Drill Bit 1/8 in mm: Conversion Guide

A 1/8 inch drill bit measures exactly 3.175 millimeters, based on the standard conversion of 25.4 mm per inch.

For the full breakdown, see our best 1 1/8 Drill Bit guide.

Working across metric and imperial drill sets happens on nearly every project. When the plans call for a 1/8-inch hole but your new kit is metric-only, the conversion itself is simple — the exact number is 3.175 mm — but choosing which metric bit actually fits the job takes a little more thought. This guide covers the precise math, the nearest metric substitutes, and where the small differences actually matter.

The Exact Conversion: 1/8 Inch in Millimeters

One inch equals exactly 25.4 millimeters — a fixed relationship defined by international agreement and used in the standards published by NIST. To convert any inch value to millimeters, multiply by 25.4.

For 1/8 inch, the math works out like this:

• 1/8 as a decimal is 0.125 inches
• 0.125 × 25.4 = 3.175 mm
• So a 1/8-inch drill bit is exactly 3.175 mm in diameter

The 3.175 mm figure is precise, not approximate. Some drill charts round it to 3.18 mm or even 3.2 mm for readability, but those are conveniences — the exact conversion is 3.175 mm.

When you need a hole that matches the intended fit — for a dowel, a tap, or a clearance fit — use the exact size rather than a rounded one.

Choosing the Nearest Metric Substitute

Metric drill sets don’t always include a 3.175 mm bit. The natural substitutes sit on either side, and neither is an exact match for 1/8 inch.

Here’s how the closest metric sizes compare:

The 3.0 mm bit is noticeably smaller than 1/8 inch and will bind if you use it where a 1/8-inch hole is required. The 3.2 mm bit is only 0.025 mm larger — a hair’s width — and often works as a practical stand-in for general drilling. But for precision work, the 0.025 mm difference can still matter on a tight fit.

A common assumption is that 3 mm equals 1/8 inch. It doesn’t — 3 mm lands 0.175 mm under the true value, a gap large enough to matter on close-tolerance holes.

When the Difference Matters

For everyday drilling into wood or plastic, the 0.025 mm difference between 3.175 mm and a 3.2 mm bit rarely causes problems. The material gives slightly, and the hole will be close enough for most purposes.

Precision work is another story. When you drill before threading, the tap spec dictates the correct hole size — an undersize hole makes tapping difficult and risks breaking the tap, while an oversize hole leaves threads that are too shallow to hold. Follow the fastener or tap specification rather than simply picking the closest metric bit.

Tolerance also matters for clearance fits where a bolt or rod passes through the hole. A 3.0 mm bit will not clear a 1/8-inch rod; the 3.2 mm bit gives that clearance. For dowel joints or press fits, the exact 3.175 mm value keeps the fit predictable. Drill-bit charts compare diameters only — actual tool compatibility depends on the material, the application, and the intended hole type.

FAQs

Is a 3 mm drill bit the same as 1/8 inch?

No. A 3 mm bit is 0.175 mm smaller than 1/8 inch, which is exactly 3.175 mm. The difference is about 5.5% of the diameter, which is enough to make a 3 mm bit bind if you need a true 1/8-inch hole.

Why do some charts list 1/8 inch as 3.18 mm?

Those charts round 3.175 mm to 3.18 mm for simplicity. The exact conversion is 3.175 mm. Rounding at the third decimal place is harmless for most work, but keep the precise figure in mind if you’re matching a tight fit.

Can a 3.2 mm bit substitute for 1/8 inch?

In most general drilling, yes. The 3.2 mm bit is only 0.025 mm larger than 1/8 inch — a difference of about 0.8%. It works fine for wood, plastic, and most soft materials, but check the fastener or tap spec for precision applications.