Tap Drill Size Chart

Name: M10 Tap Drill: 10 mm or 8.5 mm? Coarse vs Fine Pitch
Uploaded: 2026-11-10T07:17:24-08:00
Duration: 29 s
Description: An M10 hole is not drilled at 10 mm: the tap cuts the thread and the drill leaves the core. M10 × 1.5 coarse takes an 8.5 mm drill, M10 × 1.25 fine 8.8 mm and M10 × 1 fine 9.0 mm. Rule of thumb: drill ≈ major diameter − pitch, about 75% thread height. ISO coarse tap drills: M6 5.0, M8 6.8, M10 8.5, M12 10.2 mm.

Tap drill sizes for metric, UNC and UNF threads at 75 % engagement, plus 100 % and 65 % thread alternatives.

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A tap drill produces the hole a tap cuts its thread into. Too small and the tap overloads or breaks; too large and the thread loses strength. The industry default is a hole giving about 75 % of full thread depth, which retains roughly 95 % of thread strength at a fraction of the tapping torque. This chart lists the standard drills for ISO metric coarse and fine threads and for UNC and UNF inch threads.

Metric tap drill sizes (75 % thread)

Rule of thumb: tap drill diameter = major diameter − pitch. Add roughly 0.05–0.10 mm in stainless steel to reduce tapping torque.

Thread	Pitch (mm)	Tap drill (mm)	Minor dia d1 (mm)	Series
M1.6	0.35	1.25	1.221	Coarse
M2	0.4	1.60	1.567	Coarse
M2.5	0.45	2.05	2.013	Coarse
M3	0.5	2.50	2.459	Coarse
M3.5	0.6	2.90	2.850	Coarse
M4	0.7	3.30	3.242	Coarse
M5	0.8	4.20	4.134	Coarse
M6	1.0	5.00	4.917	Coarse
M8	1.25	6.80	6.647	Coarse
M10	1.5	8.50	8.376	Coarse
M12	1.75	10.20	10.106	Coarse
M14	2.0	12.00	11.835	Coarse
M16	2.0	14.00	13.835	Coarse
M18	2.5	15.50	15.294	Coarse
M20	2.5	17.50	17.294	Coarse
M22	2.5	19.50	19.294	Coarse
M24	3.0	21.00	20.752	Coarse
M27	3.0	24.00	23.752	Coarse
M30	3.5	26.50	26.211	Coarse
M33	3.5	29.50	29.211	Coarse
M36	4.0	32.00	31.670	Coarse
M39	4.0	35.00	34.670	Coarse
M42	4.5	37.50	37.129	Coarse
M45	4.5	40.50	40.129	Coarse
M48	5.0	43.00	42.587	Coarse
M8x1	1.0	7.00	6.917	Fine
M10x1.25	1.25	8.80	8.647	Fine
M10x1	1.0	9.00	8.917	Fine
M12x1.5	1.5	10.50	10.376	Fine
M12x1.25	1.25	10.80	10.647	Fine
M14x1.5	1.5	12.50	12.376	Fine
M16x1.5	1.5	14.50	14.376	Fine
M18x1.5	1.5	16.50	16.376	Fine
M20x1.5	1.5	18.50	18.376	Fine
M22x1.5	1.5	20.50	20.376	Fine
M24x2	2.0	22.00	21.835	Fine
M27x2	2.0	25.00	24.835	Fine
M30x2	2.0	28.00	27.835	Fine
M33x2	2.0	31.00	30.835	Fine
M36x3	3.0	33.00	32.752	Fine

40 rows

UNC tap drill sizes (75 % thread)

Thread	TPI	Tap drill	Drill (in)	Drill (mm)
#1-64	64	No. 53	0.0595	1.51
#2-56	56	No. 50	0.0700	1.78
#3-48	48	No. 47	0.0785	1.99
#4-40	40	No. 43	0.0890	2.26
#5-40	40	No. 38	0.1015	2.58
#6-32	32	No. 36	0.1065	2.70
#8-32	32	No. 29	0.1360	3.45
#10-24	24	No. 25	0.1495	3.80
#12-24	24	No. 16	0.1770	4.50
1/4-20	20	No. 7	0.2010	5.11
5/16-18	18	Letter F	0.2570	6.53
3/8-16	16	5/16	0.3125	7.94
7/16-14	14	Letter U	0.3680	9.35
1/2-13	13	27/64	0.4219	10.72
9/16-12	12	31/64	0.4844	12.30
5/8-11	11	17/32	0.5313	13.49
3/4-10	10	21/32	0.6563	16.67
7/8-9	9	49/64	0.7656	19.45
1-8	8	7/8	0.8750	22.23
1 1/8-7	7	63/64	0.9844	25.00
1 1/4-7	7	1 7/64	1.1094	28.18
1 3/8-6	6	1 7/32	1.2188	30.96
1 1/2-6	6	1 11/32	1.3438	34.13
2-4.5	4.5	1 25/32	1.7813	45.24

24 rows

UNF tap drill sizes (75 % thread)

No fractional drill gives a 75% thread inside class 2B on 9/16-18 and 5/8-18, so those rows use metric drills. Older charts list 33/64 and 37/64, which give about 65% thread and sit 0.0006 in above the 2B limit.

Thread	TPI	Tap drill	Drill (in)	Drill (mm)
#1-72	72	No. 53	0.0595	1.51
#2-64	64	No. 50	0.0700	1.78
#3-56	56	No. 45	0.0820	2.08
#4-48	48	No. 42	0.0935	2.37
#5-44	44	No. 37	0.1040	2.64
#6-40	40	No. 33	0.1130	2.87
#8-36	36	No. 29	0.1360	3.45
#10-32	32	No. 21	0.1590	4.04
#12-28	28	No. 14	0.1820	4.62
1/4-28	28	No. 3	0.2130	5.41
5/16-24	24	Letter I	0.2720	6.91
3/8-24	24	Letter Q	0.3320	8.43
7/16-20	20	Letter W	0.3860	9.80
1/2-20	20	29/64	0.4531	11.51
9/16-18	18	12.9 mm	0.5079	12.90
5/8-18	18	14.5 mm	0.5709	14.50
3/4-16	16	11/16	0.6875	17.46
7/8-14	14	13/16	0.8125	20.64
1-12	12	59/64	0.9219	23.42
1 1/8-12	12	1 3/64	1.0469	26.59
1 1/4-12	12	1 11/64	1.1719	29.77
1 1/2-12	12	1 27/64	1.4219	36.12

22 rows

Choosing a thread percentage

Thread engagement	Hole size effect	When to use
60–65 %	Larger hole	Deep holes, tough alloys, hand tapping, minimum tap breakage
70–75 %	Standard hole (d − p)	General engineering default, best strength-to-torque balance
80–85 %	Smaller hole	Thin material or short engagement where thread strength is critical
100 %	Hole = minor diameter	Rarely practical: very high torque and high tap breakage risk

4 rows

Watch: M10 Tap Drill: 10 mm or 8.5 mm? Coarse vs Fine Pitch

YouTube Short · 0:29

An M10 hole is not drilled at 10 mm: the tap cuts the thread and the drill leaves the core. M10 × 1.5 coarse takes an 8.5 mm drill, M10 × 1.25 fine 8.8 mm and M10 × 1 fine 9.0 mm. Rule of thumb: drill ≈ major diameter − pitch, about 75% thread height. ISO coarse tap drills: M6 5.0, M8 6.8, M10 8.5, M12 10.2 mm.

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Working with fractional drill sizes

Inch drills are sold in fractions of 1/64 in, while a calculated hole comes out as a decimal. The decimal to fraction calculator gives the nearest 1/16, 1/32 or 1/64 in and the error in inches and millimeters, which helps when the exact drill in this table is not at hand.

About the Tap Drill Size Chart

Sources

Classification: Practical shop recommendations (not a dimension standard)

Sources

ISO 2306 — Twist drills for tapping holes
ISO
Machinery’s Handbook — Tap drill sizes
Industrial Press

Tap drill sizes are shop recommendations chosen for roughly 75% thread engagement, not part of any thread dimension standard. The common rule drill ≈ major diameter − pitch (metric) or drill ≈ major diameter − 1/TPI (inch) yields that engagement. Exact drill availability (fractional/number/letter/metric) may force the nearest stocked size.

How it was compiled

A 75% engagement is the traditional compromise: enough thread depth for strength without requiring excessive tapping torque or risking tap breakage. The listed drill is the theoretical size rounded to the nearest standard drill; in practice the closest available drill is used.

How current it is

Classification: Stable (shop convention)

The 75%-engagement rule and standard drill sizes are long-established conventions. The values do not change, but the optimal engagement depends on material and thread depth requirement.

Limitations

75% engagement is a general recommendation; harder materials may need a larger drill (lower engagement) to avoid tap breakage, and softer materials a smaller one (higher engagement).
The nearest standard drill may not match the theoretical size exactly, so actual engagement varies slightly.
These sizes suit cut threads in ductile materials; formed (rolled) threads need a different, larger pre-drill.

Using it

Use the listed drill as a starting point and adjust for material hardness and tap type.
For blind holes, drill deeper than the required thread length to give the tap chips room to clear.
Pair with the correct thread standard (ISO metric or UNC/UNF) — tap drills are thread-specific.

Units and notation

Metric sizes in millimetres; inch sizes as fractional/number/letter drills with decimal-inch equivalents.

Frequently Asked Questions

What is the formula for a metric tap drill size?

Tap drill diameter ≈ major diameter − pitch. For M6 × 1 that gives a 5.0 mm drill; for M10 × 1.5 an 8.5 mm drill.

Why is 75 % thread the standard?

A 75 % thread keeps about 88 % of the load a full thread can carry while cutting tapping torque substantially compared with a full-depth thread, so taps last longer and break less.

What drill do I use for an M8 x 1.25 thread?

A 6.8 mm drill. If only imperial drills are available, 17/64 in (6.75 mm) is the closest practical substitute.

Do I need a different drill for a through hole and a blind hole?

The tap drill diameter is the same, but a blind hole must be drilled deeper than the thread length — allow at least two extra pitches plus room for chips, and use a bottoming tap to finish.

Should I use cutting fluid when tapping?

Yes. Cutting fluid reduces friction and heat, improves thread finish and significantly extends tap life, especially in stainless steel and aluminium.

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