ISO 262 selects for bolts, screws and nuts; the fine_mm column is the full ISO 261 list. Sizes not in ISO 262 (M1.1, M2.2, M4.5) show null.
TPI is the reciprocal of the pitch in inches (NBS H28) and 1 in = 25.4 mm exactly, so pitch in mm = 25.4 / TPI; 20 TPI = 1.27 mm. ISO metric and Unified threads share the 60-degree form but are not interchangeable (Guhring), even where the numbers look close.
A Unified inch thread is written size, threads per inch, series and class, for example 1/4-20 UNC-2A. NBS Handbook H28 writes the size as a decimal (.250-20 UNC-2A) and gives the fractional form as the former style.
Size: a fraction of an inch, or a number from #0 to #12 for small screws. In the H28 tables the number sizes step up 0.013 in per number from 0.060 in for #0, so #10 is 0.190 in.
TPI: threads per inch, the reciprocal of the pitch in inches. 1/4-20 has 20 threads in one inch of length.
Series: UNC (coarse), UNF (fine) or UNEF (extra-fine).
Class: A for external threads, B for internal. H28 calls 2A and 2B the most commonly used classes for general applications, including bolts, screws and nuts; 3A and 3B are for work where closeness of fit and accuracy of lead and angle matter.
UNC, UNF and UNEF: which is which
The federal screw thread handbook (NBS Handbook H28, which follows ASME B1.1) describes the three series this way:
UNC is for bulk production of bolts, screws and nuts and general engineering work. It is used for threading into lower-strength material such as cast iron, mild steel and softer metals, and where parts are assembled and taken apart quickly or may be corroded or slightly damaged.
UNF has a larger tensile stress area than the coarse thread of the same size. It suits short lengths of engagement, thin walls that need a fine pitch, and cases where a smaller lead angle is wanted.
UNEF is for even shorter engagement and for thin-walled tubes, nuts, ferrules and couplings.
Among the three series, several sizes have only a UNEF thread: 11/16, 13/16, 15/16, 1-1/16, 1-3/16, 1-5/16, 1-7/16, 1-9/16, 1-5/8 and 1-11/16 in. Above 1-1/2 in, the chart lists the coarse thread for 1-3/4 in (5 TPI) and 2 in (4-1/2 TPI).
Reading a metric thread callout
A metric thread is written M, diameter in millimeters, and pitch in millimeters: M10 x 1.25 is a 10 mm thread with 1.25 mm between crests. ISO 965-1 lets the pitch be left off for coarse threads, so M10 means the coarse pitch, here 1.5 mm.
ISO 261 lists the coarse and fine pitches for each diameter and ranks the diameters by first, second and third choice. ISO 262 is the shorter list chosen for screws, bolts and nuts. For M12, ISO 261 lists 1.5, 1.25 and 1 mm fine pitches; ISO 262 keeps 1.5 and 1.25 mm. Pitches in parentheses, such as 3 mm on M30, are to be avoided as far as possible, and M14 x 1.25 is only for engine spark plugs.
Identifying an unknown thread
Measure the major diameter of the bolt with calipers and compare it with the chart. Expect a reading a little under the basic size: in H28, a 1/4-20 UNC-2A bolt has a major diameter of 0.2408 to 0.2489 in.
Check the pitch with a thread pitch gauge, trying both the inch and the metric leaves.
Without a gauge, count the crests in 1 in for TPI, or measure the length of 10 pitches and divide by 10 for metric pitch.
Watch for near misses. Pitch in mm is 25.4 / TPI, so 20 TPI is 1.27 mm, close to the 1.25 mm of M8, and a 10-32 screw (0.794 mm pitch, 4.83 mm diameter) is close to M5 x 0.8. Guhring notes that ISO metric threads share the 60-degree form of Unified threads, but their diameters and pitches are not interchangeable.
What the chart shows
The inch section lists every UNC, UNF and UNEF size from #0 to 1-11/16 in plus 1-3/4 and 2 in, with the basic major diameter in inches and mm. The metric section lists first- and second-choice diameters from M1 to M36 with the coarse pitch, the ISO 261 fine pitches and the ISO 262 fine pitches for bolts. Tap drills for these threads are on the tap drill chart, and wrench and clearance sizes for metric bolts on the metric bolt size chart.