Tools reference
Drill bit size chart: Metric, Fractional, Number, and Letter
Actual diameters for common Metric, Fractional, Number, and Letter drill labels, plus a practical way to compare nearby sizes.
Reviewed against GaugeFox data and the sources below · Updated August 29, 2026
Four labels, one physical diameter
Number and Letter drills are standard catalog designations, not a count and not an alphabet code for a separate kind of cutting tool. The actual inch or millimeter diameter is what lets you compare them with Metric and Fractional drills.
| Series | Label | Inches | Millimeters |
|---|---|---|---|
| Number | #39 | 0.0995 in | 2.527 mm |
| Fractional | 1/8 in | 0.1250 in | 3.175 mm |
| Number | #30 | 0.1285 in | 3.264 mm |
| Number | #19 | 0.1660 in | 4.216 mm |
| Fractional | 3/16 in | 0.1875 in | 4.763 mm |
| Number | #8 | 0.1990 in | 5.055 mm |
| Number | #7 | 0.2010 in | 5.105 mm |
| Fractional | 1/4 in | 0.2500 in | 6.350 mm |
| Letter | F | 0.2570 in | 6.528 mm |
| Letter | H | 0.2660 in | 6.756 mm |
| Fractional | 5/16 in | 0.3125 in | 7.938 mm |
| Letter | R | 0.3390 in | 8.611 mm |
| Fractional | 3/8 in | 0.3750 in | 9.525 mm |
Yes, 6.8 mm is a labeled drill size
Metric drill catalogs include decimal sizes. In the range used by common tap drills, 0.1 mm steps such as 5.0, 5.1, 6.7, 6.8, and 6.9 mm are normal labels. A 6.8 mm drill is therefore not a hidden Letter or Number size.
The nearby Letter H drill is 0.2660 inch, or about 6.756 mm. It is close, but its label and diameter remain distinct. GaugeFox shows both instead of relabeling one as the other.
Identify a loose drill without trusting the worn marking
- Measure across the straight cutting body, not the tapered point or flutes at an angle.
- Keep your view square to the two reference edges to reduce parallax.
- Compare the closest catalog labels in both systems.
- Confirm with a drill index, micrometer, or caliper before precision work.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.