What thread engagement does a 1/2 drill give in 9/16"-18 UNF?

86.6% thread engagement with a cut tap. A 1/2 drill is 0.5000 in (12.700 mm) and the basic major diameter of 9/16"-18 UNF is 0.5625 in (14.287 mm), so the tap has to cut 86.6% of a full thread.

How do you calculate percent thread engagement?

Divide the depth of thread the tap has to cut by the full thread depth: %thread = (D − hole Ø) ÷ (1.29904 × P) × 100, 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 − hole Ø) × n ÷ 1.29904 × 100.

Is 75% thread engagement necessary?

Rarely. 75% is a convention carried by the published charts, not a requirement of any standard. The tapped thread is almost never the weak part of a joint — the fastener is — so dropping to 60–65% costs a few percent of thread strength while cutting tapping torque enough to make a real difference to broken taps in tough material. Tap manufacturers publish charts down to 55%.

Can I use this calculator on a hole that is already drilled?

Yes — that is the measured-hole mode. Enter the diameter a bore gauge or pin gauge reports, in inches or millimetres, and the calculator returns the engagement a cut tap will produce in it. Using the measured size rather than the drill's nominal size is the more accurate of the two, because a drill rarely makes a hole exactly its own size.

Calculator · threads

Thread Engagement Calculator — What Your Hole Actually Leaves

https://shopmath.org/calculators/thread-engagement · data verified August 1, 2026

A 1/2 drill in 9/16"-18 UNF leaves 86.6% thread engagement with a cut tap — that is the drill the traditional chart lists, and it is 21.6 points heavier than the drill nearest a computed 75% target. Pick any of the 150 threads and any standard drill, or type in a hole you have already measured, and this page reports what the tap will actually have to cut.

Threads: 150
Drills: 397
Input: Drill or measured Ø
Tap type: Cut
Verified: August 1, 2026

Thread Engagement Calculator — Drill or Measured Hole

Pick the thread, then either the drill you are about to use or the diameter you measured in a hole that is already there. The engagement comes straight out of the thread's own geometry.

Thread engagement calculator

I have

Thread to be cut

The thread the tap will cut, not the hole.

Drill in hand

397 standard drills — number, letter, fractional and metric.

Thread engagement · 9/16"-18 UNF · 1/2 hole · cut tap

Very heavy

86.6%

of a full thread

Hole Ø: 0.5000in
Hole Ø: 12.700mm
Major Ø: 0.5625in
vs 75% default: +11.6

Above 85% the hole is approaching the thread's minor diameter, torque climbs steeply and the gain in strength is negligible — the fastener fails first either way. Several traditional chart rows land here by accident, which is the documented cause of broken taps in 1/4-20. Full 9/16"-18 UNF spec page →

The formula, with your numbers in it

NBS H28 / ASME B1.1 profile geometry

```
%thread  =  (D − hole Ø) × n ÷ 1.29904 × 100
```
```
%thread  =  (0.5625 − 0.5000) × 18 ÷ 1.29904 × 100
```
= 86.6% thread
D is the basic major diameter of 9/16"-18 UNF, n its threads per inch. 1.29904 = 2 × 0.64952, twice the basic thread height per side on the 60° V profile.

What each engagement asks for on 9/16"-18 UNF

Hole diameter and nearest standard drill by thread engagement — 9/16"-18 UNF, cut tap
Property	Target hole Ø		Property
Thread %	in	mm	Nearest drill
50%	0.5264	13.371	17/32
55%	0.5228	13.279	33/64
60%	0.5192	13.188	33/64
65%	0.5156	13.096	33/64
70%	0.5120	13.005	33/64
75%	0.5084	12.913	33/64
80%	0.5048	12.822	1/2
85%	0.5012	12.730	1/2

Target hole Ø is the theoretical diameter for that engagement; the drill column is the nearest real size in the thread's own drill index. The 75% row is the assumption behind almost every published chart.

A Worked Thread Engagement Example — 9/16"-18 UNF With a 1/2 Drill

One of the three threads where the traditional published chart and the computed target genuinely disagree — so the gap below is real, and both numbers come out of the database.

9/16"-18 UNF has a basic major diameter of 0.5625 in (14.287 mm) and 18 threads per inch. The traditional wall chart lists a 1/2 drill for it, which measures 0.5000 in (12.700 mm).

Putting those into the engagement formula: (0.5625 − 0.5000) × 18 ÷ 1.29904 × 100 = 86.6% thread.

Work the other direction — ask what drill a 75% thread actually wants — and the answer is 33/64 (0.5156 in (13.097 mm)), leaving 65.0%. The two drills are 0.0156 in apart and the thread they leave differs by 21.6 points. Neither is wrong; they are answers to different questions, and until you compute the engagement you cannot tell which one you are holding.

The general reason charts and computed targets drift apart is historical. The published tap drill tables descend from an era when twist drills ran noticeably oversize, so a nominally 0.5000 in drill made a larger hole than 0.5000 in and the finished thread landed lighter than the geometry alone predicts. Modern drills cut much closer to nominal, so the same chart row now produces a heavier thread and more torque on the tap than it used to.

How to Read a Thread Engagement Percentage

What each band means, drawn from practitioner evidence rather than from a standard — no standard specifies a thread engagement for tapping.

Thread engagement bands and what each one means in practice
Engagement	Reads as	What it means
50–54%	Light	Below 55% the tap cuts very little material and the risk of breaking it is at its lowest. Thread strength falls off, but the fastener still fails before the thread strips in most steel and aluminium joints — a machinist quoted in our audience research moved his shop to a 50% chart and reports not having broken a tap in years.
55–69%	Reduced	55–70% is the tough-material and blind-hole range. Tapping torque drops sharply against 75% while thread strength gives up only a few percent, which is why tap manufacturers publish charts down to 55%.
70–77%	Standard	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.
78–85%	Heavy	78–85% asks the tap to remove noticeably more material for very little extra holding strength. It is worth it in soft or thin material where there is not much thread to begin with; in tough alloy it is where taps start breaking.
86–100%	Extreme	Above 85% the hole is approaching the thread's minor diameter, torque climbs steeply and the gain in strength is negligible — the fastener fails first either way. Several traditional chart rows land here by accident, which is the documented cause of broken taps in 1/4-20.

These bands describe what the number means. They are not a recommendation for a specific job — material, hole depth, tap type, coolant and whether the hole is blind all move the right answer, and none of them are things a web page knows.

Sources and Method for the Thread Engagement 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

Thread engagement from a hole diameter — inch threads

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

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

Thread engagement from a hole diameter — metric threads

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

  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. This is the tap drill formula solved for %thread instead of for the hole.

Assumptions

Figures are for a CUT tap. A form (roll) tap displaces material rather than removing it and needs a larger hole for the same engagement; that dataset is not in this database and this calculator does not guess at it.
The engagement is computed from the BASIC major diameter, the theoretical value before any tolerance class is applied. A thread at the bottom of its class tolerance sits slightly under it.
A drill's nominal diameter is not the hole it makes. Run-out, wear and material spring-back all move the finished hole, generally toward a heavier thread than the nominal figure says — which is what the measured-hole mode is for.
The interpretation bands come from practitioner evidence recorded in our audience research (published tap-manufacturer charts running to 55%, and reported shop practice at 50% in hard material). No standard specifies a thread engagement for tapping.
Nothing here accounts for thread engagement LENGTH — how many threads deep the hole is — which is a separate and often more important question for joint strength.
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 Thread Engagement Calculator

Citation

ShopMath. "Thread Engagement Calculator — What Your Hole Actually Leaves." ShopMath, verified August 1, 2026, https://shopmath.org/calculators/thread-engagement

BibTeX

@misc{shopmath-calculators-thread-engagement,
  title        = {Thread Engagement Calculator — What Your Hole Actually Leaves},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/calculators/thread-engagement}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/calculators/thread-engagement

Common Questions About Thread Engagement

What thread engagement does a 1/2 drill give in 9/16"-18 UNF?: 86.6% thread engagement with a cut tap. A 1/2 drill is 0.5000 in (12.700 mm) and the basic major diameter of 9/16"-18 UNF is 0.5625 in (14.287 mm), so the tap has to cut 86.6% of a full thread.
How do you calculate percent thread engagement?: Divide the depth of thread the tap has to cut by the full thread depth: %thread = (D − hole Ø) ÷ (1.29904 × P) × 100, 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 − hole Ø) × n ÷ 1.29904 × 100.
Is 75% thread engagement necessary?: Rarely. 75% is a convention carried by the published charts, not a requirement of any standard. The tapped thread is almost never the weak part of a joint — the fastener is — so dropping to 60–65% costs a few percent of thread strength while cutting tapping torque enough to make a real difference to broken taps in tough material. Tap manufacturers publish charts down to 55%.
Why does my tap chart give a different thread percentage?: Most published tap drill charts descend from tables built when twist drills ran noticeably oversize, so the nominal drill in the chart produced a larger hole than its nominal diameter and the finished thread came out lighter than the geometry alone predicts. Modern drills cut much closer to nominal, so the same chart row now leaves a heavier thread and more tapping torque. On three inch threads the chart and the computed 75% target name different drills outright; this calculator reports what the drill you name actually leaves, so you can see which one you are holding.
Can I use this calculator on a hole that is already drilled?: Yes — that is the measured-hole mode. Enter the diameter a bore gauge or pin gauge reports, in inches or millimetres, and the calculator returns the engagement a cut tap will produce in it. Using the measured size rather than the drill's nominal size is the more accurate of the two, because a drill rarely makes a hole exactly its own size.