A 1/4-20 tap uses a #7 drill (0.201 in) for about 75% thread; an M6 x 1 tap uses a 5.0 mm drill and an M8 x 1.25 tap uses 6.8 mm.
Tap drill chart: UNC, UNF and metric tap drills, thread percentage and clearance drills
UNC (Unified coarse), #1 to 1-1/2 in
Thread
Threads per inch (TPI)
Major diameter (in)
Major diameter (mm)
Tap drill
Tap drill (in)
Tap drill (mm)
Thread engagement (approx.)
Clearance drill, close fit
Close fit (in)
Close fit (mm)
Clearance drill, free fit
Free fit (in)
Free fit (mm)
#1-64
64
0.073
1.854
#53
0.0595
1.511
67
#48
0.076
1.93
#46
0.081
2.06
#2-56
56
0.086
2.184
#50
0.07
1.778
69
#43
0.089
2.26
#41
0.096
2.44
#3-48
48
0.099
2.515
#47
0.0785
1.994
76
#37
0.104
2.64
#35
0.11
2.79
#4-40
40
0.112
2.845
#43
0.089
2.261
71
#32
0.116
2.95
#30
0.1285
3.26
#5-40
40
0.125
3.175
#38
0.1015
2.578
72
#30
0.1285
3.26
#29
0.136
3.45
#6-32
32
0.138
3.505
#36
0.1065
2.705
78
#27
0.144
3.66
#25
0.1495
3.8
#8-32
32
0.164
4.166
#29
0.136
3.454
69
#18
0.1695
4.31
#16
0.177
4.5
#10-24
24
0.19
4.826
#25
0.1495
3.797
75
#9
0.196
4.98
#7
0.201
5.11
#12-24
24
0.216
5.486
#16
0.177
4.496
72
#2
0.221
5.61
#1
0.228
5.79
1/4-20
20
0.25
6.35
#7
0.201
5.105
75
F
0.257
6.53
H
0.266
6.76
5/16-18
18
0.3125
7.938
F
0.257
6.528
77
P
0.323
8.2
Q
0.332
8.43
3/8-16
16
0.375
9.525
5/16
0.3125
7.938
77
W
0.386
9.8
X
0.397
10.08
7/16-14
14
0.4375
11.112
U
0.368
9.347
75
29/64
0.453125
11.51
15/32
0.46875
11.91
1/2-13
13
0.5
12.7
27/64
0.421875
10.716
78
33/64
0.515625
13.1
17/32
0.53125
13.49
9/16-12
12
0.5625
14.287
31/64
0.484375
12.303
72
37/64
0.578125
14.68
19/32
0.59375
15.08
5/8-11
11
0.625
15.875
17/32
0.53125
13.494
79
41/64
0.640625
16.27
21/32
0.65625
16.67
3/4-10
10
0.75
19.05
21/32
0.65625
16.669
72
49/64
0.765625
19.45
25/32
0.78125
19.84
7/8-9
9
0.875
22.225
49/64
0.765625
19.447
76
57/64
0.890625
22.62
29/32
0.90625
23.02
1-8
8
1
25.4
7/8
0.875
22.225
77
1-1/64
1.015625
25.8
1-1/32
1.03125
26.19
1-1/8-7
7
1.125
28.575
63/64
0.984375
25.003
76
1-9/64
1.140625
28.97
1-5/32
1.15625
29.37
1-1/4-7
7
1.25
31.75
1-7/64
1.109375
28.178
76
1-17/64
1.265625
32.15
1-9/32
1.28125
32.54
1-3/8-6
6
1.375
34.925
1-7/32
1.21875
30.956
72
1-25/64
1.390625
35.32
1-13/32
1.40625
35.72
1-1/2-6
6
1.5
38.1
1-11/32
1.34375
34.131
72
1-33/64
1.515625
38.5
1-17/32
1.53125
38.89
UNF (Unified fine), #0 to 1-1/2 in
Thread
Threads per inch (TPI)
Major diameter (in)
Major diameter (mm)
Tap drill
Tap drill (in)
Tap drill (mm)
Thread engagement (approx.)
Clearance drill, close fit
Close fit (in)
Close fit (mm)
Clearance drill, free fit
Free fit (in)
Free fit (mm)
#0-80
80
0.06
1.524
3/64
0.046875
1.191
81
#52
0.0635
1.61
#50
0.07
1.78
#1-72
72
0.073
1.854
#53
0.0595
1.511
75
#48
0.076
1.93
#46
0.081
2.06
#2-64
64
0.086
2.184
#50
0.07
1.778
79
#43
0.089
2.26
#41
0.096
2.44
#3-56
56
0.099
2.515
#45
0.082
2.083
73
#37
0.104
2.64
#35
0.11
2.79
#4-48
48
0.112
2.845
#42
0.0935
2.375
68
#32
0.116
2.95
#30
0.1285
3.26
#5-44
44
0.125
3.175
#37
0.104
2.642
71
#30
0.1285
3.26
#29
0.136
3.45
#6-40
40
0.138
3.505
#33
0.113
2.87
77
#27
0.144
3.66
#25
0.1495
3.8
#8-36
36
0.164
4.166
#29
0.136
3.454
78
#18
0.1695
4.31
#16
0.177
4.5
#10-32
32
0.19
4.826
#21
0.159
4.039
76
#9
0.196
4.98
#7
0.201
5.11
#12-28
28
0.216
5.486
#14
0.182
4.623
73
#2
0.221
5.61
#1
0.228
5.79
1/4-28
28
0.25
6.35
#3
0.213
5.41
80
F
0.257
6.53
H
0.266
6.76
5/16-24
24
0.3125
7.938
I
0.272
6.909
75
P
0.323
8.2
Q
0.332
8.43
3/8-24
24
0.375
9.525
Q
0.332
8.433
79
W
0.386
9.8
X
0.397
10.08
7/16-20
20
0.4375
11.112
25/64
0.390625
9.922
72
29/64
0.453125
11.51
15/32
0.46875
11.91
1/2-20
20
0.5
12.7
29/64
0.453125
11.509
72
33/64
0.515625
13.1
17/32
0.53125
13.49
9/16-18
18
0.5625
14.287
33/64
0.515625
13.097
65
37/64
0.578125
14.68
19/32
0.59375
15.08
5/8-18
18
0.625
15.875
37/64
0.578125
14.684
65
41/64
0.640625
16.27
21/32
0.65625
16.67
3/4-16
16
0.75
19.05
11/16
0.6875
17.462
77
49/64
0.765625
19.45
25/32
0.78125
19.84
7/8-14
14
0.875
22.225
13/16
0.8125
20.637
67
57/64
0.890625
22.62
29/32
0.90625
23.02
1-12
12
1
25.4
59/64
0.921875
23.416
72
1-1/64
1.015625
25.8
1-1/32
1.03125
26.19
1-1/8-12
12
1.125
28.575
1-3/64
1.046875
26.591
72
1-9/64
1.140625
28.97
1-5/32
1.15625
29.37
1-1/4-12
12
1.25
31.75
1-11/64
1.171875
29.766
72
1-17/64
1.265625
32.15
1-9/32
1.28125
32.54
1-3/8-12
12
1.375
34.925
1-19/64
1.296875
32.941
72
1-25/64
1.390625
35.32
1-13/32
1.40625
35.72
1-1/2-12
12
1.5
38.1
1-27/64
1.421875
36.116
72
1-33/64
1.515625
38.5
1-17/32
1.53125
38.89
Metric coarse (ISO), M1.6 to M30
Thread
Pitch (mm)
Major diameter (in)
Major diameter (mm)
Tap drill
Tap drill (in)
Tap drill (mm)
Thread engagement (approx.)
Clearance drill, close fit
Close fit (in)
Close fit (mm)
Clearance drill, free fit
Free fit (in)
Free fit (mm)
M1.6 x 0.35
0.35
0.063
1.6
1.25 mm
0.0492
1.25
77
1.7 mm
0.0669
1.7
1.8 mm
0.0709
1.8
M2 x 0.4
0.4
0.0787
2
1.6 mm
0.063
1.6
77
2.2 mm
0.0866
2.2
2.4 mm
0.0945
2.4
M2.5 x 0.45
0.45
0.0984
2.5
2.05 mm
0.0807
2.05
77
2.7 mm
0.1063
2.7
2.9 mm
0.1142
2.9
M3 x 0.5
0.5
0.1181
3
2.5 mm
0.0984
2.5
77
3.2 mm
0.126
3.2
3.4 mm
0.1339
3.4
M3.5 x 0.6
0.6
0.1378
3.5
2.9 mm
0.1142
2.9
77
3.7 mm
0.1457
3.7
3.9 mm
0.1535
3.9
M4 x 0.7
0.7
0.1575
4
3.3 mm
0.1299
3.3
77
4.3 mm
0.1693
4.3
4.5 mm
0.1772
4.5
M5 x 0.8
0.8
0.1969
5
4.2 mm
0.1654
4.2
77
5.3 mm
0.2087
5.3
5.5 mm
0.2165
5.5
M6 x 1
1
0.2362
6
5.0 mm
0.1969
5
77
6.4 mm
0.252
6.4
6.6 mm
0.2598
6.6
M8 x 1.25
1.25
0.315
8
6.8 mm
0.2677
6.8
74
8.4 mm
0.3307
8.4
9 mm
0.3543
9
M10 x 1.5
1.5
0.3937
10
8.5 mm
0.3346
8.5
77
10.5 mm
0.4134
10.5
11 mm
0.4331
11
M12 x 1.75
1.75
0.4724
12
10.2 mm
0.4016
10.2
79
13 mm
0.5118
13
13.5 mm
0.5315
13.5
M14 x 2
2
0.5512
14
12.0 mm
0.4724
12
77
15 mm
0.5906
15
15.5 mm
0.6102
15.5
M16 x 2
2
0.6299
16
14.0 mm
0.5512
14
77
17 mm
0.6693
17
17.5 mm
0.689
17.5
M18 x 2.5
2.5
0.7087
18
15.5 mm
0.6102
15.5
77
19 mm
0.748
19
20 mm
0.7874
20
M20 x 2.5
2.5
0.7874
20
17.5 mm
0.689
17.5
77
21 mm
0.8268
21
22 mm
0.8661
22
M22 x 2.5
2.5
0.8661
22
19.5 mm
0.7677
19.5
77
23 mm
0.9055
23
24 mm
0.9449
24
M24 x 3
3
0.9449
24
21.0 mm
0.8268
21
77
25 mm
0.9843
25
26 mm
1.0236
26
M27 x 3
3
1.063
27
24.0 mm
0.9449
24
77
28 mm
1.1024
28
30 mm
1.1811
30
M30 x 3.5
3.5
1.1811
30
26.5 mm
1.0433
26.5
77
31 mm
1.2205
31
33 mm
1.2992
33
Metric fine (ISO 262 selected pitches), M8 to M30
Thread
Pitch (mm)
Major diameter (in)
Major diameter (mm)
Tap drill
Tap drill (in)
Tap drill (mm)
Thread engagement (approx.)
Clearance drill, close fit
Close fit (in)
Close fit (mm)
Clearance drill, free fit
Free fit (in)
Free fit (mm)
M8 x 1
1
0.315
8
7.0 mm
0.2756
7
77
8.4 mm
0.3307
8.4
9 mm
0.3543
9
M10 x 1.25
1.25
0.3937
10
8.8 mm
0.3465
8.8
74
10.5 mm
0.4134
10.5
11 mm
0.4331
11
M10 x 1
1
0.3937
10
9.0 mm
0.3543
9
77
10.5 mm
0.4134
10.5
11 mm
0.4331
11
M12 x 1.5
1.5
0.4724
12
10.5 mm
0.4134
10.5
77
13 mm
0.5118
13
13.5 mm
0.5315
13.5
M12 x 1.25
1.25
0.4724
12
10.8 mm
0.4252
10.8
74
13 mm
0.5118
13
13.5 mm
0.5315
13.5
M14 x 1.5
1.5
0.5512
14
12.5 mm
0.4921
12.5
77
15 mm
0.5906
15
15.5 mm
0.6102
15.5
M16 x 1.5
1.5
0.6299
16
14.5 mm
0.5709
14.5
77
17 mm
0.6693
17
17.5 mm
0.689
17.5
M18 x 1.5
1.5
0.7087
18
16.5 mm
0.6496
16.5
77
19 mm
0.748
19
20 mm
0.7874
20
M20 x 1.5
1.5
0.7874
20
18.5 mm
0.7283
18.5
77
21 mm
0.8268
21
22 mm
0.8661
22
M22 x 1.5
1.5
0.8661
22
20.5 mm
0.8071
20.5
77
23 mm
0.9055
23
24 mm
0.9449
24
M24 x 2
2
0.9449
24
22.0 mm
0.8661
22
77
25 mm
0.9843
25
26 mm
1.0236
26
M27 x 2
2
1.063
27
25.0 mm
0.9843
25
77
28 mm
1.1024
28
30 mm
1.1811
30
M30 x 2
2
1.1811
30
28.0 mm
1.1024
28
77
31 mm
1.2205
31
33 mm
1.2992
33
Inch tap drills are the standard number, letter or fractional drill nearest the exact 75% diameter, drill = major - 75 x 0.01299 / TPI = major - 0.9743 / TPI (OSG formula). This rule reproduces the drill most published charts list; the resulting thread percentage is shown for each.
Thread percentage: inch % = (major - drill) x TPI / 0.01299 (OSG); metric % = 76.98 x (major - drill) / pitch (Sandvik Coromant). The constant is 1.299/100: a hole 1.299 x pitch smaller than the major diameter counts as 100% thread, and 1.299 x pitch is twice 0.6495 x pitch, three quarters of the 60-degree sharp-V height (NBS H28: 0.649519p = 0.75H). Both assume the drill cuts exactly to size; twist drills generally cut slightly oversize, so actual engagement can be a few points lower.
Metric tap drills follow Sandvik Coromant's basic cutting tap rule, drill = major - pitch (about 77% thread), rounded to a stocked 0.1 mm drill where that falls between sizes (M8 x 1.25: 6.8 mm, M12 x 1.75: 10.2 mm, M10 x 1.25: 8.8 mm, M12 x 1.25: 10.8 mm).
Guhring notes that most tap drill charts list one drill, giving about 75% thread, and suggests 60-70% for most applications; OSG says 55-65% is sufficient for most requirements. The hole minor diameter must stay within the thread class limits (class 2B minor diameters are listed by OSG; ISO 965-1 makes 6H the default metric internal class). Forming (roll) taps need a larger drill; use the tap maker's chart.
Inch clearance drills are the close fit and free fit columns of TR Fastenings (matching Little Machine Shop). For metric threads, this site puts the ISO 273 fine series in the close column and the medium series in the free column; TR Fastenings lists the medium series as its one metric clearance hole.
Cross-checked against Fastenal, Tap/Drill Recommendations for cutting taps (Metalworking Decimal Equivalents Chart, poster 9701309) and Little Machine Shop, Tap & Clearance Drill Sizes (inch) and Metric Tap & Clearance Drill Sizes (PDF) and TR Fastenings, Tapping sizes and clearance holes (Unified and metric tables) and Albany County Fasteners, Clearance Hole Chart (inch close fit column): Inch: 44 of 47 tap drills match. Fastenal picks the next smaller drill for #10-24 (#26, 79% thread), #3-56 (#46, 78%) and #12-28 (#15, 78%); Little Machine Shop and TR Fastenings agree with the table (#25, #45, #14). Metric: 27 of 29 match; Fastenal gives 6.7 mm for M8 x 1.25 and 8.7 mm for M10 x 1.25 (about 80% thread) where the table, Little Machine Shop and TR give 6.8 and 8.8 mm.; Inch tap drills: 38 of 39 match (#0 to 1 in). For 1-12 UNF it lists 15/16 (58% thread); the table uses 59/64 (72%), as Fastenal does. Inch clearance drills: 39 of 39 close/free pairs match. Metric tap drills: 22 of 24 match; it lists 10.3 mm for M12 x 1.75 and 12.1 mm for M14 x 2 where the table, Fastenal and TR give 10.2 and 12 mm. Its metric clearance columns are its own, not ISO 273, so they were not compared.; Inch tap drills: 46 of 47 match (only 1-12, where TR lists 15/16). Metric coarse tap drills: 17 of 19 match; TR prints 1.5 mm for M1.6 and 2 mm for M2.5, which would leave 8% and 85% thread, so the 1.25 and 2.05 mm values given by Fastenal and Little Machine Shop are used.; Inch close-fit clearance: 33 of 39 match. Albany uses slightly larger close-fit drills for #3 (#38), 5/16 (21/64) and 3/8 (25/64); its normal and loose columns follow a different scheme from the close/free pairs used here..
How to read the chart
Each row is one thread. The columns give the drill to use before tapping, its decimal size in inches and millimeters, the approximate thread engagement that drill leaves, and two clearance drills for a bolt of that size.
Tap drill: a number drill (#7), a letter drill (F) or a fraction (27/64) for inch threads; a metric drill in millimeters for metric threads.
Thread %: how much of a full-depth thread the drill leaves for the tap to cut, calculated from the drill size. Guhring notes that most tap drill charts list a single drill, one that gives about 75% thread.
Close and free clearance: the smaller and the larger of the two clearance drills.
The chart covers UNC and UNF from #0 to 1-1/2 in, metric coarse threads from M1.6 to M30, and fine pitches from the ISO 262 selection for bolts, M8 to M30. Thread series and pitches for every size are on the thread pitch chart.
How the tap drill is chosen
OSG, the tap maker, gives the percentage of thread as threads per inch x (tap major diameter - drill diameter) / 0.01299. Turned around, the drill for 75% thread is major diameter - 0.9743 / TPI.
For a 1/4-20 tap that is 0.250 - 0.9743 / 20 = 0.2013 in. The nearest standard drill is a #7 at 0.201 in, which leaves (0.250 - 0.201) x 20 / 0.01299 = 75.4% thread. The chart picks the nearest number, letter or fractional drill this way for every inch thread, and that rule reproduces the drill most published charts list.
Sandvik Coromant gives the same relation in millimeters: hole diameter = major diameter - thread height % x pitch / 76.98, so % = 76.98 x (major - drill) / pitch. Its basic rule for a metric cutting tap is drill = major - pitch, which works out to about 77% thread. An M10 x 1.5 tap takes 10 - 1.5 = 8.5 mm. Where that lands between stocked sizes the chart uses the drill most charts list: 6.8 mm for M8 x 1.25 and 10.2 mm for M12 x 1.75.
Why 75%, and when to use less
More thread is not much stronger. Guhring states that a 100% thread is only about 5% stronger than a 75% thread but takes three times the torque to tap, and suggests 60% to 70% for most applications to lengthen tap life. OSG says 55% to 65% is sufficient for most requirements and advises the largest drill that still gives adequate strength, to minimize tapping problems and lengthen tool life.
OSG suggests these ranges:
Free-cutting material (aluminum, brass, bronze, cast iron, copper, mild steel, tool steel): 65% to 70% for average commercial work, 60% to 70% for deep holes, 75% to 85% in thin sheet or stampings.
Hard or tough material (cast steel, drop forgings, Monel, nickel steel, stainless): 60% to 70% for average work, 55% to 65% for deep holes.
OSG treats a hole as deep when it is generally more than 1-1/2 times its diameter. To drop the percentage, pick the next larger drill and check the result with the formula. The finished minor diameter still has to fall inside the thread class limits, which OSG notes sometimes calls for a higher percentage. OSG prints the class 2B limits beside its drill sizes; for metric threads, ISO 965-1 makes 6H the default internal class from M1.6 up.
Where published charts disagree
Charts from different suppliers do not always list the same drill. These are the differences found while checking this chart:
10-24: Fastenal lists #26 (79% thread). Little Machine Shop and TR Fastenings list #25 (75%), used here.
3-56 and 12-28: Fastenal lists #46 and #15 (78% each). Little Machine Shop and TR list #45 and #14 (73% each).
1-12 UNF: Little Machine Shop and TR list 15/16 in, which leaves only 58%. Fastenal lists 59/64 in (72%), used here.
M8 x 1.25: Fastenal lists 6.7 mm (80%); Little Machine Shop and TR list 6.8 mm (74%).
M12 x 1.75: Little Machine Shop lists 10.3 mm (75%); Fastenal and TR list 10.2 mm (79%).
The thread % column shows what each drill leaves.
Clearance holes
For inch screws the close and free clearance drills are the close fit and free fit columns of TR Fastenings and Little Machine Shop, for example F (0.257 in) close and H (0.266 in) free for 1/4 in. For metric screws the close and free columns are the ISO 273 fine and medium series, for example 6.4 mm and 6.6 mm for M6, and 11 mm free for M10. The full ISO 273 table, with the coarse series, is on the metric bolt size chart.
What this chart does not cover
Forming (roll) taps need a larger hole than cutting taps. Sandvik Coromant's basic forming tap hole is major diameter - half the pitch, and Fastenal lists a #1 drill (0.228 in) for a 1/4-20 forming tap. Use the forming tap maker's chart.
Pipe taps (NPT, NPTF, NPSM and others) have their own drill sizes, which Fastenal lists in a separate table.
A twist drill generally cuts a hole slightly larger than its own diameter; a Government of India machinist training text allows 0.05 mm for this. On a 1/4-20 that is 0.002 in, worth about 3 points of thread, so actual engagement can run a little below the number shown. If a thread must be gauged, tap a test hole and check it with thread plug gauges before running parts.