Fractional drill · 25/64
25/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes
https://shopmath.org/drills/fractional/25-64 · data verified August 1, 2026
A 25/64" drill bit is 0.3906 in (9.922 mm) in diameter. It is the tap drill for 7/16"-20 UNF at 75% thread engagement with a cut tap, where it leaves 72.2% thread. Diameters are nominal.
- Series
- Fractional
- Designation
- 25/64
- Inch
- 0.3906
- mm
- 9.922
- Standard
- ANSI/ASME B94.11M
Fractional Drill · 25/64 · nominal diameter
0.3906 in
- Millimetre
- 9.922mm
- Fraction
- 25/64in
- Taps
- 7/16"-20 UNF
Nominal diameter. As a tap drill it leaves 72.2% thread in 7/16"-20 UNF with a cut tap.
25/64" Drill Bit Size in Inches and Millimetres
The nominal diameter of a 25/64" drill bit. Drill sizes are nominal — a worn drill, run-out or a soft material that closes in behind the cut all move the finished hole.
| Property | Value |
|---|---|
| Diameter, decimal inch | 0.3906 in |
| Diameter, millimetre | 9.922 mm |
| Fraction | 25/64 in |
| Cross-sectional area | 0.11984 in² |
1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.
Threads a 25/64" Drill Bit Taps
Every thread in this reference whose tap drill is the 25/64" drill bit, and the thread engagement it leaves. The engagement asked for and the engagement the drill actually produces are different numbers — that gap is why both columns are here.
| Thread | Series | Thread % asked for | Thread % this drill leaves |
|---|---|---|---|
| 7/16"-20 UNF | Unified Fine (UNF) | 75%default | 72.2 |
| 7/16"-20 UNF | Unified Fine (UNF) | 70% | 72.2 |
| 7/16"-14 UNC | Unified Coarse (UNC) | 50% | 50.5 |
Values are for cut taps. Rows marked default are the shop-default engagement for that thread.
25/64" Drill Bit Equivalents in Every Other Drill Series
The closest size to the 25/64" drill bit in each of the other three indexes, with the difference in inches. None of them is the same drill — check the difference before substituting one into a tapped hole.
| Property | Property | Diameter | Property | |
|---|---|---|---|---|
| Drill | Series | in | mm | Difference, in |
| #1 | Number | 0.2280 | 5.791 | −0.1626 |
| W | Letter | 0.3860 | 9.804 | −0.0046 |
| 9.9 mm | Metric | 0.3898 | 9.900 | −0.0009 |
A positive difference means the equivalent drill is larger, which leaves less thread in a tapped hole. Sizes without a link are stored for the nearest-drill calculation but do not carry their own page.
Drill Sizes Either Side of the 25/64" Drill Bit
Ordered by diameter across all four indexes, so the next size up may be a letter, a fraction or a metric drill rather than the next one in this series.
Sources and Method for the 25/64" Drill Bit Size
- Drill diametercomputed
Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.
Verified against Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed
Computed from the standard's formulas, then checked against published tables.
- Tap drill usagecomputed+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
Fractional drill diameter
Ø = 25/64 in = 0.3906 in
Fractional drills are exact fractions of an inch in 1/64 in steps; the decimal is the fraction evaluated, not a measurement.
- Assumptions
- Diameters are nominal. Drill manufacturing tolerance, wear and run-out all cut oversize, and every one of those moves a tapped hole toward less thread than the table says.
- Tap drill usage assumes cut taps. 7/16"-20 UNF at 75% requested engagement is what this drill is normally bought for.
- Dataset 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 to Cite This 25/64" Drill Bit Reference
Citation
ShopMath. "25/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/25-64
BibTeX
@misc{shopmath-drills-fractional-25-64,
title = {25/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/drills/fractional/25-64}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/drills/fractional/25-64
Common Questions About the 25/64" Drill Bit
What size is a 25/64" drill bit?
- A 25/64" drill bit is 0.3906 in in diameter, which is 9.922 mm. It is the 25/64 in fractional size.
What thread does a 25/64" drill bit tap?
- A 25/64" drill bit is the tap drill for 7/16"-20 UNF at 75% requested thread engagement, where it leaves 72.2% thread with a cut tap.
What is the metric equivalent of a 25/64" drill bit?
- The closest metric drill to a 25/64" drill bit (9.922 mm) is 9.9 mm, which is 9.900 mm — 0.0009 in smaller.
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