Fractional drill · 1/4
1/4" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes
https://shopmath.org/drills/fractional/1-4 · data verified August 1, 2026
A 1/4" drill bit is 0.2500 in (6.350 mm) in diameter. Diameters are nominal.
- Series
- Fractional
- Designation
- 1/4
- Inch
- 0.2500
- mm
- 6.350
- Standard
- ANSI/ASME B94.11M
Fractional Drill · 1/4 · nominal diameter
0.2500 in
- Millimetre
- 6.350mm
- Fraction
- 1/4in
Nominal diameter. A worn or run-out drill cuts oversize, which leaves less thread in a tapped hole than the table says.
1/4" Drill Bit Size in Inches and Millimetres
The nominal diameter of a 1/4" 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 | 0.2500 in |
| Diameter, millimetre | 6.350 mm |
| Fraction | 1/4 in |
| Cross-sectional area | 0.04909 in² |
1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.
1/4" Drill Bit Equivalents in Every Other Drill Series
The closest size to the 1/4" 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 | −0.0220 |
| E | Letter | 0.2500 | 6.350 | +0.0000 |
| 6.3 mm | Metric | 0.2480 | 6.300 | −0.0020 |
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 1/4" 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 1/4" 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.
- Formula
Fractional drill diameter
Ø = 1/4 in = 0.2500 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.
- 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 1/4" Drill Bit Reference
Citation
ShopMath. "1/4" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/1-4
BibTeX
@misc{shopmath-drills-fractional-1-4,
title = {1/4" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/drills/fractional/1-4}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/drills/fractional/1-4
Common Questions About the 1/4" Drill Bit
What size is a 1/4" drill bit?
- A 1/4" drill bit is 0.2500 in in diameter, which is 6.350 mm. It is the 1/4 in fractional size.
What is the metric equivalent of a 1/4" drill bit?
- The closest metric drill to a 1/4" drill bit (6.350 mm) is 6.3 mm, which is 6.300 mm — 0.0020 in smaller.
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