Metric drill · 8 mm
8 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes
https://shopmath.org/drills/metric/8mm · data verified August 1, 2026
A 8 mm drill bit is 0.3150 in (8.000 mm) in diameter. The nearest fractional size is 5/16" at 0.3125 in. Diameters are nominal.
- Series
- Metric
- Designation
- 8 mm
- Inch
- 0.3150
- mm
- 8.000
- Standard
- ANSI/ASME B94.11M
Metric Drill · 8 mm · nominal diameter
0.3150 in
- Millimetre
- 8.000mm
Nominal diameter. A worn or run-out drill cuts oversize, which leaves less thread in a tapped hole than the table says.
8 mm Drill Bit Size in Inches and Millimetres
The nominal diameter of a 8 mm 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 | 0.3150 in |
| Diameter, millimetre | 8.000 mm |
| Cross-sectional area | 0.07791 in² |
1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.
8 mm Drill Bit Equivalents in Every Other Drill Series
The closest size to the 8 mm 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 | −0.0870 |
| O | Letter | 0.3160 | 8.026 | +0.0010 |
| 5/16" | Fractional | 0.3125 | 7.938 | −0.0025 |
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 8 mm 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 8 mm 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.
- 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 8 mm Drill Bit Reference
Citation
ShopMath. "8 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/metric/8mm
BibTeX
@misc{shopmath-drills-metric-8mm,
title = {8 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/drills/metric/8mm}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/drills/metric/8mm
Common Questions About the 8 mm Drill Bit
What size is a 8 mm drill bit?
- A 8 mm drill bit is 0.3150 in in diameter, which is 8.000 mm.
What fraction is a 8 mm drill bit closest to?
- A 8 mm drill bit (0.3150 in) sits closest to the 5/16" fractional drill at 0.3125 in, a difference of 0.0025 in.
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