Tap & Drill Size Chart

Recommended tap drill sizes for Unified inch threads (UNC & UNF) at ~75% thread engagement — drill designation plus decimal size in inches and millimeters, from #2-56 up to 3/4-10.

Tap drill sizes for UNC and UNF threads at 75 percent thread
Thread	Series	Threads/in	Tap drill	Decimal (in)	mm
#2-56	UNC	56	#50	0.0700	1.78
#3-48	UNC	48	#47	0.0785	1.99
#4-40	UNC	40	#43	0.0890	2.26
#5-40	UNC	40	#38	0.1015	2.58
#6-32	UNC	32	#36	0.1065	2.71
#8-32	UNC	32	#29	0.1360	3.45
#10-24	UNC	24	#25	0.1495	3.80
#10-32	UNF	32	#21	0.1590	4.04
#12-24	UNC	24	#16	0.1770	4.50
1/4-20	UNC	20	#7	0.2010	5.11
1/4-28	UNF	28	#3	0.2130	5.41
5/16-18	UNC	18	F	0.2570	6.53
5/16-24	UNF	24	I	0.2720	6.91
3/8-16	UNC	16	5/16	0.3125	7.94
3/8-24	UNF	24	Q	0.3320	8.43
7/16-14	UNC	14	U	0.3680	9.35
7/16-20	UNF	20	25/64	0.3906	9.92
1/2-13	UNC	13	27/64	0.4219	10.72
1/2-20	UNF	20	29/64	0.4531	11.51
5/8-11	UNC	11	17/32	0.5312	13.49
3/4-10	UNC	10	21/32	0.6562	16.67

~75% thread engagement. Most shops use 50–65% in steel; confirm against your tap manufacturer's spec.

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Frequently asked questions

What tap drill should I use?: These sizes target ~75% thread engagement, the traditional default. In steel and harder alloys most shops drop to 50–65% engagement (a slightly larger drill) to cut tapping torque and reduce tap breakage with minimal strength loss; softer materials like aluminum and brass take the higher engagement. When in doubt, follow your tap manufacturer’s recommendation.
How is tap drill size calculated?: For ~75% thread the inch formula is: tap drill ≈ major diameter − (0.974 / threads-per-inch). For 1/4-20 that is 0.250 − (0.974/20) = 0.2013", closest to a #7 drill (0.201").
UNC vs UNF?: UNC is coarse (fewer threads per inch), UNF is fine (more threads per inch). Fine threads have a larger minor diameter, so they use a larger tap drill than the coarse thread of the same nominal size.

Tap & Drill Size Chart

Recommended tap drill sizes for Unified inch threads (UNC & UNF) at ~75% thread engagement — drill designation plus decimal size in inches and millimeters, from #2-56 up to 3/4-10.

Tap drill sizes for UNC and UNF threads at 75 percent thread
Thread	Series	Threads/in	Tap drill	Decimal (in)	mm
#2-56	UNC	56	#50	0.0700	1.78
#3-48	UNC	48	#47	0.0785	1.99
#4-40	UNC	40	#43	0.0890	2.26
#5-40	UNC	40	#38	0.1015	2.58
#6-32	UNC	32	#36	0.1065	2.71
#8-32	UNC	32	#29	0.1360	3.45
#10-24	UNC	24	#25	0.1495	3.80
#10-32	UNF	32	#21	0.1590	4.04
#12-24	UNC	24	#16	0.1770	4.50
1/4-20	UNC	20	#7	0.2010	5.11
1/4-28	UNF	28	#3	0.2130	5.41
5/16-18	UNC	18	F	0.2570	6.53
5/16-24	UNF	24	I	0.2720	6.91
3/8-16	UNC	16	5/16	0.3125	7.94
3/8-24	UNF	24	Q	0.3320	8.43
7/16-14	UNC	14	U	0.3680	9.35
7/16-20	UNF	20	25/64	0.3906	9.92
1/2-13	UNC	13	27/64	0.4219	10.72
1/2-20	UNF	20	29/64	0.4531	11.51
5/8-11	UNC	11	17/32	0.5312	13.49
3/4-10	UNC	10	21/32	0.6562	16.67

~75% thread engagement. Most shops use 50–65% in steel; confirm against your tap manufacturer's spec.

Frequently asked questions

What tap drill should I use?

These sizes target ~75% thread engagement, the traditional default. In steel and harder alloys most shops drop to 50–65% engagement (a slightly larger drill) to cut tapping torque and reduce tap breakage with minimal strength loss; softer materials like aluminum and brass take the higher engagement. When in doubt, follow your tap manufacturer’s recommendation.

How is tap drill size calculated?

For ~75% thread the inch formula is: tap drill ≈ major diameter − (0.974 / threads-per-inch). For 1/4-20 that is 0.250 − (0.974/20) = 0.2013", closest to a #7 drill (0.201").

UNC vs UNF?

UNC is coarse (fewer threads per inch), UNF is fine (more threads per inch). Fine threads have a larger minor diameter, so they use a larger tap drill than the coarse thread of the same nominal size.