Verified by Rachel Mayfield, Supply Chain Analyst - June 2026
Photo by cottonbro studio on Pexels
Unified National Coarse (UNC) and Unified National Fine (UNF) thread sizes from #4 to 1 inch, with threads per inch and tapping drill sizes from ASME B1.1. UNC and UNF Thread Sizes Chart UNC is the standard coarse series used for general assembly into steel and aluminium; UNF is the fine series used where a higher tensile stress area, finer adjustment, or thinner wall section matters. Both share a 60-degree thread angle and the same major diameter for a given size - only the pitch differs.
| Size |
Major dia (in) |
UNC TPI |
UNC tap drill |
UNF TPI |
UNF tap drill |
| #4 |
0.112 |
40 |
#43 (2.26mm) |
48 |
#42 (2.37mm) |
| #6 |
0.138 |
32 |
#36 (2.71mm) |
40 |
#33 (2.87mm) |
| #8 |
0.164 |
32 |
#29 (3.45mm) |
36 |
#29 (3.45mm) |
| #10 |
0.190 |
24 |
#25 (3.80mm) |
32 |
#21 (4.04mm) |
| 1/4" |
0.250 |
20 |
#7 (5.11mm) |
28 |
#3 (5.41mm) |
| 5/16" |
0.3125 |
18 |
F (6.53mm) |
24 |
I (6.91mm) |
| 3/8" |
0.375 |
16 |
5/16" (7.94mm) |
24 |
Q (8.43mm) |
| 7/16" |
0.4375 |
14 |
U (9.35mm) |
20 |
25/64" (9.92mm) |
| 1/2" |
0.500 |
13 |
27/64" (10.72mm) |
20 |
29/64" (11.51mm) |
| 9/16" |
0.5625 |
12 |
31/64" (12.30mm) |
18 |
33/64" (13.10mm) |
| 5/8" |
0.625 |
11 |
17/32" (13.49mm) |
18 |
37/64" (14.68mm) |
| 3/4" |
0.750 |
10 |
21/32" (16.67mm) |
16 |
11/16" (17.46mm) |
| 7/8" |
0.875 |
9 |
49/64" (19.45mm) |
14 |
13/16" (20.64mm) |
| 1" |
1.000 |
8 |
7/8" (22.23mm) |
12 |
15/16" (23.81mm) |
How to read the UNC and UNF chart
Below 1/4 inch, sizes use a number gauge (#4, #6, #8, #10). The major diameter in inches for a numbered size is 0.060 + 0.013 times the number, so a #10 is 0.060 + 0.130 = 0.190 inch. From 1/4 inch up, the size is the actual major diameter in fractional inches. A thread is written as size-TPI, so 1/4-20 is a 1/4 inch UNC and 1/4-28 is a 1/4 inch UNF.
UNF has more threads per inch, which gives it a slightly larger tensile stress area and better resistance to vibration loosening, but it strips more easily in soft materials and tolerates less dirt. UNC is more forgiving for tapping into cast iron, aluminium, and plastics. Pick the tap drill from the matching column - the listed drills target roughly 75 percent thread engagement, which balances strength against tapping torque.
UNC and UNF aren't interchangeable with metric threads. A 1/4-20 UNC is close to an M6 in diameter but the pitch and angle differ, so they cross-thread. They're also distinct from BSW (Whitworth), which uses a 55-degree angle.
Related standards
- ASME B1.1 - Unified Inch Screw Threads (UN and UNR thread form)
- ASME B18.3 - Socket cap screws dimensioned in the UNC/UNF series
- ASME B1.2 - Gauges and gauging for unified inch screw threads
References
- ASME B1.1 - Unified Inch Screw Threads
- ASME B18.3 - Socket cap, shoulder, and set screws (inch)