A tap drill is the drill bit you use before tapping a hole. It makes a hole just the right size so the tap can cut threads into it. The tap drill size is always a little smaller than the thread size. This leaves enough material for the tap to form strong threads.

Tap Drill Chart: Inch + Metric Sizes (UNC/UNF/ISO)

Q: How do I pick between a close fit and free fit drill?

A close fit drill makes a hole just big enough for the bolt to pass through with little play. Use it when you need precise alignment. A free fit drill makes a slightly larger hole for easy bolt insertion. Use it when alignment is less critical or when you need to slide parts into place quickly.

Q: What thread percentage should I use for CNC tapping?

Most CNC shops tap to 75% thread. This gives about 90% of the strength of a full thread but is much easier on the tap. Going above 75% adds very little strength and wears out taps much faster. For soft metals like aluminum, 60-65% thread works well and extends tap life.

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

The tap drill size for a 1/4-20 UNC thread is a #7 drill bit (0.2010"). This gives 75% thread engagement, which is the standard for most applications. If you do not have a #7 drill, a 13/64" (0.2031") works as a close substitute. For a 1/4-28 UNF (fine thread), use a #3 drill bit (0.2130").

Q: What is the difference between coarse and fine metric threads?

Coarse pitch (like M10×1.5) is the default metric thread with deeper, further-apart threads. Fine pitch (like M10×1.25 or M10×1.0) has shallower, closer threads. Fine pitch threads are stronger in thin-wall parts, less likely to loosen from vibration, and allow finer adjustment. But they are harder to tap and easier to cross-thread. Use coarse pitch unless you have a specific reason to use fine.

Q: What happens if I use the wrong tap drill size?

If the drill is too small, the tap has to cut deeper threads. This increases force, generates more heat, and can break the tap — especially in hard materials. If the drill is too large, the threads will be too shallow and the bolt may strip out under load. Using the correct tap drill from a tap drill chart gives you the right balance of thread strength and tapping ease.

Thread Size ▲	TPI / Pitch ▲	Tap Drill ▲	Tap Drill Decimal (in) ▲	Close Fit Drill ▲	Free Fit Drill ▲
#1-64 UNC	64	#53	0.0595	#46	#43
#2-56 UNC	56	#50	0.0700	#43	#38
#3-48 UNC	48	#47	0.0785	#37	#33
#4-40 UNC	40	#43	0.0890	#31	#28
#5-40 UNC	40	#38	0.1015	#29	#24
#6-32 UNC	32	#36	0.1065	#25	#18
#8-32 UNC	32	#29	0.1360	#18	#13
#10-24 UNC	24	#25	0.1495	#9	#2
#12-24 UNC	24	#16	0.1770	#2	1/4"
1/4"-20 UNC	20	#7	0.2010	17/64"	9/32"
5/16"-18 UNC	18	F	0.2570	21/64"	11/32"
3/8"-16 UNC	16	5/16"	0.3125	25/64"	13/32"
7/16"-14 UNC	14	U	0.3680	29/64"	15/32"
1/2"-13 UNC	13	27/64"	0.4219	33/64"	17/32"
9/16"-12 UNC	12	31/64"	0.4844	37/64"	19/32"
5/8"-11 UNC	11	17/32"	0.5312	41/64"	21/32"
3/4"-10 UNC	10	21/32"	0.6562	49/64"	25/32"
7/8"-9 UNC	9	49/64"	0.7656	57/64"	29/32"
1"-8 UNC	8	7/8"	0.8750	1-1/64"	1-1/32"
#0-80 UNF	80	3/64"	0.0469	#51	#49
#1-72 UNF	72	#53	0.0595	#48	#44
#2-64 UNF	64	#50	0.0700	#42	#37
#3-56 UNF	56	#45	0.0820	#36	#32
#4-48 UNF	48	#42	0.0935	#30	#27
#6-40 UNF	40	#33	0.1130	#23	#16
#8-36 UNF	36	#29	0.1360	#16	#10
#10-32 UNF	32	#21	0.1590	#7	#1
1/4"-28 UNF	28	#3	0.2130	17/64"	9/32"
5/16"-24 UNF	24	I	0.2720	21/64"	11/32"
3/8"-24 UNF	24	Q	0.3320	25/64"	13/32"
7/16"-20 UNF	20	25/64"	0.3906	29/64"	15/32"
1/2"-20 UNF	20	29/64"	0.4531	33/64"	17/32"
9/16"-18 UNF	18	33/64"	0.5156	37/64"	19/32"
5/8"-18 UNF	18	37/64"	0.5781	41/64"	21/32"
3/4"-16 UNF	16	11/16"	0.6875	49/64"	25/32"
1"-12 UNF	12	59/64"	0.9219	1-1/64"	1-1/32"
M2×0.4	0.4 mm	1.6 mm	0.0630	2.1 mm	2.3 mm
M2.5×0.45	0.45 mm	2.05 mm	0.0807	2.6 mm	2.8 mm
M3×0.5	0.5 mm	2.5 mm	0.0984	3.2 mm	3.4 mm
M3.5×0.6	0.6 mm	2.9 mm	0.1142	3.6 mm	3.9 mm
M4×0.7	0.7 mm	3.3 mm	0.1299	4.2 mm	4.5 mm
M5×0.8	0.8 mm	4.2 mm	0.1654	5.2 mm	5.5 mm
M6×1.0	1.0 mm	5.0 mm	0.1969	6.2 mm	6.6 mm
M7×1.0	1.0 mm	6.0 mm	0.2362	7.2 mm	7.6 mm
M8×1.25	1.25 mm	6.8 mm	0.2677	8.2 mm	8.6 mm
M8×1.0 (fine)	1.0 mm	7.0 mm	0.2756	8.2 mm	8.6 mm
M10×1.5	1.5 mm	8.5 mm	0.3346	10.2 mm	10.8 mm
M10×1.25 (fine)	1.25 mm	8.8 mm	0.3465	10.2 mm	10.8 mm
M10×1.0 (fine)	1.0 mm	9.0 mm	0.3543	10.2 mm	10.8 mm
M12×1.75	1.75 mm	10.2 mm	0.4016	12.2 mm	13.0 mm
M12×1.5 (fine)	1.5 mm	10.5 mm	0.4134	12.2 mm	13.0 mm
M12×1.25 (fine)	1.25 mm	10.8 mm	0.4252	12.2 mm	13.0 mm
M14×2.0	2.0 mm	12.0 mm	0.4724	14.5 mm	15.0 mm
M14×1.5 (fine)	1.5 mm	12.5 mm	0.4921	14.5 mm	15.0 mm
M16×2.0	2.0 mm	14.0 mm	0.5512	16.5 mm	17.0 mm
M16×1.5 (fine)	1.5 mm	14.5 mm	0.5709	16.5 mm	17.0 mm
M18×2.5	2.5 mm	15.5 mm	0.6102	18.5 mm	19.0 mm
M18×1.5 (fine)	1.5 mm	16.5 mm	0.6496	18.5 mm	19.0 mm
M20×2.5	2.5 mm	17.5 mm	0.6890	20.5 mm	21.0 mm
M20×1.5 (fine)	1.5 mm	18.5 mm	0.7283	20.5 mm	21.0 mm
M22×2.5	2.5 mm	19.5 mm	0.7677	22.5 mm	23.0 mm
M24×3.0	3.0 mm	21.0 mm	0.8268	24.5 mm	25.0 mm
M27×3.0	3.0 mm	24.0 mm	0.9449	27.5 mm	28.0 mm
M30×3.5	3.5 mm	26.5 mm	1.0433	30.5 mm	31.0 mm

No sizes match your search. Try a different term or filter.

How to Use This Tap Drill Chart

Using a tap drill chart is simple once you understand the steps. Here is how to find the right drill bit size for any tapped hole:

Find your thread size. Look at your drawing or the bolt you need to fit. It will say something like "1/4-20 UNC" or "M10×1.5" (see our threading standards guide for help reading callouts).
Find that row in the chart. Use the search box above or scroll to find your thread size. You can also filter by UNC, UNF, or Metric.
Read the "Tap Drill" column. This is the drill bit size you need to drill the hole before tapping. For example, the drill bit size for an M10 tap (M10×1.5 standard pitch) is 8.5 mm (0.3346″).
Drill the hole. Use the tap drill to drill your hole to the correct depth. For a blind hole, drill slightly deeper than the thread length you need.
Tap the hole. Run the tap into the drilled hole. The tap cuts threads into the walls of the hole.

If you need a clearance hole instead (for a bolt to pass through without threads), use the "Close Fit Drill" or "Free Fit Drill" column.

Drill Bit Size for M10 Tap — Quick Reference

M10 is one of the most commonly used metric thread sizes. Here are the tap drill sizes for all three M10 pitch options:

Thread	Pitch	Tap Drill (mm)	Tap Drill (inch)	Best For
M10×1.5	1.5 mm (coarse)	8.5 mm	0.3346″	General purpose — most common M10 thread
M10×1.25	1.25 mm (fine)	8.8 mm	0.3465″	Stronger in thin walls, better vibration resistance
M10×1.0	1.0 mm (extra fine)	9.0 mm	0.3543″	Precision applications, adjustment screws, thin-wall parts

Tip: If your drawing just says “M10” with no pitch specified, it means M10×1.5 (the coarse/standard pitch). Use an 8.5 mm drill bit. The closest imperial size is a letter “Q” drill (0.332″) or 21/64″ (0.3281″), but the 8.5 mm metric drill is the correct choice for a proper 75% thread.

1/4-20 Tap Drill Size — Quick Reference

The 1/4-20 UNC thread is one of the most common thread sizes in the United States. You will find it everywhere — machine screws, brackets, fixtures, jigs, and consumer products. Here is everything you need to know about the 1/4-20 tap drill size:

Thread	TPI	Tap Drill	Decimal (in)	Close Fit	Free Fit
1/4″-20 UNC	20	#7 drill	0.2010″	17/64″	9/32″
1/4″-28 UNF	28	#3 drill	0.2130″	17/64″	9/32″

The 1/4-20 tap drill size is a #7 drill bit (0.2010″). This gives approximately 75% thread engagement, which is the standard for most applications. If you do not have a #7 drill, a 13/64″ drill (0.2031″) is very close and works fine in most situations.

For the fine-thread version (1/4-28 UNF), use a #3 drill bit (0.2130″). The UNF version is common in automotive, aerospace, and applications where vibration resistance matters.

Clearance holes for 1/4″ bolts: If you need a bolt to pass through without threads, drill a 17/64″ (0.2656″) hole for a close fit or a 9/32″ (0.2812″) hole for a free fit.

Understanding Thread Percentage

Thread percentage tells you how deep the threads are compared to a "perfect" full thread. Here is what you need to know:

100% thread = Full depth threads. Very hard to tap. Breaks taps easily. Almost never used.
75% thread = The standard. Gives about 90% of full thread strength. This is what the tap drill sizes in our chart are based on.
65% thread = Slightly shallower threads. Still very strong (about 85% of full). Great for aluminum, brass, and other soft metals. Use a slightly larger drill bit.
50% thread = Shallow threads. Used for very hard materials like hardened steel or titanium where tapping forces need to be low.

The key insight: going from 75% to 100% thread only adds about 10% more strength, but it doubles the tapping force and dramatically increases the chance of breaking a tap. That is why 75% is the sweet spot for almost every application.

CNC Tapping Tips from the Shop Floor

Use cutting fluid. Always. It reduces friction, extends tap life, and gives cleaner threads. For aluminum, use a cutting oil or mist coolant. For steel, use a sulfur-based cutting oil.
Watch your speed. Tapping RPM should be much slower than drilling RPM. A common starting point is 50-100 SFM for steel and 100-200 SFM for aluminum.
Peck tap deep holes. For holes deeper than 2× the diameter, use a pecking cycle to break and clear chips. Chip buildup is the #1 cause of broken taps.
Check your drill depth. For blind holes (holes that do not go all the way through), drill at least 1.5× the tap diameter deeper than the thread length. The tap needs room at the bottom.
Chamfer the hole entry. A small chamfer (about 0.5 mm) at the top of the hole helps the tap start straight and prevents raised edges.
Use the right tap style. Spiral point taps push chips forward (good for through holes). Spiral flute taps pull chips up and out (good for blind holes). Form taps displace metal without cutting (strongest threads, no chips).
Replace worn taps. A dull tap makes oversized threads and requires more force. If threads feel loose with a go/no-go gauge, the tap is worn.

Frequently Asked Questions

What is a tap drill?

A tap drill is the drill bit you use before tapping a hole. It makes a hole that is slightly smaller than the thread size. This leaves enough metal on the walls for the tap to cut threads into. Without drilling the right size hole first, the tap cannot create proper threads.

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

The tap drill size for a 1/4-20 UNC thread is a #7 drill bit (0.2010″). This gives 75% thread engagement, which is the standard for most applications. If you do not have a #7 drill, a 13/64″ (0.2031″) works as a close substitute. For a 1/4-28 UNF (fine thread), use a #3 drill bit (0.2130″).

What is the drill bit size for an M10 tap?

The drill bit size for an M10 tap depends on the thread pitch:

M10×1.5 (standard coarse pitch): Use an 8.5 mm drill bit (0.3346″)
M10×1.25 (fine pitch): Use an 8.8 mm drill bit (0.3465″)
M10×1.0 (extra fine pitch): Use a 9.0 mm drill bit (0.3543″)

If your drawing just says “M10” without specifying the pitch, it means M10×1.5 (the standard coarse pitch). Use an 8.5 mm drill.

What is the difference between UNC and UNF threads?

UNC stands for Unified National Coarse. It has fewer threads per inch, which makes it easier to assemble by hand and more forgiving with slightly damaged threads. UNF stands for Unified National Fine. It has more threads per inch, which makes it stronger in thin materials and more resistant to loosening from vibration. UNC is the most common choice for general-purpose fastening. UNF is used in aerospace, automotive, and precision applications.

What is the difference between coarse and fine metric threads?

Coarse pitch (like M10×1.5) is the default metric thread. The threads are deeper and further apart. Fine pitch (like M10×1.25 or M10×1.0) has shallower, closer threads. Fine pitch threads are stronger in thin-wall parts, less likely to loosen from vibration, and allow finer adjustment. But they are harder to tap and easier to cross-thread. Use coarse pitch unless you have a specific reason to use fine.

How do I pick between a close fit and free fit drill?

A close fit drill makes a hole just barely bigger than the bolt. The bolt fits through, but with very little wiggle room. Use close fit when you need precise alignment between parts — like dowel pins or shoulder screws.

A free fit drill makes a bigger hole so the bolt drops in easily. Use free fit when quick assembly matters more than exact alignment — like bolting a cover plate or bracket.

What thread percentage should I use for CNC tapping?

Use 75% thread for most applications. It gives about 90% of full thread strength and is much easier on the tap. For soft metals like aluminum or brass, drop to 60-65% thread — the threads will still be plenty strong, and your taps will last much longer. Going above 75% adds very little strength but dramatically increases the risk of breaking a tap.

Can I use an imperial drill bit for a metric tap?

You can if the imperial size is close enough. For example, a 21/64″ drill (0.3281″) is close to the 8.5 mm (0.3346″) needed for an M10×1.5 tap. But “close” means you get slightly more than 75% thread engagement, which increases tapping force. For best results, use the correct metric drill bit. Most machine shops stock both imperial and metric drill sets.

What happens if I use the wrong tap drill size?

If the drill is too small, the tap has to cut deeper threads. This increases force, generates more heat, and can break the tap — especially in hard materials. If the drill is too large, the threads will be too shallow and the bolt may strip out under load. Using the correct tap drill from this chart gives you the right balance of thread strength and tapping ease.

Tap & Drill Size Chart

Notes on This Chart

How to Use This Tap Drill Chart

Drill Bit Size for M10 Tap — Quick Reference

1/4-20 Tap Drill Size — Quick Reference

Understanding Thread Percentage

CNC Tapping Tips from the Shop Floor

Frequently Asked Questions

More Resources

Get Your Parts Made Right

Tap & Drill Size Chart

Notes on This Chart

How to Use This Tap Drill Chart

Drill Bit Size for M10 Tap — Quick Reference

1/4-20 Tap Drill Size — Quick Reference

Understanding Thread Percentage

CNC Tapping Tips from the Shop Floor

Frequently Asked Questions

More Resources

Related Resources

Thread Chart | Major, Minor & Pitch Diameters for Common Threads

Drill Bit Size Chart | Number, Letter & Fractional Sizes

Thread Pitch Chart | UNC, UNF & Metric Pitch Reference

Get Your Parts Made Right

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