What size drill do I use to tap a 1/4-20 thread?

For a standard 75% thread on 1/4-20 UNC, use a #7 drill (0.201 in). If you only have fractional drills, a 13/64 in (0.2031 in) is the usual substitute and gives a slightly easier-tapping ~70% thread. Do not use 1/4 in — that is the clearance size for a bolt to pass through, not the tap drill.

What is the tap drill size for M6?

For M6 x 1.0 (standard coarse metric), the tap drill is 5.0 mm. The quick rule for metric threads is tap drill = major diameter minus the pitch: 6.0 - 1.0 = 5.0 mm. For the fine M6 x 0.75, use 5.2 mm.

Why do charts use 75% thread engagement instead of 100%?

Going from 75% to 100% thread engagement adds only about 5% more thread strength, but it roughly doubles the tapping torque and dramatically increases the chance of breaking the tap. 75% is the industry standard because it captures almost all of the available strength while keeping tapping safe. A larger tap drill (lower % engagement) makes tapping easier and is common in tough or gummy materials; a smaller drill (higher %) is only worth it in thin material where thread depth is limited.

What is the formula for tap drill size?

For inch threads at roughly 75% engagement, tap drill diameter is approximately the major diameter minus 1/TPI (one over the threads per inch). For metric threads, tap drill diameter is approximately the major diameter minus the thread pitch in millimetres. These rules land within a thousandth or two of the standard chart values; use the chart for the exact named drill.

Can I use a metric drill for an inch tap, or an inch drill for a metric tap?

Yes. Tapping tolerates a small hole-size difference, so the nearest drill within about 0.1 mm (0.004 in) of the listed size works fine. For example an 8.5 mm tap drill for M10 x 1.5 is almost identical to a letter Q drill; a #7 for 1/4-20 is close to 5.1 mm. Pick the nearest size you own and stay on the slightly-larger side if you are unsure, which errs toward easier tapping.

Tap Drill Size Chart: UNC, UNF & Metric

The drill size for every common tap at 75% thread engagement — Unified coarse (UNC), Unified fine (UNF) and metric — with the named number, letter or fractional drill and its decimal size. The four figures marked with a tighter-fit alternative are where published charts differ by one drill number; both are given. Sources are listed at the bottom.

Inch — coarse thread (UNC)

Standard 75% thread engagement. The "tighter fit" column is the next-smaller drill some charts list for a higher-percentage thread — harder to tap, only worth it in thin material.

Thread	Tap drill	Drill Ø (in)	Tighter fit
#1-64	#53	0.0595	—
#2-56	#50	0.0700	#51 (.067)
#3-48	#47	0.0785	5/64 (.0781)
#4-40	#43	0.0890	—
#5-40	#38	0.1015	—
#6-32	#36	0.1065	—
#8-32	#29	0.1360	—
#10-24	#25	0.1495	—
#12-24	#16	0.1770	#17 (.173)
1/4-20	#7	0.2010	13/64 (.2031)
5/16-18	F	0.2570	—
3/8-16	5/16	0.3125	—
7/16-14	U	0.3680	—
1/2-13	27/64	0.4219	—
9/16-12	31/64	0.4844	—
5/8-11	17/32	0.5312	—
3/4-10	21/32	0.6562	—
7/8-9	49/64	0.7656	—
1-8	7/8	0.8750	—

Inch — fine thread (UNF)

Fine threads use a larger tap drill than the coarse thread of the same nominal size, because the thread is shallower. Standard 75% engagement.

Thread	Tap drill	Drill Ø (in)	Tighter fit
#0-80	3/64	0.0469	—
#1-72	#53	0.0595	—
#2-64	#50	0.0700	—
#3-56	#45	0.0820	#46 (.081)
#4-48	#42	0.0935	—
#5-44	#37	0.1040	—
#6-40	#33	0.1130	—
#8-36	#29	0.1360	—
#10-32	#21	0.1590	—
#12-28	#14	0.1820	#15 (.180)
1/4-28	#3	0.2130	—
5/16-24	I	0.2720	—
3/8-24	Q	0.3320	—
7/16-20	25/64	0.3906	—
1/2-20	29/64	0.4531	—
9/16-18	33/64	0.5156	—
5/8-18	37/64	0.5781	—
3/4-16	11/16	0.6875	—
7/8-14	13/16	0.8125	—
1-12	59/64	0.9219	—

Metric — coarse thread

Tap drill in millimetres. The quick rule tap drill = major diameter − pitch reproduces every value below. Two rows note the exact theoretical figure some charts round differently.

Thread	Tap drill (mm)	Pitch (mm)	Note
M2 × 0.4	1.60	0.40	—
M2.5 × 0.45	2.05	0.45	—
M3 × 0.5	2.50	0.50	—
M3.5 × 0.6	2.90	0.60	—
M4 × 0.7	3.30	0.70	—
M5 × 0.8	4.20	0.80	—
M6 × 1.0	5.00	1.00	—
M8 × 1.25	6.80	1.25	theoretical 6.75
M10 × 1.5	8.50	1.50	—
M12 × 1.75	10.20	1.75	theoretical 10.25
M14 × 2.0	12.00	2.00	—
M16 × 2.0	14.00	2.00	—
M18 × 2.5	15.50	2.50	—
M20 × 2.5	17.50	2.50	—

Metric — fine thread

Common fine-pitch metric sizes. Same rule: tap drill = major diameter − pitch, rounded to a stocked drill.

Thread	Tap drill (mm)	Pitch (mm)	Note
M8 × 1.0	7.00	1.00	—
M10 × 1.25	8.80	1.25	—
M12 × 1.25	10.80	1.25	—
M12 × 1.5	10.50	1.50	—

Drilled the hole? → Set the right spindle RPM for your drill diameter · then → work out the drilling feed rate

How to read this chart

The tap drill is the hole you drill before cutting an internal thread — not the hole a bolt passes through. Drill it too big and the threads are shallow and strip; too small and the tap binds and snaps. The workflow is:

1. Find your thread in the table above (inch coarse, inch fine, or metric).
2. Drill the listed tap drill size — the named number/letter/fractional drill for inch, or the millimetre size for metric.
3. Tap the hole, backing off every half-turn to break the chip, with cutting fluid.

Number, letter and fractional drills

Inch tap drills use three overlapping size systems, which is why a chart is faster than a calculator:

Number drills #1–#80 — the smallest, #1 (0.228 in) down to #80 (0.0135 in). Counter-intuitively, a higher number is a smaller drill.
Letter drills A–Z — mid-range, A (0.234 in) up to Z (0.413 in). A higher letter is a larger drill.
Fractional drills — 1/16 in and up in 1/64 in steps, used for the bigger taps.

The decimal column lets you substitute across systems: any drill within about 0.002 in of the listed diameter taps the same.

The formula behind the chart

Inch: tap drill ≈ major diameter − (1 / TPI). For 1/4-20 that is 0.25 − 1/20 = 0.200 in, essentially the #7 (0.201 in) in the table.
Metric: tap drill ≈ major diameter − pitch. For M6 × 1.0 that is 6.0 − 1.0 = 5.0 mm.

Both rules target roughly 75% thread engagement and land within a thousandth or two of the standard values. Use the rule to sanity-check, use the chart for the exact stocked drill.

Why 75% — and when to change it

75% thread engagement is the default because the jump from 75% to 100% adds only about 5% thread strength while roughly doubling tapping torque and tap breakage. Deviate deliberately:

Go bigger (50–65%) in tough, gummy or work-hardening materials (stainless, titanium) and for hand-tapping deep holes — easier and safer, with almost no strength loss.
Go smaller (higher %) only in thin material where there are too few threads to reach full strength, or in soft materials like aluminium and plastic that hold a fuller thread cleanly.

Tap drill vs clearance drill

A common and expensive mistake: drilling the clearance size (the diameter that lets the bolt slide through, i.e. the major diameter or a hair over) when you meant to tap. A 1/4-20 tap drill is a #7 (0.201 in); the 1/4-20 clearance drill is 17/64 in (0.2656 in) — a completely different hole. This page is the tap drill; if you need the through-hole for the screw, that is the clearance size, not here.

Frequently asked questions

What size drill do I use to tap 1/4-20?

A #7 drill (0.201 in) for a standard 75% thread. A 13/64 in (0.2031 in) is the common fractional substitute and taps a little easier. Never use 1/4 in — that is clearance, not tap drill.

What is the tap drill for M6?

5.0 mm for M6 × 1.0 coarse (major diameter 6.0 − pitch 1.0). For fine M6 × 0.75, use 5.2 mm.

Why do charts use 75% thread and not 100%?

100% engagement adds only ~5% strength over 75% but roughly doubles tapping torque and the risk of snapping the tap. 75% captures almost all the strength while keeping tapping safe.

What is the tap drill formula?

Inch: major diameter − 1/TPI. Metric: major diameter − pitch. Both target ~75% engagement and match the chart within a thousandth or two.

Can I swap a metric drill for an inch tap drill?

Yes — the nearest drill within about 0.1 mm (0.004 in) taps the same. When unsure, take the slightly larger drill; it errs toward easier tapping.

Sources

Every value above was cross-checked against at least two published references. Where two charts disagreed by one drill number — #2-56, #12-24, #3-56 and #12-28 — the mainstream 75% value is shown in the main column and the tighter alternative is flagged. All figures assume standard Unified and ISO metric thread forms.

American Fastener Technologies — Tap & Drill Size Chart — primary inch (UNC/UNF) source; values match the Machinery's Handbook 75% table.
6GTools — Standard Tap Drill Chart and Metric Tap Drill Chart — cross-check for inch and metric; states ~70–75% engagement.
Newman Tools — Tap Drill Sizes — inch cross-check; states approx. 75% thread.
AmesWeb — Metric Tap Drill Chart — metric coarse and fine, using the major-diameter-minus-pitch rule.
TTP Hard Drills — Drill Sizes for Metric Taps — metric cross-check; explicitly states standard 75% thread engagement.