What size drill do I need to tap 1/4"-20 UNC?

A #7 drill — 0.2010 in (5.105 mm) — which leaves 75.4% thread engagement with a cut tap. The calculator on this page gives the same answer for any of the 150 inch and metric threads in the database, at any engagement from 50% to 100%.

How do you calculate tap drill size?

Subtract the engaged thread depth from the major diameter: hole Ø = D − (%thread ÷ 100) × 1.29904 × P, where D is the basic major diameter, P is the pitch, and 1.29904 is twice the basic thread height per side on the 60° V profile. For inch threads P = 1/n, so the same formula is usually written D − (%thread ÷ 100) × (1.29904 ÷ n). Then pick the nearest standard drill and recompute what that real drill actually leaves.

What percent thread engagement should a tap drill give?

75% is the shop default and what almost every published chart assumes without saying so. Dropping to 60–65% sharply reduces tapping torque and broken taps in tough material while costing only a few percent of thread strength, because the fastener nearly always fails before the thread strips. Above 85% torque climbs steeply for almost no gain — several traditional chart rows land there by accident, which is a documented cause of broken taps.

What drill should I use for 1/4"-20 UNC at 60% thread?

A #3 drill — 0.2130 in (5.410 mm) — which leaves 57.0% thread. That is the lighter-engagement choice for tough material or a blind hole; the 75% answer is #7.

Does this tap drill calculator handle metric threads?

Yes — 68 metric threads and 82 inch threads are in the same picker, and every result is given in both inches and millimetres. Metric threads default to the metric drill index; the drill-index control will search the inch index instead if that is what is in the cabinet.

Is the metric rule 'drill size = diameter minus pitch' accurate?

It is exactly the 77% thread case. Running D − P through the engagement formula returns 77.0% for M3×0.5, M4×0.7, M5×0.8, M6×1.0, M10×1.5 and M16×2 alike, so the rule of thumb is a slightly heavier thread than the 75% a chart implies — close enough for most work and worth knowing when a tap is breaking.

Calculator · threads

Tap Drill Size Calculator — Any Thread, Any Engagement

https://shopmath.org/calculators/tap-drill · data verified August 1, 2026

The tap drill for 1/4"-20 UNC at 75% thread engagement is #7 — 0.2010 in (5.105 mm) — and that drill actually leaves 75.4% thread, not 75%. Change either the thread or the engagement below and every number moves with it, including the one your wall chart does not print.

Threads: 150
Drills: 397
Engagement: 50–100%
Tap type: Cut
Verified: August 1, 2026

Tap Drill Size Calculator — Inch and Metric Threads

Every thread in the database, at any engagement from 50% to 100%. The drill index defaults to the one the thread's own system uses — inch drills for inch threads, metric for metric — which is the assumption every published chart makes silently.

Tap drill calculator

Drill index

Thread

150 inch and metric threads. Type a size — “m8” or “1/4” — to filter.

Thread engagement

70–77% is what a published chart means when it says nothing. 75% is the classic default and the assumption behind almost every tap drill chart in circulation, including the one on this site's thread pages.

Tap drill · 1/4"-20 UNC · 75% thread · cut tap

nearest of the number, letter and fractional drills

Drill Ø: 0.2010in
Drill Ø: 5.105mm
Actual thread: 75.4%
Target hole Ø: 0.2013in

The theoretical hole for 75% thread is 0.2013 in; #7 is the nearest drill that exists, and it leaves 75.4% — 0.4 points more than asked for. Figures are for a cut tap. Full 1/4"-20 UNC spec page →

The formula, with your numbers in it

NBS H28 / ASME B1.1 profile geometry

```
hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)
```
```
hole Ø  =  0.2500 − (75 ÷ 100) × (1.29904 ÷ 20)
```
= 0.2013 in
1.29904 = 2 × 0.64952, twice the basic thread height per side on the 60° V profile that the Unified and ISO systems share. D is the basic major diameter, n the threads per inch.
```
%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100
```
```
%thread  =  (0.2500 − 0.2010) × 20 ÷ 1.29904 × 100
```
= 75.4% thread
The first line gives a theoretical hole; #7 is the nearest real drill to it, so this line reports what that drill actually leaves. The gap between the two is the number every published chart leaves out.

The drills either side of it

Standard drills near the 75% thread target for 1/4"-20 UNC
Property	Drill Ø		Property	Property
Drill	in	mm	Thread %	vs target
#10	0.1935	4.915	87.0	+12.0
#9	0.1960	4.978	83.1	+8.1
#8	0.1990	5.055	78.5	+3.5
#7nearest	0.2010	5.105	75.4	+0.4
13/64	0.2031	5.159	72.2	−2.8
#6	0.2040	5.182	70.8	−4.2
#5	0.2055	5.220	68.5	−6.5

Every drill within three sizes of the target, with the engagement each one leaves. The nearest is not always the right choice: a blind hole in tough steel is a good reason to take the next size up and a lighter thread, and this table is what lets you make that call with the numbers in front of you.

A Worked Tap Drill Example — 1/4"-20 UNC

The same calculation done in prose, so the answer is on the page whether or not anything runs.

1/4"-20 UNC has a basic major diameter of 0.2500 in (6.350 mm) and 20 threads per inch. At 75% thread engagement the formula asks for a hole of 0.2013 in (5.113 mm): 0.2500 − (75 ÷ 100) × (1.29904 ÷ 20).

No drill is made at exactly that size. The nearest one in the inch index is #7 at 0.2010 in (5.105 mm), which is 0.0003 in from the target and leaves 75.4% thread rather than exactly 75%. On this size the rounding costs almost nothing; on others it costs a great deal, and the only way to know which you have is to compute it. A wall chart prints the drill and stops.

Drop the engagement to 60% — the range tap manufacturers publish for tough material — and the answer moves to #3 (0.2130 in (5.410 mm)), leaving 57.0% thread. The joint gives up a few percent of thread strength and the tap sees far less torque.

Sources and Method for the Tap Drill Calculator

Inch thread dimensionscomputed

NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)

Verified against threads-unified: 1064 checks passed across 3 harness(es)

Computed from the standard's formulas, then checked against published tables.

Metric thread dimensionscomputed+compiled

ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled

Verified against threads-metric: 215 checks passed across 3 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Standard drill indexcomputed+compiled

bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns

Verified against drills: 31 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Formula

Tap drill hole diameter — inch threads

hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch

Tap drill hole diameter — metric threads

hole Ø  =  D − (%thread ÷ 100) × 1.29904 × P

  D = basic major diameter (mm)
  P = pitch (mm)

1.29904 = 2 × 0.64952, twice the basic thread height per side on the 60° V profile shared by the Unified and ISO systems.

Engagement the chosen drill actually produces

%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100        [inch]
%thread  =  (D − drill Ø) ÷ (1.29904 × P) × 100      [metric]

The recommendation rounds to a real drill; this reports what that drill leaves.

Assumptions

Every figure is for a CUT tap. Form (roll) taps need a larger hole and are not in this dataset.
The drill index defaults to the thread's own system — inch drills for inch threads, metric for metric. That is the assumption behind every published chart, and the drill-index control makes it explicit and changeable.
The recommendation is the nearest standard drill in either direction, not the next size up. Selecting nearest-to-75% reproduces 36 of the 39 rows of the classic published UNC/UNF chart; every tie-break variant tested made the agreement worse.
Drill diameters are nominal. A worn drill, spindle run-out, or a soft material closing in behind the drill all move the finished hole toward a heavier thread than the table says.
The engagement bands described beside the slider come from practitioner evidence in our audience research, not from a standard. They describe what the number means; they are not a recommendation for a specific job.
Inch thread dimensions scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
Metric thread dimensions scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).
Standard drill index scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Tap Drill Calculator

Citation

ShopMath. "Tap Drill Size Calculator — Any Thread, Any Engagement." ShopMath, verified August 1, 2026, https://shopmath.org/calculators/tap-drill

BibTeX

@misc{shopmath-calculators-tap-drill,
  title        = {Tap Drill Size Calculator — Any Thread, Any Engagement},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/calculators/tap-drill}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/calculators/tap-drill

Common Questions About Tap Drill Sizes

What size drill do I need to tap 1/4"-20 UNC?: A #7 drill — 0.2010 in (5.105 mm) — which leaves 75.4% thread engagement with a cut tap. The calculator on this page gives the same answer for any of the 150 inch and metric threads in the database, at any engagement from 50% to 100%.
How do you calculate tap drill size?: Subtract the engaged thread depth from the major diameter: hole Ø = D − (%thread ÷ 100) × 1.29904 × P, where D is the basic major diameter, P is the pitch, and 1.29904 is twice the basic thread height per side on the 60° V profile. For inch threads P = 1/n, so the same formula is usually written D − (%thread ÷ 100) × (1.29904 ÷ n). Then pick the nearest standard drill and recompute what that real drill actually leaves.
What percent thread engagement should a tap drill give?: 75% is the shop default and what almost every published chart assumes without saying so. Dropping to 60–65% sharply reduces tapping torque and broken taps in tough material while costing only a few percent of thread strength, because the fastener nearly always fails before the thread strips. Above 85% torque climbs steeply for almost no gain — several traditional chart rows land there by accident, which is a documented cause of broken taps.
What drill should I use for 1/4"-20 UNC at 60% thread?: A #3 drill — 0.2130 in (5.410 mm) — which leaves 57.0% thread. That is the lighter-engagement choice for tough material or a blind hole; the 75% answer is #7.
Does this tap drill calculator handle metric threads?: Yes — 68 metric threads and 82 inch threads are in the same picker, and every result is given in both inches and millimetres. Metric threads default to the metric drill index; the drill-index control will search the inch index instead if that is what is in the cabinet.
Is the metric rule 'drill size = diameter minus pitch' accurate?: It is exactly the 77% thread case. Running D − P through the engagement formula returns 77.0% for M3×0.5, M4×0.7, M5×0.8, M6×1.0, M10×1.5 and M16×2 alike, so the rule of thumb is a slightly heavier thread than the 75% a chart implies — close enough for most work and worth knowing when a tap is breaking.