Tap Drill Size Chart: UNC, UNF and Metric

The right drill for every common inch and metric tap, with decimal and mm sizes, the formula behind it, tap drills for NPT pipe threads, and clearance hole sizes for bolts that pass through.

Updated October 17, 2026

Cross-sections of three tapped holes drawn to scale, M6, 1/4-20 and M10, each showing the tap drill hole inside the thread's outside diameter

Before you cut threads with a tap, you drill a hole a little smaller than the bolt. The tap then cuts the threads into the material left around the hole. Drill too big and the threads are shallow and strip; drill too small and the tap binds and can snap. The sizes below give about 75% thread depth, the standard for most metal, plastic and hardwood.

Quick rule: tap drill = bolt diameter − pitch. For M6 × 1.0: 6 − 1 = 5 mm. Not sure which thread you have? Measure the bolt and use the thread identifier.

UNC and UNF tap drill chart

Thread	Tap drill	Decimal (in)	mm
#0-80	3/64	0.0469	1.19
#1-64	#53	0.0595	1.51
#2-56	#50	0.0700	1.78
#3-48	#47	0.0785	1.99
#4-40	#43	0.0890	2.26
#4-48	#42	0.0935	2.37
#5-40	#38	0.1015	2.58
#6-32	#36	0.1065	2.71
#6-40	#33	0.1130	2.87
#8-32	#29	0.1360	3.45
#8-36	#29	0.1360	3.45
#10-24	#25	0.1495	3.80
#10-32	#21	0.1590	4.04
#12-24	#16	0.1770	4.50
#12-28	#14	0.1820	4.62
1/4-20	#7	0.2010	5.11
1/4-28	#3	0.2130	5.41
5/16-18	F	0.2570	6.53
5/16-24	I	0.2720	6.91
3/8-16	5/16	0.3125	7.94
3/8-24	Q	0.3320	8.43
7/16-14	U	0.3680	9.35
7/16-20	25/64	0.3906	9.92
1/2-13	27/64	0.4219	10.72
1/2-20	29/64	0.4531	11.51
9/16-12	31/64	0.4844	12.30
9/16-18	33/64	0.5156	13.10
5/8-11	17/32	0.5313	13.49
5/8-18	37/64	0.5781	14.68
3/4-10	21/32	0.6563	16.67
3/4-16	11/16	0.6875	17.46
7/8-9	49/64	0.7656	19.45
7/8-14	13/16	0.8125	20.64
1-8	7/8	0.8750	22.23
1-12	59/64	0.9219	23.42

The first number is the screw size and the second is threads per inch: 1/4-20 is a 1/4 inch screw with 20 threads per inch. In each pair, the lower number is UNC (coarse), the common hardware-store thread, and the higher is UNF (fine). Number and letter drills are listed with their sizes in the number and letter drill size chart.

Metric tap drill chart

Thread	Pitch	Tap drill	Inch equivalent
M2	0.4	1.6 mm	#52 (0.0635)
M2.5	0.45	2.05 mm	#46 (0.081)
M3	0.5	2.5 mm	#40 (0.098)
M3.5	0.6	2.9 mm	#33 (0.113)
M4	0.7	3.3 mm	#30 (0.1285)
M5	0.8	4.2 mm	#19 (0.166)
M6	1.0	5.0 mm	#9 (0.196)
M8	1.25	6.8 mm	H (0.266)
M10	1.5	8.5 mm	Q (0.332)
M12	1.75	10.2 mm	Y (0.404)
M14	2.0	12.0 mm	15/32 (0.469)
M16	2.0	14.0 mm	35/64 (0.547)
M18	2.5	15.5 mm	39/64 (0.609)
M20	2.5	17.5 mm	11/16 (0.6875)
M24	3.0	21.0 mm	53/64 (0.828)

These are the standard coarse threads, the ones you get when no pitch is given. For fine metric threads, use the same rule:

Fine thread	Tap drill
M8 × 1.0	7.0 mm
M10 × 1.25	8.8 mm
M10 × 1.0	9.0 mm
M12 × 1.5	10.5 mm
M12 × 1.25	10.8 mm
M14 × 1.5	12.5 mm
M16 × 1.5	14.5 mm
M20 × 1.5	18.5 mm

Not sure of the pitch? See the thread pitch chart.

NPT pipe tap drill sizes

Pipe threads are tapered and sized by the pipe, not the hole, so the drill is much bigger than the name suggests.

NPT size	Tap drill	Decimal (in)	mm
1/8-27	R	0.339	8.6
1/4-18	7/16	0.4375	11.1
3/8-18	37/64	0.5781	14.7
1/2-14	23/32	0.7188	18.3
3/4-14	59/64	0.9219	23.4
1-11 1/2	1 5/32	1.1563	29.4

Tap until a fitting threads in by hand about 3 to 4 turns, then stop: the taper does the sealing. The pipe size chart explains why 1/2 inch pipe is 0.84 inch across.

Clearance holes

A clearance hole is the opposite of a tap drill: it goes in the part being bolted down, so the bolt slides through. A close fit locates parts accurately; a free fit is easier to line up.

Bolt	Close fit	Free fit
#4	#32 (0.116)	#30 (0.1285)
#6	#27 (0.144)	#25 (0.1495)
#8	#18 (0.1695)	#16 (0.177)
#10	#9 (0.196)	#7 (0.201)
1/4	F (0.257)	H (0.266)
5/16	P (0.323)	Q (0.332)
3/8	W (0.386)	X (0.397)
1/2	33/64 (0.516)	17/32 (0.531)

Metric bolt	Close (fine)	Normal (medium)	Loose (coarse)
M3	3.2 mm	3.4 mm	3.6 mm
M4	4.3 mm	4.5 mm	4.8 mm
M5	5.3 mm	5.5 mm	5.8 mm
M6	6.4 mm	6.6 mm	7.0 mm
M8	8.4 mm	9.0 mm	10.0 mm
M10	10.5 mm	11.0 mm	12.0 mm
M12	13.0 mm	13.5 mm	14.5 mm
M16	17.0 mm	17.5 mm	18.5 mm

Metric clearance holes are from ISO 273.

Tapping tips

Drill straight. A drill press or a guide block keeps the hole square; a crooked hole makes a crooked thread.
Use cutting oil on steel and aluminum (not on brass or cast iron, which tap best dry).
Turn the tap a half turn forward, then a quarter turn back to break the chips, especially in blind holes.
Soft or thin material? In aluminum, plastic or thin sheet, a drill one size smaller gives stronger threads. In hard steel, one size bigger makes tapping much easier with little loss of strength.

For drill bit sizes in fractions and mm, see the drill bit size chart; to convert any size, use inches to mm.

More DIY charts: number and letter drill sizes · bolt torque · washer sizes · screw and bolt sizes · thread pitch.

Frequently asked questions

What size drill for a 3/8-16 tap?

A 5/16 inch drill (0.3125 in, 7.94 mm). That leaves about 75% thread depth, the standard for steel and aluminum. For the fine thread 3/8-24, use a letter Q drill (0.332 in).

What size drill for a 1/4-20 tap?

A #7 drill (0.201 in, 5.11 mm). If you don't have number drills, 13/64 inch (0.203 in) is the nearest fractional bit and works in most materials. For 1/4-28, use a #3 drill (0.213 in).

How do you calculate the tap drill size?

Subtract the pitch from the diameter. For metric: M8 × 1.25 needs 8 − 1.25 = 6.75, rounded to a 6.8 mm drill. For inch threads, the pitch is 1 ÷ TPI: 1/2-13 needs 0.500 − 1/13 = 0.423 in, so a 27/64 drill (0.422 in).

What size drill for an M8 tap?

6.8 mm for the standard coarse M8 × 1.25 thread. For fine M8 × 1.0, use 7.0 mm. A 17/64 inch bit (6.75 mm) is the closest inch size if you don't have a 6.8 mm drill.

What size drill for a 1/8 NPT tap?

A letter R drill (0.339 in, 8.6 mm), or 11/32 inch (0.344 in) if you don't have letter drills. NPT pipe threads are tapered, so drill to depth and tap until a fitting threads in snugly by hand about 3 to 4 turns.