Tap Drill Chart

Name: Tap Drill Chart
Creator: Manufacturing Processes

Tap drill sizes for UNC and UNF threads from #0 to 1/2 in at ~75% thread engagement, and for metric coarse threads M1.6 to M12 by the D−P rule.

Tables: 2
Rows: 27
Units: in, mm
References: 4
Revised: 2026-08-11

UNC / UNF tap drills, #0 to 1/2 in (~75% thread engagement)

15 rows

Size	Major diain	UNC TPI	UNC tap drill	UNC drillin	UNC drillmm	UNF TPI	UNF tap drill	UNF drillin	UNF drillmm
#0	0.06	—	—	—	—	80	3/64	0.046875	1.191
#1	0.073	64	#53	0.0595	1.511	72	#53	0.0595	1.511
#2	0.086	56	#50	0.07	1.778	64	#50	0.07	1.778
#3	0.099	48	#47	0.0785	1.994	56	#45	0.082	2.083
#4	0.112	40	#43	0.089	2.261	48	#42	0.0935	2.375
#5	0.125	40	#38	0.1015	2.578	44	#37	0.104	2.642
#6	0.138	32	#36	0.1065	2.705	40	#33	0.113	2.87
#8	0.164	32	#29	0.136	3.454	36	#29	0.136	3.454
#10	0.19	24	#25	0.1495	3.797	32	#21	0.159	4.039
#12	0.216	24	#16	0.177	4.496	28	#14	0.182	4.623
1/4	0.25	20	#7	0.201	5.105	28	#3	0.213	5.41
5/16	0.3125	18	F	0.257	6.528	24	I	0.272	6.909
3/8	0.375	16	5/16	0.3125	7.938	24	Q	0.332	8.433
7/16	0.4375	14	U	0.368	9.347	20	25/64	0.390625	9.922
1/2	0.5	13	27/64	0.421875	10.716	20	29/64	0.453125	11.509
No rows match that filter.

15 rows × 10 columnsUnits: in, mm

Metric coarse tap drills, M1.6 to M12 (D − P rule)

12 rows

Thread	Major diamm	Pitchmm	Tap drillmm	D − Pmm	Tap drillin	Nearest inch drill
M1.6x0.35	1.6	0.35	1.25	1.25	0.0492	#55 (0.0520)
M2x0.4	2	0.4	1.6	1.6	0.063	1/16 (0.0625)
M2.5x0.45	2.5	0.45	2.05	2.05	0.0807	#46 (0.0810)
M3x0.5	3	0.5	2.5	2.5	0.0984	#40 (0.0980)
M3.5x0.6	3.5	0.6	2.9	2.9	0.1142	#33 (0.1130)
M4x0.7	4	0.7	3.3	3.3	0.1299	#30 (0.1285)
M5x0.8	5	0.8	4.2	4.2	0.1654	#19 (0.1660)
M6x1	6	1	5	5	0.1969	#9 (0.1960)
M7x1	7	1	6	6	0.2362	15/64 (0.2344)
M8x1.25	8	1.25	6.8	6.75	0.2677	H (0.2660)
M10x1.5	10	1.5	8.5	8.5	0.3346	Q (0.3320)
M12x1.75	12	1.75	10.2	10.25	0.4016	Y (0.4040)
No rows match that filter.

12 rows × 7 columnsUnits: mm, in

Basis and references

Read before use

A tap drill is sized to leave a chosen percentage of full thread depth in the hole. The inch column below uses the Machinery's Handbook / ASME B1.1 standard of approximately 75% thread engagement; the metric column uses the ISO 2306 recommended drills, which follow the major diameter minus pitch rule and land at roughly 76–77% engagement.

Why 75% and not 100%. Going from 75% to 100% engagement buys only about 5% more thread strength but roughly triples tapping torque, and is the leading cause of broken taps. In tough or gummy materials — stainless, titanium, Inconel — drop to 60–65% engagement by moving up one drill size.

Drill decimals here are the true drill diameters from the ASME B94.11M drill chart, not the three-place approximations most tap charts print.

References

Every value on this page was set against at least two of these sources and recomputed rather than transcribed. Where they disagree, the disagreement is recorded in the notes below.

Revision and twin

2026-08-11

Last reviewed 2026-08-11. This page carries 27 rows across 2 tables, all of them in the HTML — nothing is paginated, gated or fetched.

Plain-Markdown twin: /charts/tap-drill-chart.md

Notes to the tables

4 notes

Where this data disagrees with itself, with other published charts, or with what the number looks like it means.

The metric rule is exact enough to memorise. Tap drill = major diameter − pitch. Every listed metric drill is within 0.05 mm of D − P, and only M8 (6.80 against a computed 6.75) and M12 (10.20 against 10.25) are rounded to the nearest stocked drill. Both of those are the standard catalog sizes.
Published tap-drill decimals are usually rounded to three places. Common charts print #42 as .094, #38 as .102, #36 as .107 and #25 as .150. The true diameters are 0.0935, 0.1015, 0.1065 and 0.1495. There is no conflict here, only precision — but if you are computing thread engagement, use the true value.
#0-80 has two tap drills in circulation. Machinery's Handbook gives 3/64 in (0.0469), while several shop charts give #56 (0.0465). The two differ by 0.0004 in, about 3% of thread engagement, and either works. This chart uses the Machinery's Handbook value.
Numbered-size major diameters were independently confirmed by formula: major diameter = (screw number × 0.013) + 0.060 in.

Questions

6 questions

What is the tap drill for 1/4-20?

A #7 drill, 0.2010 in (5.105 mm), for approximately 75% thread engagement. For 1/4-28 UNF the tap drill is a #3, 0.2130 in.

What is the tap drill for M6?

5.00 mm for M6 × 1 coarse. That follows directly from the metric rule: tap drill = major diameter − pitch, so 6 − 1 = 5 mm. The nearest inch drill is a #9 at 0.1960 in, which is 0.4% undersize.

How do I work out a metric tap drill without a chart?

Subtract the pitch from the major diameter. M8 × 1.25 gives 6.75 mm, which rounds to the stocked 6.8 mm drill; M10 × 1.5 gives 8.5 mm exactly. The rule holds across the whole coarse series and produces about 76–77% thread engagement.

Why is 75% thread engagement the standard target?

Because the last 25% is not worth paying for. Going from 75% to 100% engagement adds only about 5% to thread strength while roughly tripling tapping torque, which is the main cause of broken taps.

What tap drill should I use in stainless steel?

Go one drill size larger than the 75% value to drop to roughly 60–65% engagement. The same applies to titanium and Inconel. The loss in thread strength is small; the reduction in tapping torque is not.

What is the tap drill for 10-32?

A #21 drill, 0.1590 in (4.039 mm). For 10-24 UNC it is a #25, 0.1495 in.

How to cite this page

Manufacturing Processes. “Tap Drill Chart.” manufacturingprocesses.org, last reviewed 2026-08-11. https://manufacturingprocesses.org/charts/tap-drill-chart

If you reproduce a table, cite the standard it derives from as well — the references are the primary sources, this page is a cross-checked transcription of them.