Fractional drill · 53/64
53/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes
https://shopmath.org/drills/fractional/53-64 · data verified August 1, 2026
A 53/64" drill bit is 0.8281 in (21.034 mm) in diameter. It is the tap drill for 7/8"-20 UNEF at 75% thread engagement with a cut tap, where it leaves 72.2% thread. Diameters are nominal.
- Series
- Fractional
- Designation
- 53/64
- Inch
- 0.8281
- mm
- 21.034
- Standard
- ANSI/ASME B94.11M
Fractional Drill · 53/64 · nominal diameter
0.8281 in
- Millimetre
- 21.034mm
- Fraction
- 53/64in
- Taps
- 7/8"-20 UNEF
Nominal diameter. As a tap drill it leaves 72.2% thread in 7/8"-20 UNEF with a cut tap.
53/64" Drill Bit Size in Inches and Millimetres
The nominal diameter of a 53/64" 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 | 0.8281 in |
| Diameter, millimetre | 21.034 mm |
| Fraction | 53/64 in |
| Cross-sectional area | 0.53862 in² |
1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.
Threads a 53/64" Drill Bit Taps
Every thread in this reference whose tap drill is the 53/64" 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 |
|---|---|---|---|
| 7/8"-20 UNEF | Unified Extra Fine (UNEF) | 75%default | 72.2 |
| 7/8"-20 UNEF | Unified Extra Fine (UNEF) | 65% | 72.2 |
| 7/8"-20 UNEF | Unified Extra Fine (UNEF) | 70% | 72.2 |
| 7/8"-20 UNEF | Unified Extra Fine (UNEF) | 80% | 72.2 |
| 7/8"-14 UNF | Unified Fine (UNF) | 50% | 50.5 |
Values are for cut taps. Rows marked default are the shop-default engagement for that thread.
53/64" Drill Bit Equivalents in Every Other Drill Series
The closest size to the 53/64" 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 | −0.6001 |
| Z | Letter | 0.4130 | 10.490 | −0.4151 |
| 21 mm | Metric | 0.8268 | 21.000 | −0.0014 |
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 53/64" 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 53/64" 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 = 53/64 in = 0.8281 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. 7/8"-20 UNEF 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 53/64" Drill Bit Reference
Citation
ShopMath. "53/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/53-64
BibTeX
@misc{shopmath-drills-fractional-53-64,
title = {53/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/drills/fractional/53-64}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/drills/fractional/53-64
Common Questions About the 53/64" Drill Bit
What size is a 53/64" drill bit?
- A 53/64" drill bit is 0.8281 in in diameter, which is 21.034 mm. It is the 53/64 in fractional size.
What thread does a 53/64" drill bit tap?
- A 53/64" drill bit is the tap drill for 7/8"-20 UNEF at 75% requested thread engagement, where it leaves 72.2% thread with a cut tap.
What is the metric equivalent of a 53/64" drill bit?
- The closest metric drill to a 53/64" drill bit (21.034 mm) is 21 mm, which is 21.000 mm — 0.0014 in smaller.
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