What size drill do I use for a 1/4-20 tap?

A #7 drill, 0.2010 in diameter. The pilot diameter is 0.2500 − 0.0500 = 0.2000 in, and #7 at 0.2010 is the nearest standard drill.

What is the tap drill for M10?

8.5 mm for M10x1.5 coarse. It is 8.8 mm for M10x1.25 and 9.0 mm for M10x1 fine — a finer pitch needs a larger drill because less material is removed.

Is there a difference between tap drill and clearance drill?

Yes. A tap drill is the size for cutting the thread; a clearance drill is the larger hole a bolt passes through freely. Clearance holes are normally tap drill plus roughly 1/16 in or 1.5 mm, and are the size used in the unthreaded part of a bolted joint.

What happens if I use the wrong tap drill?

Too small and the tap cuts an over-full thread — torque rises sharply and the tap may break, which usually scraps the part. Too large and the thread is shallow: engagement falls, and below about 60% the joint is meaningfully weaker than the design assumed.

Why does the table give a different drill than major minus pitch exactly?

Because drills come in discrete sizes. The computed pilot diameter sits between two available drills and has to be rounded. The table shows the pilot diameter and the drill actually chosen so the rounding is visible.

Tap Drill Size Chart

The tap drill for every thread size on this site — computed from major diameter minus pitch and matched to the nearest standard drill.

Data verified 2026-09-29 · based on ASME B1.1-2019

Quick Answer

Tap drill is chosen so that thread engagement is about 75%. The rule is pilot diameter = major diameter − pitch, then rounded to the nearest standard drill. For 1/4-20 that gives 0.2000 in, which lands on a #7 drill at 0.2010 in.

Find a thread size

Tap Drill Sizes for All Thread Series

Thread^[3]	Series^[3]	Nominal dia in / mm^[3]	Pitch in / mm^[3]	Tap drill^[2]	Pilot dia in / mm^[1]
#0-80 #	UNC	0.0600	0.0125	3/64	0.0475
#0-80 #	UNF	0.0600	0.0125	3/64	0.0475
#1-72 #	UNF	0.0730	0.0139	1/16	0.0591
#1-64 #	UNC	0.0730	0.0156	1/16	0.0574
#2-64 #	UNF	0.0860	0.0156	5/64	0.0704
#2-56 #	UNC	0.0860	0.0179	1/16	0.0681
#3-56 #	UNF	0.0990	0.0179	5/64	0.0811
#3-48 #	UNC	0.0990	0.0208	5/64	0.0782
#4-48 #	UNF	0.1120	0.0208	3/32	0.0912
#4-40 #	UNC	0.1120	0.0250	3/32	0.0870
#5-44 #	UNF	0.1250	0.0227	#38	0.1023
#5-40 #	UNC	0.1250	0.0250	#39	0.1000
#6-40 #	UNF	0.1380	0.0250	#33	0.1130
#6-32 #	UNC	0.1380	0.0312	#36	0.1068
#8-36 #	UNF	0.1640	0.0278	#29	0.1362
#8-32 #	UNC	0.1640	0.0312	#29	0.1328
#10-32 #	UNF	0.1900	0.0312	#21	0.1588
#10-24 #	UNC	0.1900	0.0417	#25	0.1483
#12-28 #	UNF	0.2160	0.0357	#15	0.1803
#12-24 #	UNC	0.2160	0.0417	#17	0.1743
1/4-32 #	UNEF	0.2500	0.0312	7/32	0.2188
1/4-28 #	UNF	0.2500	0.0357	#3	0.2143
1/4-20 #	UNC	0.2500	0.0500	#7	0.2000
5/16-32 #	UNEF	0.3125	0.0312	9/32	0.2812
5/16-24 #	UNF	0.3125	0.0417	I	0.2708
5/16-18 #	UNC	0.3125	0.0556	F	0.2569
3/8-32 #	UNEF	0.3750	0.0312	11/32	0.3438
3/8-24 #	UNF	0.3750	0.0417	Q	0.3333
3/8-16 #	UNC	0.3750	0.0625	5/16	0.3125
7/16-28 #	UNEF	0.4375	0.0357	Y	0.4018
7/16-20 #	UNF	0.4375	0.0500	W	0.3875
7/16-14 #	UNC	0.4375	0.0714	U	0.3661
1/2-28 #	UNEF	0.5000	0.0357	15/32	0.4643
1/2-20 #	UNF	0.5000	0.0500	29/64	0.4500
1/2-13 #	UNC	0.5000	0.0769	27/64	0.4231
9/16-24 #	UNEF	0.5625	0.0417	17/32	0.5208
9/16-18 #	UNF	0.5625	0.0556	1/2	0.5069
9/16-12 #	UNC	0.5625	0.0833	31/64	0.4792
5/8-24 #	UNEF	0.6250	0.0417	37/64	0.5833
5/8-18 #	UNF	0.6250	0.0556	9/16	0.5694
5/8-11 #	UNC	0.6250	0.0909	17/32	0.5341
11/16-24 #	UNEF	0.6875	0.0417	41/64	0.6458
3/4-20 #	UNEF	0.7500	0.0500	45/64	0.7000
3/4-16 #	UNF	0.7500	0.0625	11/16	0.6875
3/4-10 #	UNC	0.7500	0.1000	21/32	0.6500
13/16-20 #	UNEF	0.8125	0.0500	49/64	0.7625
7/8-20 #	UNEF	0.8750	0.0500	53/64	0.8250
7/8-14 #	UNF	0.8750	0.0714	51/64	0.8036
7/8-9 #	UNC	0.8750	0.1111	49/64	0.7639
1-20 #	UNEF	1.0000	0.0500	61/64	0.9500
1-12 #	UNF	1.0000	0.0833	59/64	0.9167
1-8 #	UNC	1.0000	0.1250	7/8	0.8750
1 1/16-18 #	UNEF	1.0625	0.0556	1	1.0069
1 1/8-18 #	UNEF	1.1250	0.0556	1	1.0694
1 1/8-12 #	UNF	1.1250	0.0833	1	1.0417
1 1/8-7 #	UNC	1.1250	0.1429	63/64	0.9821
1 3/16-18 #	UNEF	1.1875	0.0556	1	1.1319
1 1/4-18 #	UNEF	1.2500	0.0556	1	1.1944
1 1/4-12 #	UNF	1.2500	0.0833	1	1.1667
1 1/4-7 #	UNC	1.2500	0.1429	1	1.1071
1 3/8-18 #	UNEF	1.3750	0.0556	1	1.3194
1 3/8-12 #	UNF	1.3750	0.0833	1	1.2917
1 3/8-6 #	UNC	1.3750	0.1667	1	1.2083
1 1/2-18 #	UNEF	1.5000	0.0556	1	1.4444
1 1/2-12 #	UNF	1.5000	0.0833	1	1.4167
1 1/2-6 #	UNC	1.5000	0.1667	1	1.3333
M1.6x0.35 #	Metric Coarse Threads	1.6	0.35	1.3 mm	1.250
1 3/4-5 #	UNC	1.7500	0.2000	1	1.5500
2-4 1/2 #	UNC	2.0000	0.2222	1	1.7778
M2x0.4 #	Metric Coarse Threads	2	0.4	1.6 mm	1.600
M2.5x0.45 #	Metric Coarse Threads	2.5	0.45	2.1 mm	2.050
M3x0.35 #	Metric Fine Threads	3	0.35	2.7 mm	2.650
M3x0.5 #	Metric Coarse Threads	3	0.5	2.5 mm	2.500
M4x0.5 #	Metric Fine Threads	4	0.5	3.5 mm	3.500
M4x0.7 #	Metric Coarse Threads	4	0.7	3.3 mm	3.300
M5x0.5 #	Metric Fine Threads	5	0.5	4.5 mm	4.500
M5x0.8 #	Metric Coarse Threads	5	0.8	4.2 mm	4.200
M6x0.75 #	Metric Fine Threads	6	0.75	5.3 mm	5.250
M6x1 #	Metric Coarse Threads	6	1	5 mm	5.000
M8x1 #	Metric Fine Threads	8	1	7 mm	7.000
M8x1.25 #	Metric Coarse Threads	8	1.25	6.8 mm	6.750
M10x1 #	Metric Fine Threads	10	1	9 mm	9.000
M10x1.25 #	Metric Fine Threads	10	1.25	8.8 mm	8.750
M10x1.5 #	Metric Coarse Threads	10	1.5	8.5 mm	8.500
M12x1.25 #	Metric Fine Threads	12	1.25	10.8 mm	10.750
M12x1.5 #	Metric Fine Threads	12	1.5	10.5 mm	10.500
M12x1.75 #	Metric Coarse Threads	12	1.75	10.3 mm	10.250
M14x1.5 #	Metric Fine Threads	14	1.5	12.5 mm	12.500
M14x2 #	Metric Coarse Threads	14	2	12 mm	12.000
M16x1.5 #	Metric Fine Threads	16	1.5	14.5 mm	14.500
M16x2 #	Metric Coarse Threads	16	2	14 mm	14.000
M18x1.5 #	Metric Fine Threads	18	1.5	16.5 mm	16.500
M18x2 #	Metric Fine Threads	18	2	16 mm	16.000
M20x1.5 #	Metric Fine Threads	20	1.5	18.5 mm	18.500
M20x2 #	Metric Fine Threads	20	2	18 mm	18.000
M20x2.5 #	Metric Coarse Threads	20	2.5	17.5 mm	17.500
M22x2 #	Metric Fine Threads	22	2	20 mm	20.000
M24x2 #	Metric Fine Threads	24	2	22 mm	22.000
M24x3 #	Metric Coarse Threads	24	3	21 mm	21.000
M27x2 #	Metric Fine Threads	27	2	25 mm	25.000
M30x2 #	Metric Fine Threads	30	2	28 mm	28.000
M30x3.5 #	Metric Coarse Threads	30	3.5	26.5 mm	26.500
M33x2 #	Metric Fine Threads	33	2	31 mm	31.000
M36x3 #	Metric Fine Threads	36	3	33 mm	33.000
M36x4 #	Metric Coarse Threads	36	4	32 mm	32.000
M39x3 #	Metric Fine Threads	39	3	36 mm	36.000
M42x3 #	Metric Fine Threads	42	3	39 mm	39.000
M42x4.5 #	Metric Coarse Threads	42	4.5	37.5 mm	37.500
M48x3 #	Metric Fine Threads	48	3	45 mm	45.000
M48x5 #	Metric Coarse Threads	48	5	43 mm	43.000
M56x5.5 #	Metric Coarse Threads	56	5.5	50.5 mm	50.500
M64x6 #	Metric Coarse Threads	64	6	58 mm	58.000

Pilot diameter is major diameter minus pitch. Where two standard drills are equidistant from it, the larger drill is chosen, which keeps engagement just under 75% rather than over it. Increasing engagement above 75% raises tapping torque sharply and risks tap breakage.

How Tap Drill Size Is Chosen

A tapped hole must leave enough material to carry the load but enough clearance for the tap to cut without seizing. The convention that satisfies both is 75% thread engagement, and it is produced by a simple rule:

pilot diameter = major diameter − pitch

The result is then swapped for the nearest standard drill — number, letter, fractional for inch threads, or the metric series for metric threads. Because engagement varies with the drill actually chosen, the achieved percentage differs slightly from exactly 75%.

Why not use the nearest drill to the minor diameter?

The nominal minor diameter would give near-100% engagement, which sounds better but is not — the tap has to remove the full thread volume in one pass, torque rises steeply, and tap breakage in a finished part is far more expensive than a slightly weaker thread. Most published tables are built around 75% for this reason.

Thread Engagement Percentage

Engagement is the fraction of the full thread height actually formed. It is a trade-off:

Above 75% — stronger thread, but sharply higher tapping torque, more tap wear and a real risk of breaking the tap in the hole.
At about 75% — the standard compromise, and what normal drill selection produces.
Below 60% — noticeably weaker; acceptable only in soft materials or where the fastener is lightly loaded.

In hard materials, or in deep holes where chip clearance is poor, it is normal to go one drill size larger than the table suggests and accept reduced engagement in exchange for a tap that survives.

Frequently Asked Questions

What size drill do I use for a 1/4-20 tap?: A #7 drill, 0.2010 in diameter. The pilot diameter is 0.2500 − 0.0500 = 0.2000 in, and #7 at 0.2010 is the nearest standard drill.
What is the tap drill for M10?: 8.5 mm for M10x1.5 coarse. It is 8.8 mm for M10x1.25 and 9.0 mm for M10x1 fine — a finer pitch needs a larger drill because less material is removed.
Is there a difference between tap drill and clearance drill?: Yes. A tap drill is the size for cutting the thread; a clearance drill is the larger hole a bolt passes through freely. Clearance holes are normally tap drill plus roughly 1/16 in or 1.5 mm, and are the size used in the unthreaded part of a bolted joint.
What happens if I use the wrong tap drill?: Too small and the tap cuts an over-full thread — torque rises sharply and the tap may break, which usually scraps the part. Too large and the thread is shallow: engagement falls, and below about 60% the joint is meaningfully weaker than the design assumed.
Why does the table give a different drill than major minus pitch exactly?: Because drills come in discrete sizes. The computed pilot diameter sits between two available drills and has to be rounded. The table shows the pilot diameter and the drill actually chosen so the rounding is visible.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	Pilot diameter	derived	major diameter − pitch.
[2]	Standard twist drill series (number, letter, fractional, metric)	standard	n/a — industry standard drill series — source
[3]	Thread dimension data on this site	derived	Computed from the ASME B1.1 and ISO 68-1 basic profiles — see the individual series charts.

Data Sources

Standard	Revision	What it covers on this page
ASME B1.1 — Unified Inch Screw Threads	ASME B1.1-2019	the inch thread series this table draws on
ISO 68-1 — Basic profile	ISO 68-1:2023	the metric thread series this table draws on

Cross-checked against:

Each tap drill verified at build time against the standard drill series rather than transcribed
Spot-checked against published tap drill charts for 1/4-20 (#7), 5/16-18 (F), 3/8-16 (5/16) and M10x1.5 (8.5 mm)

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Pilot diameter and tap drill	Pilot = major diameter − pitch; drill = nearest standard drill, taking the larger on a tie.

Engagement is approximately 75% for the drills listed. Hard materials, deep holes and poor chip clearance may justify going one drill size larger.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Tap Drill Size Chart." https://refmanual.com/charts/tap-drill-sizes/ (accessed 2026-09-29). Data from ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019.

BibTeX

@misc{tap-drill-sizes,
  title        = {Tap Drill Size Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/tap-drill-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Tap Drill Size Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/tap-drill-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.