What drill is listed for 2-4½ UNC?

1 25/32 in (1.78125 in), also listed in the ICS chart. The geometric D − pitch estimate is 1.7778 in; a 1 inch drill is not a suitable rounding of that estimate.

Why is #2-56 listed with a #50 drill?

This row uses the ICS wire-gauge lookup, #50 at 0.0700 in, rather than the nearest fractional drill. Alternative columns in a supplier chart are not exact equivalents.

Is a tap drill the same as a clearance drill?

No. A tap drill leaves material for an internal thread. A clearance hole lets the fastener pass without threading. Select clearance dimensions from the drawing and applicable fit requirements.

Does this chart apply to forming taps?

No. These are cutting-tap starting sizes. Forming taps displace material and require a different hole range specified by the tool manufacturer for the material and required thread.

Tap Drill Size Chart

Compare cutting-tap starting sizes with the geometric pilot estimate. Rows marked as supplier lookups use the stated chart; other rows use the nearest supported drill.

Page last changed 2026-10-19 · based on ASME B1.1-2019

Quick Answer

For 2-4½ UNC, the geometric pilot is 1.7778 in and the listed drill is 1 25/32 in (1.78125 in). For #2-56, the supplier lookup is #50 (0.0700 in). Major diameter minus pitch does not specify an exact thread percentage.

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]	Selection basis^[2]
#0-80 #	UNF	0.0600	0.0125	3/64	0.0475	Nearest to D − pitch
#1-72 #	UNF	0.0730	0.0139	#53	0.0591	ICS wire-gauge lookup
#1-64 #	UNC	0.0730	0.0156	#53	0.0574	ICS wire-gauge lookup
#2-64 #	UNF	0.0860	0.0156	#50	0.0704	ICS wire-gauge lookup
#2-56 #	UNC	0.0860	0.0179	#50	0.0681	ICS wire-gauge lookup
#3-56 #	UNF	0.0990	0.0179	#45	0.0811	ICS wire-gauge lookup
#3-48 #	UNC	0.0990	0.0208	#47	0.0782	ICS wire-gauge lookup
#4-48 #	UNF	0.1120	0.0208	#42	0.0912	ICS wire-gauge lookup
#4-40 #	UNC	0.1120	0.0250	#43	0.0870	ICS wire-gauge lookup
#5-44 #	UNF	0.1250	0.0227	#37	0.1023	ICS wire-gauge lookup
#5-40 #	UNC	0.1250	0.0250	#38	0.1000	ICS wire-gauge lookup
#6-40 #	UNF	0.1380	0.0250	#33	0.1130	ICS wire-gauge lookup
#6-32 #	UNC	0.1380	0.0312	#35	0.1068	ICS wire-gauge lookup
#8-36 #	UNF	0.1640	0.0278	#29	0.1362	ICS wire-gauge lookup
#8-32 #	UNC	0.1640	0.0312	#29	0.1328	ICS wire-gauge lookup
#10-32 #	UNF	0.1900	0.0312	#21	0.1588	ICS wire-gauge lookup
#10-24 #	UNC	0.1900	0.0417	#25	0.1483	ICS wire-gauge lookup
#12-28 #	UNF	0.2160	0.0357	#14	0.1803	ICS wire-gauge lookup
#12-24 #	UNC	0.2160	0.0417	#16	0.1743	ICS wire-gauge lookup
1/4-32 #	UNEF	0.2500	0.0312	7/32	0.2188	Nearest to D − pitch
1/4-28 #	UNF	0.2500	0.0357	#3	0.2143	Nearest to D − pitch
1/4-20 #	UNC	0.2500	0.0500	#7	0.2000	Nearest to D − pitch
5/16-32 #	UNEF	0.3125	0.0312	9/32	0.2812	Nearest to D − pitch
5/16-24 #	UNF	0.3125	0.0417	I	0.2708	Nearest to D − pitch
5/16-18 #	UNC	0.3125	0.0556	F	0.2569	Nearest to D − pitch
3/8-32 #	UNEF	0.3750	0.0312	11/32	0.3438	Nearest to D − pitch
3/8-24 #	UNF	0.3750	0.0417	Q	0.3333	Nearest to D − pitch
3/8-16 #	UNC	0.3750	0.0625	5/16	0.3125	Nearest to D − pitch
7/16-28 #	UNEF	0.4375	0.0357	Y	0.4018	Nearest to D − pitch
7/16-20 #	UNF	0.4375	0.0500	W	0.3875	Nearest to D − pitch
7/16-14 #	UNC	0.4375	0.0714	U	0.3661	Nearest to D − pitch
1/2-28 #	UNEF	0.5000	0.0357	15/32	0.4643	Nearest to D − pitch
1/2-20 #	UNF	0.5000	0.0500	29/64	0.4500	Nearest to D − pitch
1/2-13 #	UNC	0.5000	0.0769	27/64	0.4231	Nearest to D − pitch
9/16-24 #	UNEF	0.5625	0.0417	33/64	0.5208	Nearest to D − pitch
9/16-18 #	UNF	0.5625	0.0556	1/2	0.5069	Nearest to D − pitch
9/16-12 #	UNC	0.5625	0.0833	31/64	0.4792	Nearest to D − pitch
5/8-24 #	UNEF	0.6250	0.0417	37/64	0.5833	Nearest to D − pitch
5/8-18 #	UNF	0.6250	0.0556	9/16	0.5694	Nearest to D − pitch
5/8-11 #	UNC	0.6250	0.0909	17/32	0.5341	Nearest to D − pitch
11/16-24 #	UNEF	0.6875	0.0417	41/64	0.6458	Nearest to D − pitch
3/4-20 #	UNEF	0.7500	0.0500	45/64	0.7000	Nearest to D − pitch
3/4-16 #	UNF	0.7500	0.0625	11/16	0.6875	Nearest to D − pitch
3/4-10 #	UNC	0.7500	0.1000	21/32	0.6500	Nearest to D − pitch
13/16-20 #	UNEF	0.8125	0.0500	49/64	0.7625	Nearest to D − pitch
7/8-20 #	UNEF	0.8750	0.0500	53/64	0.8250	Nearest to D − pitch
7/8-14 #	UNF	0.8750	0.0714	51/64	0.8036	Nearest to D − pitch
7/8-9 #	UNC	0.8750	0.1111	49/64	0.7639	Nearest to D − pitch
1-20 #	UNEF	1.0000	0.0500	61/64	0.9500	Nearest to D − pitch
1-12 #	UNF	1.0000	0.0833	59/64	0.9167	Nearest to D − pitch
1-8 #	UNC	1.0000	0.1250	7/8	0.8750	Nearest to D − pitch
1 1/16-18 #	UNEF	1.0625	0.0556	1	1.0069	Nearest to D − pitch
1 1/8-18 #	UNEF	1.1250	0.0556	1 1/16	1.0694	Nearest to D − pitch
1 1/8-12 #	UNF	1.1250	0.0833	1 3/64	1.0417	Nearest to D − pitch
1 1/8-7 #	UNC	1.1250	0.1429	63/64	0.9821	Nearest to D − pitch
1 3/16-18 #	UNEF	1.1875	0.0556	1 1/8	1.1319	Nearest to D − pitch
1 1/4-18 #	UNEF	1.2500	0.0556	1 3/16	1.1944	Nearest to D − pitch
1 1/4-12 #	UNF	1.2500	0.0833	1 11/64	1.1667	Nearest to D − pitch
1 1/4-7 #	UNC	1.2500	0.1429	1 7/64	1.1071	Nearest to D − pitch
1 3/8-18 #	UNEF	1.3750	0.0556	1 5/16	1.3194	Nearest to D − pitch
1 3/8-12 #	UNF	1.3750	0.0833	1 19/64	1.2917	Nearest to D − pitch
1 3/8-6 #	UNC	1.3750	0.1667	1 13/64	1.2083	Nearest to D − pitch
1 1/2-18 #	UNEF	1.5000	0.0556	1 7/16	1.4444	Nearest to D − pitch
1 1/2-12 #	UNF	1.5000	0.0833	1 27/64	1.4167	Nearest to D − pitch
1 1/2-6 #	UNC	1.5000	0.1667	1 21/64	1.3333	Nearest to D − pitch
M1.6x0.35 #	Metric Coarse Threads	1.6	0.35	1.3 mm	1.250	Nearest to D − pitch
1 3/4-5 #	UNC	1.7500	0.2000	1 35/64	1.5500	Nearest to D − pitch
2-4 1/2 #	UNC	2.0000	0.2222	1 25/32	1.7778	Nearest to D − pitch
M2x0.4 #	Metric Coarse Threads	2	0.4	1.6 mm	1.600	Nearest to D − pitch
M2.5x0.45 #	Metric Coarse Threads	2.5	0.45	2.1 mm	2.050	Nearest to D − pitch
M3x0.35 #	Metric Fine Threads	3	0.35	2.7 mm	2.650	Nearest to D − pitch
M3x0.5 #	Metric Coarse Threads	3	0.5	2.5 mm	2.500	Nearest to D − pitch
M4x0.5 #	Metric Fine Threads	4	0.5	3.5 mm	3.500	Nearest to D − pitch
M4x0.7 #	Metric Coarse Threads	4	0.7	3.3 mm	3.300	Nearest to D − pitch
M5x0.5 #	Metric Fine Threads	5	0.5	4.5 mm	4.500	Nearest to D − pitch
M5x0.8 #	Metric Coarse Threads	5	0.8	4.2 mm	4.200	Nearest to D − pitch
M6x0.75 #	Metric Fine Threads	6	0.75	5.3 mm	5.250	Nearest to D − pitch
M6x1 #	Metric Coarse Threads	6	1	5 mm	5.000	Nearest to D − pitch
M8x1 #	Metric Fine Threads	8	1	7 mm	7.000	Nearest to D − pitch
M8x1.25 #	Metric Coarse Threads	8	1.25	6.8 mm	6.750	Nearest to D − pitch
M10x1 #	Metric Fine Threads	10	1	9 mm	9.000	Nearest to D − pitch
M10x1.25 #	Metric Fine Threads	10	1.25	8.8 mm	8.750	Nearest to D − pitch
M10x1.5 #	Metric Coarse Threads	10	1.5	8.5 mm	8.500	Nearest to D − pitch
M12x1.25 #	Metric Fine Threads	12	1.25	10.8 mm	10.750	Nearest to D − pitch
M12x1.5 #	Metric Fine Threads	12	1.5	10.5 mm	10.500	Nearest to D − pitch
M12x1.75 #	Metric Coarse Threads	12	1.75	10.3 mm	10.250	Nearest to D − pitch
M14x1.5 #	Metric Fine Threads	14	1.5	12.5 mm	12.500	Nearest to D − pitch
M14x2 #	Metric Coarse Threads	14	2	12 mm	12.000	Nearest to D − pitch
M16x1.5 #	Metric Fine Threads	16	1.5	14.5 mm	14.500	Nearest to D − pitch
M16x2 #	Metric Coarse Threads	16	2	14 mm	14.000	Nearest to D − pitch
M18x1.5 #	Metric Fine Threads	18	1.5	16.5 mm	16.500	Nearest to D − pitch
M18x2 #	Metric Fine Threads	18	2	16 mm	16.000	Nearest to D − pitch
M20x1.5 #	Metric Fine Threads	20	1.5	18.5 mm	18.500	Nearest to D − pitch
M20x2 #	Metric Fine Threads	20	2	18 mm	18.000	Nearest to D − pitch
M20x2.5 #	Metric Coarse Threads	20	2.5	17.5 mm	17.500	Nearest to D − pitch
M22x2 #	Metric Fine Threads	22	2	20 mm	20.000	Nearest to D − pitch
M24x2 #	Metric Fine Threads	24	2	22 mm	22.000	Nearest to D − pitch
M24x3 #	Metric Coarse Threads	24	3	21 mm	21.000	Nearest to D − pitch
M27x2 #	Metric Fine Threads	27	2	25 mm	25.000	Nearest to D − pitch
M30x2 #	Metric Fine Threads	30	2	28 mm	28.000	Nearest to D − pitch
M30x3.5 #	Metric Coarse Threads	30	3.5	26.5 mm	26.500	Nearest to D − pitch
M33x2 #	Metric Fine Threads	33	2	31 mm	31.000	Nearest to D − pitch
M36x3 #	Metric Fine Threads	36	3	33 mm	33.000	Nearest to D − pitch
M36x4 #	Metric Coarse Threads	36	4	32 mm	32.000	Nearest to D − pitch
M39x3 #	Metric Fine Threads	39	3	36 mm	36.000	Nearest to D − pitch
M42x3 #	Metric Fine Threads	42	3	39 mm	39.000	Nearest to D − pitch
M42x4.5 #	Metric Coarse Threads	42	4.5	37.5 mm	37.500	Nearest to D − pitch
M48x3 #	Metric Fine Threads	48	3	45 mm	45.000	Nearest to D − pitch
M48x5 #	Metric Coarse Threads	48	5	43 mm	43.000	Nearest to D − pitch
M56x5.5 #	Metric Coarse Threads	56	5.5	50.5 mm	50.500	Nearest to D − pitch
M64x6 #	Metric Coarse Threads	64	6	58 mm	58.000	Nearest to D − pitch

Inch-thread dimensions are inches; metric-thread dimensions are millimetres. The pilot column is D − pitch, not necessarily the selected drill diameter. Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

Selection methods and coverage

Rows marked ICS wire-gauge lookup use the corresponding column of the ICS tap chart. Other rows select the nearest size to D − pitch from number drills #80–#1, letters A–Z and a 1/64-inch grid through 2 inches; metric candidates use 0.1 mm steps from 1–20 mm and 0.5 mm steps to 80 mm. Actual stock availability must be confirmed. An out-of-range pilot must not be clamped to the smallest or largest drill.

Thread height is not axial engagement

D − pitch is a geometric shortcut, not an exact 75% rule. The Guhring chart, page 2 gives theoretical cut-thread percentage as (major − hole) × TPI / 0.01299 for inch threads. This describes a thread-height convention, not axial engagement length, joint strength or an acceptance limit. Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

Frequently Asked Questions

What drill is listed for 2-4½ UNC?: 1 25/32 in (1.78125 in), also listed in the ICS chart. The geometric D − pitch estimate is 1.7778 in; a 1 inch drill is not a suitable rounding of that estimate.
Why is #2-56 listed with a #50 drill?: This row uses the ICS wire-gauge lookup, #50 at 0.0700 in, rather than the nearest fractional drill. Alternative columns in a supplier chart are not exact equivalents.
Is a tap drill the same as a clearance drill?: No. A tap drill leaves material for an internal thread. A clearance hole lets the fastener pass without threading. Select clearance dimensions from the drawing and applicable fit requirements.
Does this chart apply to forming taps?: No. These are cutting-tap starting sizes. Forming taps displace material and require a different hole range specified by the tool manufacturer for the material and required thread.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	Pilot diameter	derived	major diameter − pitch.
[2]	Drill candidate series and lookup method	derived	Number/letter sizes plus a fractional grid through 2 inches; nearest D − pitch except the explicitly labelled ICS numbered-thread lookups. See selection methods above.
[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.
[4]	ICS Cutting Tools — tap drill chart	reference	n/a — undated supplier chart, selected rows checked 2026-10-19 — source

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

Verification scope

2026-10-19 — Numbered machine-screw drill labels and the 2-4½ UNC fractional drill compared with the corresponding ICS columns. Other rows are generated estimates, not individually verified supplier recommendations. Evidence

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

Value	How it is derived
Geometric pilot and non-lookup drill candidates	Pilot = D − pitch. Select the nearest supported candidate, with larger diameter on a tie. Supplier-lookup rows are marked separately.

Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. 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

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "Tap Drill Size Chart." https://refmanual.com/charts/tap-drill-sizes/ (accessed 2026-10-19). 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: 2026-10-19},
  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  - 2026-10-19
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.