Tap drill sizes for UNC, UNF, and metric coarse/fine threads — major diameter, pitch/TPI, and recommended tap drill. NPT and BSP pipe threads included.
KEY —Ø diameter ·TPI threads per inch ·#N / A–Z number & letter gauge drills · fractions & plain decimals = inches ·mm = millimeters · ( ) = decimal-inch equivalent ·G = BSP parallel pipe
UNC — Unified National Coarse
18 sizes
Designation
Pitch
Major dia
Tap drill
#1-64
64
0.0730
#53 (0.0595)
#2-56
56
0.0860
#50 (0.0700)
#4-40
40
0.1120
#43 (0.0890)
#5-40
40
0.1250
#38 (0.1015)
#6-32
32
0.1380
#36 (0.1065)
#8-32
32
0.1640
#29 (0.1360)
#10-24
24
0.1900
#25 (0.1495)
#12-24
24
0.2160
#16 (0.1770)
1/4-20
20
0.2500
#7 (0.2010)
5/16-18
18
0.3125
F (0.2570)
3/8-16
16
0.3750
5/16 (0.3125)
7/16-14
14
0.4375
U (0.3680)
1/2-13
13
0.5000
27/64 (0.4219)
9/16-12
12
0.5625
31/64 (0.4844)
5/8-11
11
0.6250
17/32 (0.5313)
3/4-10
10
0.7500
21/32 (0.6563)
7/8-9
9
0.8750
49/64 (0.7656)
1-8
8
1.0000
7/8 (0.8750)
UNF — Unified National Fine
19 sizes
Designation
Pitch
Major dia
Tap drill
#0-80
80 TPI
0.0600
3/64 (0.0469)
#1-72
72
0.0730
#53 (0.0595)
#2-64
64
0.0860
#50 (0.0700)
#3-56
56
0.0990
#45 (0.0820)
#4-48
48
0.1120
#42 (0.0935)
#5-44
44
0.1250
#37 (0.1040)
#6-40
40
0.1380
#33 (0.1130)
#8-36
36
0.1640
#29 (0.1360)
#10-32
32
0.1900
#21 (0.1590)
1/4-28
28
0.2500
#3 (0.2130)
5/16-24
24
0.3125
I (0.2720)
3/8-24
24
0.3750
Q (0.3320)
7/16-20
20
0.4375
25/64 (0.3906)
1/2-20
20
0.5000
29/64 (0.4531)
9/16-18
18
0.5625
33/64 (0.5156)
5/8-18
18
0.6250
37/64 (0.5781)
3/4-16
16
0.7500
11/16 (0.6875)
7/8-14
14
0.8750
13/16 (0.8125)
1-12
12
1.0000
59/64 (0.9219)
Metric — Coarse (ISO)
14 sizes
Designation
Pitch
Major dia
Tap drill
M1.6 × 0.35
0.35 mm
1.60 (0.0630)
1.25 mm (0.0492)
M2 × 0.4
0.40
2.00 (0.0787)
1.6 mm (0.0630)
M2.5 × 0.45
0.45
2.50 (0.0984)
2.05 mm (0.0807)
M3 × 0.5
0.50
3.00 (0.1181)
2.5 mm (0.0984)
M4 × 0.7
0.70
4.00 (0.1575)
3.3 mm (0.1299)
M5 × 0.8
0.80
5.00 (0.1969)
4.2 mm (0.1654)
M6 × 1.0
1.00
6.00 (0.2362)
5.0 mm (0.1969)
M8 × 1.25
1.25
8.00 (0.3150)
6.8 mm (0.2677)
M10 × 1.5
1.50
10.00 (0.3937)
8.5 mm (0.3346)
M12 × 1.75
1.75
12.00 (0.4724)
10.2 mm (0.4016)
M14 × 2.0
2.00
14.00 (0.5512)
12.0 mm (0.4724)
M16 × 2.0
2.00
16.00 (0.6299)
14.0 mm (0.5512)
M20 × 2.5
2.50
20.00 (0.7874)
17.5 mm (0.6890)
M24 × 3.0
3.00
24.00 (0.9449)
21.0 mm (0.8268)
Metric — Fine
10 sizes
Designation
Pitch
Major dia
Tap drill
M3 × 0.35
0.35 mm
3.00 (0.1181)
2.65 mm (0.1043)
M4 × 0.5
0.50
4.00 (0.1575)
3.5 mm (0.1378)
M5 × 0.5
0.50
5.00 (0.1969)
4.5 mm (0.1772)
M6 × 0.75
0.75
6.00 (0.2362)
5.25 mm (0.2067)
M8 × 1.0
1.00
8.00 (0.3150)
7.0 mm (0.2756)
M10 × 1.25
1.25
10.00 (0.3937)
8.8 mm (0.3465)
M12 × 1.5
1.50
12.00 (0.4724)
10.5 mm (0.4134)
M14 × 1.5
1.50
14.00 (0.5512)
12.5 mm (0.4921)
M16 × 1.5
1.50
16.00 (0.6299)
14.5 mm (0.5709)
M20 × 1.5
1.50
20.00 (0.7874)
18.5 mm (0.7283)
NPT — National Pipe Thread (tapered)
7 sizes
Designation
Pitch
Major dia
Tap drill
1/16-27
27 TPI
0.3125
C (0.2420)
1/8-27
27
0.4050
R (0.3390)
1/4-18
18
0.5400
7/16 (0.4375)
3/8-18
18
0.6750
37/64 (0.5781)
1/2-14
14
0.8400
23/32 (0.7188)
3/4-14
14
1.0500
59/64 (0.9219)
1-11.5
11.5
1.3150
1-5/32 (1.1563)
BSP — British Standard Pipe (parallel)
6 sizes
Designation
Pitch
Major dia
Tap drill
G1/8 (28 TPI)
28
9.728 mm
8.7 mm
G1/4 (19 TPI)
19
13.157 mm
11.7 mm
G3/8 (19 TPI)
19
16.662 mm
15.2 mm
G1/2 (14 TPI)
14
20.955 mm
19.0 mm
G3/4 (14 TPI)
14
26.441 mm
24.2 mm
G1 (11 TPI)
11
33.249 mm
30.5 mm
Pitch shown as TPI for inch series, mm for metric. Major diameter in inches (UNC/UNF/NPT) or mm with decimal-inch equivalent (metric). Tap drills are typical handbook sizes (~65–80% thread; 9/16-18, 5/8-18 and 7/8-14 land near 65%); reduce engagement for harder materials. NPT is tapered (1°47′) and seals by interference. Heli-Coil / STI inserts need an oversize pre-drill per the insert manufacturer’s chart. For a searchable, filterable version of these sizes, see thetap-drill calculator; for the full series reference, see thethread spec library.
Free engineering reference from Digital Machine digitalmachine.com · 805-697-7829 · Located in California
Typical handbook tap drills (~65–80% thread) per ASME B1.1 / ISO 261 / ISO 262 — verify against current standards for design-critical work. · September 2026
