How to Use the Drill Bit Size Converter
Drill bits are labeled in several different ways — fractional inches (1/16, 1/8, etc.), millimeters, or numbered/lettered gauges. US tools typically use fractional inch sizes, while most metric-region machines and drawings use mm, so you often need to switch between the two.
Enter a value and pick its unit to instantly see the equivalent in mm, decimal inches, and the nearest standard inch fraction (to the nearest 1/64). Use it to confirm the right drill size before a bolt hole, or to compare an imported tool's inch spec against a metric drill set.
The chart below lists common tap drill sizes matching metric (M) and UNC (Unified Coarse) screw standards. Actual recommended drill sizes can vary slightly depending on material and thread depth, so for precision work always cross-check with your tooling manufacturer's official tap drill chart.
| Metric (M) | Tap Drill (mm) | UNC Equivalent |
|---|---|---|
| M3 (0.5 pitch) | 2.5mm | #4-40 |
| M4 (0.7 pitch) | 3.3mm | #8-32 |
| M5 (0.8 pitch) | 4.2mm | #10-24 |
| M6 (1.0 pitch) | 5.0mm | 1/4-20 |
| M8 (1.25 pitch) | 6.8mm | 5/16-18 |
| M10 (1.5 pitch) | 8.5mm | 3/8-16 |
| M12 (1.75 pitch) | 10.2mm | 1/2-13 |
Frequently Asked Questions
1/4 inch equals 6.35mm. Multiply any inch fraction by 25.4 to convert it to mm.
For an M6 screw (1.0mm pitch), the standard tap drill size is about 5.0mm. Using a 5.0mm bit is the common choice for a proper thread engagement.
UNC (Unified Coarse) has wider thread spacing and is the general-purpose standard, while UNF (Unified Fine) has tighter spacing for precision applications. The same diameter can require a different tap drill size depending on which one you use.