"What tap drill do I use for a 1/4-20 thread?"

"A #7 drill, which is 0.201 inch. That gives roughly 75 percent thread engagement on a 1/4-20 UNC tapped hole, which is the standard general-purpose target. Some shops drill 13/64 inch (0.2031) instead because a number-drill index is not always on hand, and the result is close enough for most bolted joints. If you need the exact engagement for a critical part, follow the tap manufacturer's chart for the specific tap, because it accounts for the thread form and the tolerance class in ASME B1.1."

"Why is 75 percent thread engagement the standard, not 100 percent?"

"Going from 75 percent to 100 percent engagement adds only a few percent of pull-out strength, but it roughly triples the torque needed to cut the thread. That extra torque is what breaks taps, especially small ones in deep or blind holes. 75 percent engagement gets you nearly all of the strength with far less tap breakage, so it is the convention almost every published tap drill chart is built on. If you ever see two different drills listed for the same thread, one is usually a 75 percent value and the other a 50 percent value for tougher materials where tap life matters more than maximum strength."

"What is the difference between number, letter, and fractional drill bits?"

"They are three overlapping size series that fill the gaps between round fractions. Fractional drills step in 1/64 inch increments. Number drills run from #1 (0.228 inch) down to #80 (0.0135 inch) and cover the small end. Letter drills run from A (0.234 inch) up to Z (0.413 inch) and bridge the range just above 1/4 inch. The systems are defined in ANSI/ASME B94.11M. The numbers and letters exist because many tap drills and clearance holes land between the common fractions, and you need a finer step than 1/64 inch to hit them."

"Can I just use the nearest fractional drill instead of a number or letter drill?"

"Often yes, for general work. The decimal columns let you see how close the nearest fraction is. For 3/8-16, the chart already calls out 5/16 inch as the tap drill, so no number drill is needed. For 1/4-20 the proper drill is a #7 at 0.201, and the nearest fraction, 13/64 at 0.2031, is only three thousandths over, which is fine for most bolted connections. The closer the hole runs to the listed tap drill, the closer you land to 75 percent engagement. For thin material, soft alloys, or anything structural, drill the size the chart actually lists."

Drill Bit and Tap Drill Size Chart: Fractional, Number, Letter, and Metric

Two charts in one. The first set converts a drill designation, fractional, number, or letter, into its decimal-inch and millimeter size. The second tells you which drill to run before a tap for the common Unified threads. The tap drill values target about 75 percent thread engagement, which is the standard general-shop convention and what almost every published tap chart is built on.

Number drills run small (#1 is 0.228 inch, #80 is 0.0135 inch). Letter drills pick up just above 1/4 inch (A is 0.234 inch, Z is 0.413 inch). They exist because tap drills and clearance holes rarely land on a clean fraction. The series are defined in ANSI/ASME B94.11M, and the thread dimensions behind the tap drill sizes come from ASME B1.1, the Unified Inch Screw Threads standard. These are reference numbers for layout and setup. For a critical hole, the tap maker’s own recommendation for that specific tap wins.

Tap Drill Sizes for Common UNC and UNF Threads

This is the table fabricators reach for. Find your thread, drill the listed size, then tap. UNC is the coarse series and UNF is the fine series. The decimal column is the actual drill diameter so you can see how close the nearest fraction sits if you do not have the exact bit.

Tap drill sizes for Unified threads, approximately 75 percent engagement (per ASME B1.1 thread dimensions)
Thread Size	Series	Tap Drill	Drill Diameter (inch)	Drill Diameter (mm)
4-40	UNC	#43	0.0890	2.26
6-32	UNC	#36	0.1065	2.71
8-32	UNC	#29	0.1360	3.45
10-24	UNC	#25	0.1495	3.80
10-32	UNF	#21	0.1590	4.04
1/4-20	UNC	#7	0.2010	5.11
1/4-28	UNF	#3	0.2130	5.41
5/16-18	UNC	F	0.2570	6.53
5/16-24	UNF	I	0.2720	6.91
3/8-16	UNC	5/16	0.3125	7.94
3/8-24	UNF	Q	0.3320	8.43
7/16-14	UNC	U	0.3680	9.35
1/2-13	UNC	27/64	0.4219	10.72
1/2-20	UNF	29/64	0.4531	11.51
5/8-11	UNC	17/32	0.5312	13.49
3/4-10	UNC	21/32	0.6562	16.67

Notice how many of these are number or letter drills, not fractions. That is the whole reason those series exist. A 1/4-20 wants a #7, and the closest fraction (13/64 at 0.2031) is three thousandths oversize. For a general bolted bracket that is fine. For a thin part or a fastener that has to carry real load, drill the listed size so you actually get the engagement the chart assumes.

How to Read This Chart: A Worked Example

Say you are tapping a 3/8-16 hole into a piece of 1/2 inch plate to bolt a fixture down. Drop into the 3/8-16 row. The tap drill is 5/16 inch, a clean fraction, so no special bit needed. Drill 5/16, run the tap, and you land near 75 percent engagement.

Now take 1/4-20, the most common thread in a hobby shop. The row says #7, 0.201 inch. If you do not own a number-drill index, the decimal column lets you judge the substitute. The nearest fraction above is 13/64 (0.2031), three thousandths over, which lowers engagement slightly but works for most fasteners. The nearest below is 3/16 (0.1875), which is too tight and will load the tap hard or break it. When in doubt, go to the slightly larger fraction, not the smaller one.

Fractional Drill Sizes

Standard fractional drills step in 1/64 inch increments. These are the everyday sizes for clearance holes, hole saw pilots, and the larger tap drills.

Fractional drill sizes, decimal and metric equivalents
Fraction	Inch	mm
1/16	0.0625	1.59
3/32	0.0938	2.38
1/8	0.1250	3.18
5/32	0.1563	3.97
3/16	0.1875	4.76
7/32	0.2188	5.56
1/4	0.2500	6.35
5/16	0.3125	7.94
3/8	0.3750	9.53
7/16	0.4375	11.11
1/2	0.5000	12.70
9/16	0.5625	14.29
5/8	0.6250	15.88
3/4	0.7500	19.05
7/8	0.8750	22.23
1	1.0000	25.40

Number Drill Sizes (#1 through #43)

Number drills fill the gaps below 1/4 inch where most small tap drills and tight clearance holes live. The full series runs to #80, but the sizes below are the ones that matter for the threads on this chart. Remember the count runs backward, a higher number is a smaller drill.

Number drill sizes #1 to #43, decimal and metric
Number	Inch	mm
#1	0.2280	5.79
#2	0.2210	5.61
#3	0.2130	5.41
#7	0.2010	5.11
#10	0.1935	4.91
#21	0.1590	4.04
#25	0.1495	3.80
#29	0.1360	3.45
#36	0.1065	2.71
#43	0.0890	2.26

Letter Drill Sizes (A through Z)

Letter drills bridge the range from just above 1/4 inch up past 13/32 inch, exactly where several coarse tap drills land. F, I, Q, and U on this list are all tap drills for the threads in the table above.

Letter drill sizes A to Z, decimal and metric
Letter	Inch	mm
A	0.2340	5.94
C	0.2420	6.15
F	0.2570	6.53
I	0.2720	6.91
L	0.2900	7.37
O	0.3160	8.03
Q	0.3320	8.43
U	0.3680	9.35
X	0.3970	10.08
Z	0.4130	10.49

Where Drilled Holes Show Up in Fab Work

Most of the time you reach for this chart it is because something on the bench told you to drill a specific hole and you wanted to confirm the size. Plug welds are the obvious case. The hole has to be big enough to fuse the bottom sheet but not so big it blows out, and the plug welding sheet metal guide walks through hole sizing for panel work. Building a workbench is another. The DIY welding table plans call out tapped and clearance holes for the legs and accessories, and the tap drill table above is what turns a thread callout into the right bit.

If you are newer to laying out and drilling steel before you ever strike an arc, the metal fabrication basics walkthrough covers measuring, marking, and hole work as part of the whole layout-to-weld sequence.

Notes and Caveats

A few things that trip people up:

The tap drill values are for cut taps removing material. Forming taps (roll taps), which displace metal instead of cutting it, use a larger drill and have their own separate chart. Do not use this table for a forming tap.
75 percent engagement is the general convention. In hard or gummy material, a 50 percent value (a slightly larger drill) saves the tap and still holds. The threads on this chart assume 75 percent.
Decimal sizes here are nominal drill diameters. A worn or oversize-grinding drill cuts a hole larger than its marked size, which lowers thread engagement. For a critical hole, drill on scrap and measure before you commit to the part.
The thread dimensions behind these numbers come from ASME B1.1. The drill series come from ANSI/ASME B94.11M. When a fastener drawing references a different standard or tolerance class, get the tap drill from that source rather than this general reference.

For machine settings and cutting work tied to the rest of a job, the other welding reference charts on this site follow the same printable-table format.

How to Read This Chart