Tap Drill and Clearance Hole Chart: Metric, UNC and UNF

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Reviewed by MLC Editorial Team | 22 August 2026

Verified by Rachel Mayfield, Supply Chain Analyst - August 2026

A tap drill is the hole you drill before cutting an internal thread. A clearance drill is the through hole a bolt passes freely. For a metric coarse thread the tap drill is the major diameter minus the pitch, so an M8 coarse at 1.25 mm pitch taps with a 6.8 mm drill. For a Unified thread at 75 percent engagement it is the major diameter minus 0.974 divided by threads per inch, so a 1/4-20 taps with a #7 drill at 0.201 inches. Both tables below are built from those two formulas and checked against the published standards.

Metric tap drill sizes, M1.6 to M42

Coarse pitch threads at roughly 75 percent engagement. Pitches follow ISO 261, clearance holes follow ISO 273.

Size	Coarse pitch (mm)	Tap drill (mm)	Close clearance (mm)	Medium clearance (mm)
M1.6	0.35	1.25	1.7	1.8
M2	0.40	1.6	2.2	2.4
M2.5	0.45	2.05	2.7	2.9
M3	0.50	2.5	3.2	3.4
M4	0.70	3.3	4.3	4.5
M5	0.80	4.2	5.3	5.5
M6	1.00	5	6.4	6.6
M8	1.25	6.8	8.4	9
M10	1.50	8.5	10.5	11
M12	1.75	10.2	13	13.5
M14	2.00	12	15	15.5
M16	2.00	14	17	17.5
M18	2.50	15.5	19	20
M20	2.50	17.5	21	22
M22	2.50	19.5	23	24
M24	3.00	21	25	26
M27	3.00	24	28	30
M30	3.50	26.5	31	33
M36	4.00	32	37	39
M42	4.50	37.5	43	45

Imperial UNC tap drill sizes, 1/4 inch to 3/4 inch

Unified National Coarse at 75 percent engagement, with the metric equivalent of each drill so a workshop with metric drills can still tap a UNC thread. Clearance is the close fit, which is the major diameter plus 1/64 inch.

Thread	Major dia (in)	TPI	Tap drill	Tap drill (in)	Tap drill (mm)	Close clearance
1/4-20	0.2500	20	#7	0.201	5.11	17/64"
5/16-18	0.3125	18	F	0.257	6.53	21/64"
3/8-16	0.3750	16	5/16"	0.3125	7.94	25/64"
7/16-14	0.4375	14	U	0.368	9.35	29/64"
1/2-13	0.5000	13	27/64"	0.4219	10.72	33/64"
9/16-12	0.5625	12	31/64"	0.4844	12.30	37/64"
5/8-11	0.6250	11	17/32"	0.5312	13.49	41/64"
3/4-10	0.7500	10	21/32"	0.6562	16.67	49/64"

Machine screw sizes, #4 to #12

Numbered screw sizes use the rule that a #0 is 0.060 inches and every number adds 0.013 inches, so a #10 is 0.190 inches. The tap drills below are calculated from that diameter at 75 percent engagement for both the coarse and fine series.

Size	Major dia (in)	UNC	UNC tap drill (in)	UNF	UNF tap drill (in)	Close clearance (in)
#4	0.1120	#4-40	0.0876	#4-48	0.0917	0.1276 (1/8")
#6	0.1380	#6-32	0.1076	#6-40	0.1136	0.1536 (5/32")
#8	0.1640	#8-32	0.1336	#8-36	0.1369	0.1796 (11/64")
#10	0.1900	#10-24	0.1494	#10-32	0.1596	0.2056 (13/64")
#12	0.2160	#12-24	0.1754	#12-28	0.1812	0.2316 (15/64")

One honest limitation on this table. The values above are calculated diameters, not drill designations. Machinists normally reach for the nearest number drill, and MLC does not publish a number drill index it has not verified, so pick the closest size from your own drill index rather than assuming a designation. The fractional UNC table above does give named drills, because those come from published MLC bolt specification pages.

How these numbers are calculated

Both formulas describe how much of the thread height the drill leaves behind. On a Unified thread the full thread height is 0.649519 times the pitch, and the tap drill removes the chosen percentage of it from both flanks:

Tap drill = major diameter - (engagement x 1.299038 / TPI)

At 75 percent engagement on a 1/4-20 that gives 0.2500 minus 0.0487, which is 0.2013 inches. A #7 drill measures 0.201 inches, which is why #7 is the standard answer.

The formula gives a theoretical diameter, and drills come in fixed sizes, so the listed drill is the nearest one in the index rather than an exact match. A 5/8-11 is the clearest example on this page: the formula asks for 0.5364 inches, and the drill you actually reach for is 17/32 at 0.5312, which is five thousandths under. That leaves slightly more thread than 75 percent, not less. Every row in the UNC table above sits within half a 64th of its calculated size, which is the most any standard drill index can offer.

The metric rule is simpler because the pitch is already in millimetres. Major diameter minus pitch lands close to 76 percent engagement, which is why it has survived as a workshop rule.

Engagement is a choice, not a constant. Dropping from 75 percent to 60 percent enlarges the tap drill, cuts tapping torque by roughly half and costs only a few percent of thread strength, because a fastener in a tapped hole almost always fails in the bolt rather than the threads. In tough materials or blind holes that trade is usually worth taking.

Clearance holes

Clearance holes are sized to the bolt shank, not the thread. A close fit locates parts precisely and suits dowelled or machined assemblies. A medium fit is the general purpose choice and absorbs small misalignments between drilled parts. For an M10 bolt, drill 10.5 mm for a close fit or 11.0 mm for a medium fit. On the imperial side the close fit is consistently the major diameter plus 1/64 inch, which holds for every size from 1/4 inch to 3/4 inch.

Fine pitch threads need a larger tap drill than coarse, because a smaller pitch removes less material. An M8 coarse taps at 6.8 mm while M8 fine at 1.0 mm pitch taps at 7.0 mm. Check the pitch stamped on the tap before drilling.

Related standards

ISO 261 - ISO general purpose metric screw threads, general plan
ISO 273 - Fasteners, clearance holes for bolts and screws
ISO 2306 - Drills for use before tapping screw threads
ASME B1.1 - Unified inch screw threads, UN and UNR thread form

References

ISO 261:1998 - ISO general purpose metric screw threads
ISO 273:1979 - Clearance holes for bolts and screws

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Number-size machine screw cards

Below 1/4" the imperial series switches to screw numbers. Each card gives the tap drill, clearance drills, pan and flat head sizes, socket head and hex key, nut size and inch-pound torque: #4-40, #6-32, #8-32, #10-24 and #12-24.

Frequently Asked Questions

What drill size do I need to tap an M6 thread?

To tap a standard M6 coarse thread (1.0 mm pitch) use a 5.0 mm drill, which gives about 75 percent thread engagement. The rule for coarse metric threads is tap drill equals major diameter minus pitch, so 6 minus 1.0 equals 5.0 mm. The clearance hole for an M6 bolt is 6.4 mm (close fit) or 6.6 mm (medium fit) per ISO 273.

What is the difference between a tap drill and a clearance drill?

A tap drill makes the hole you cut an internal thread into - it is smaller than the bolt so the tap can form threads. A clearance drill makes the through hole a bolt passes freely without threading - it is larger than the bolt. For M8: tap drill 6.8 mm, clearance drill 8.4 mm (close) or 9.0 mm (medium).

How do you calculate metric tap drill size?

For a coarse-pitch metric thread, tap drill size equals the major diameter minus the thread pitch. An M10 coarse thread has a 1.5 mm pitch, so the tap drill is 10 minus 1.5 equals 8.5 mm. This targets about 75 percent thread engagement. Fine-pitch threads have a smaller pitch, so they need a slightly larger tap drill.

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