ENGINEERING TOOL
#10-32 UNF Tap Drill Size: #21 — 0.1590 in (4.039 mm)
Answer for this thread
Standard tap drill (75% thread)
#210.1590 in(4.039 mm)
(0.1900 − 0.1590) ÷ (1.299 × 0.0313) × 100 = 76.4%
Rule value d − P
0.1587in(4.032 mm)
major diameter − pitch = 0.1900 − 0.0313 = 0.1587 in · 77.0% Percent of thread at the rule value
Nearest drill in this set
0.1590in
#21 · 4.039 mm
(0.1900 − 0.1590) ÷ (1.299 × 0.0313) × 100 = 76.4% · +0.0003 in
The standard drill in the first column is the closest entry to a 75% thread. The rule value d − P is not a drill size itself; the nearest entry is a conversion only.
Basic minor diameter 0.1562 in (3.967 mm) · Percent of thread ceiling 83.3%
Drill the standard tap drill (75% thread). The rule value d − P is the formula baseline (77.0% of thread); the nearest drill is only a conversion for sets that lack the standard size.
Below: the three entries of the inch drill set closest to this rule value, each with its own percent of thread.
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Entries of the inch drill set nearest to this rule value
Ranked by distance from the rule value, larger diameter first when the two are equally close, because a larger hole means less tap torque and less risk of breaking the tap.
| Drill entry (in) | Percent of thread | Difference from rule value (in) | Status |
|---|---|---|---|
| #21 0.1590 in | 76.4% | +0.0003 in | within the thread form |
| #22 0.1570 in | 81.3% | -0.0017 in | within the thread form |
| #20 0.1610 in | 71.4% | +0.0023 in | within the thread form |
Percent of thread here is that of the entry, not that of the rule value. A flagged entry is smaller than the basic minor diameter: the tap would remove metal that the thread profile needs.
Basis & references
How to read this chart
Percent of thread = actual thread height ÷ 0.75H, where H is the height of the fundamental triangle of a 60° thread form. A Unified thread's basic height is 0.625H, so the ceiling is 83.3%; the basic minor diameter is d − 1.082532·P. (Definition: FED-STD-H28/2B §20.2.3.)
The nearest entry is taken from the 169 inch drill sizes (number, letter and fractional drills, 0.0135–0.9844 in, i.e. 0.343–25.004 mm; steps 0.0000–0.0157 in); 8 rows take a nearest entry smaller than the basic minor diameter and are flagged in the table rather than offered as usable drills.
The inch drill set stops at 0.9844 in: 4 threads have no drill that can make their tap drill hole (for 1 1/8, the nearest entry 0.9844 in is 0.0573 in below the rule value 1.0417 in and would give 129.9% of thread, far past the ceiling). These rows still print the rule value, marked.
Why the drilled hole differs from the chart
- A drill typically cuts a hole larger than its stamped diameter, so bore tolerance should allow for the probable average enlargement; the largest sizes are usually reamed instead.
- Use a bore near the minimum for short engagement; open the bore up for long engagement to lower the risk of breaking the tap.
- In soft materials (especially with fine pitches) metal is squeezed toward the tap root, so the finished minor diameter can end up smaller than the drilled hole.
From FED-STD-H28/2B §20.2.2, §30.1 and §40.3.
Nature of the values
The figures are engineering guidance, not mandatory values, given for a cutting tap in a through hole; adjust for material, tap type, length of engagement and through vs blind holes. Of the appendix carrying figures of this kind, FED-STD-H28/2B says it is not a mandatory part of the standard and is intended for guidance only (§10).
Every figure in the chart is computed from the thread geometry by a single calculation module — one consistent basis, reproducible value by value.
The inch thread list (diameter – threads per inch – series) is taken from the appendix of FED-STD-H28/2B, a US Government publication; the tap drill, millimetre pitch, percent of thread and basic minor diameter are all computed from the thread geometry. The same standard notes that this appendix is not a mandatory part of the standard and is intended for guidance only (§10).
Cite this chart
YAXIIO. #10-32 UNF Tap Drill Size: #21 — 0.1590 in (4.039 mm). https://www.yaxiio.com/en/tools/tap-drill-chart/unified-unf/10-32 (accessed 2026-10-28).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Other UNF threads
Related References
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Every drill size this site carries: number, letter, fractional and metric series, with the diameter in both systems.
Bolt Torque Chart
Tightening torque by property class for the same thread sizes, with the friction basis stated.
Bolt Size Chart
Head dimensions across the preferred and non-preferred metric sizes, both tolerance grades side by side.
Threaded Rod Weight Chart
Unit weight by diameter on the same disclosed basis, for costing a threaded order.
Stud Bolt Torque Chart
Tightening torque for stud bolts across three nut factors and six property classes, with the target preload stated.
Wrench Size & Socket Chart
Which wrench or socket fits each hex bolt: width across flats for the bolt, the hex nut and the flange nut side by side, with the standard named row by row.
Hex Key Size Chart
Which hex key fits a socket head cap screw: the socket width across flats plus the key long and short arm lengths, over the same metric sizes as this page.
FAQ
What tap drill does #10-32 UNF take?
The rule value is 0.1587 in (4.032 mm): the nominal diameter minus the pitch, 0.1900 − 0.0313 in. That gives 77.0% of thread, the same percent as every other size in this chart. The nearest entry in the inch drill set is #21 at 0.1590 in (4.039 mm), which gives 76.4% and sits +0.0003 in from the rule value.
Is the nearest drill the right one to use?
It is the right conversion, not a second answer, and it is not what a printed inch tap drill table would give you: those tables often print the theoretical figure d − P itself, whereas this column takes the closest entry of the drill set, larger first when the two are equally close. Use the rule value as the target and the nearest entry as the drill in your hand, and re-measure if the thread is critical.
What are the limits of this figure?
It is a reference value for a cutting tap in a through hole, not a design value: a drill cuts oversize, bore limits loosen as the length of engagement grows, and a soft material can be squeezed so the finished minor diameter is smaller than the hole drilled. The percent of thread ceiling is 83.3%, at the basic minor diameter 0.1562 in for this thread; below that the tap cuts into the form. Adjust for the material and the tap you are using, and drill a test piece first.
Disclaimer: the figures on this page are computed reference values and text rules, not standard values, design values or an acceptance basis. Tap drill size depends on the material, the tap and the length of engagement; for work that carries load, follow the thread standard for the joint and drill a test piece before running the job.