1/8-Inch Drill Bit: Size, Metric Equivalent and Uses
A 1/8-inch drill bit is 0.125 inch or exactly 3.175 mm in nominal diameter. Choose the cutting type for the material, not the size alone.
A 1/8-inch drill bit has a nominal cutting diameter of 0.125 inch, or exactly 3.175 mm. The conversion is 25.4 ÷ 8, using the international inch definition. The printed size describes the intended diameter; an actual drilled hole also depends on the bit, tool, material, and technique. NIST inch definition.
Is 3 mm or 3.2 mm the Same?
No. A 3.2 mm bit is the closer nominal size, but it is still 0.025 mm larger than 1/8 inch. A 3 mm bit is 0.175 mm smaller.
| Bit Size | Nominal Millimeters | Difference From 1/8 Inch |
|---|---|---|
| 3 mm | 3.000 | 0.175 mm smaller |
| 1/8 in | 3.175 | Exact conversion |
| 3.2 mm | 3.200 | 0.025 mm larger |
For a loose, noncritical hole, that difference may be acceptable. For a fastener pilot hole, tapped hole, tight-fitting pin, or manufacturer-specified hole, use the required size and tolerance. “Almost the same” is not a compatibility rule.
Choose the Cutting Type for the Material
Diameter alone does not tell you what the bit can cut. DEWALT’s DW1702 1/8-inch brad-point bit is designed for wood, with cutting edges intended to reduce splintering. That product is a wood-bit example, not a universal bit for steel or concrete. DEWALT DW1702.
Check the bit package for the material and drilling mode. A general twist bit, brad-point wood bit, masonry bit, and rotary-tool cutting bit can all carry a small diameter while doing different jobs. Our guide to drill-bit types explains the families.
Also distinguish the cutting diameter from the shank that fits the chuck. An accessory can have a 1/4-inch hex shank and a smaller cutting diameter. Match the tool connection as well as the hole size.
Drilling Without Snapping the Bit
Secure the workpiece, wear eye protection, and follow the drill’s manual. Keep the bit aligned with the intended hole; avoid using it as a lever to enlarge the opening sideways. Start with control rather than forcing the feed. DEWALT’s drill manual discusses securing work and reducing pressure as the bit breaks through to limit stalling. DEWALT drilling instructions.
A backing scrap can be useful for a wood through-hole, but material-specific technique still matters. Do not assume the same speed, lubricant, or hammer setting applies to wood, metal, tile, and masonry.
Is It the Right Pilot Hole for My Screw?
Only if the screw’s instructions specify it for that substrate. Screw diameter, thread form, material thickness, and wood hardness can change pilot requirements. Buy a 1/8-inch bit because the connection calls for a 1/8-inch hole—not because it is a common size in the kit.