What Size Is a 1/8 Drill Bit? | Exact Diameter and Uses

A 1/8 drill bit measures exactly 0.125 inches in diameter, which converts to precisely 3.175 millimeters.

Whether you’re setting up a pilot hole or working from metric drawings, the exact size of a 1/8 drill bit matters more than most DIY guides admit. The number 1/8 refers to a fractional-inch measurement, not a number, letter, or wire gauge size, and getting that straight prevents a surprising amount of fit problems. Here is what the spec means, how it converts to metric, and when it’s actually the right bit for your project.

Exact Dimensions of a 1/8 Drill Bit

The decimal equivalent tells the real story: 1/8 inch equals 0.125 inch. Multiply by 25.4 and you get precisely 3.175 mm. That’s the exact metric conversion, though most metric bits stop at 3.2 mm—a close substitute that isn’t identical.

Beyond the cutting diameter, standard 1/8 bits follow common physical proportions. McMaster-Carr lists typical dimensions for a jobber-length 1/8 bit: a flute length of 1-5/8 inches and an overall length of 2-3/4 inches. The shank size usually matches the cutting diameter on standard bits, but it doesn’t have to, and confusing the two is a common DIY mistake.

A 1/8 bit belongs to the fractional family in standard drill-size charts. Those charts separate bits into four families—fractional, number, letter, and metric—and knowing which family you’re working with removes a whole class of errors. For the 1/8 size, the applicable chart is always the fractional one.

Is 3.2 mm the Same as 1/8 Inch?

No. Three-point-two millimeters is approximately 1/8 inch, but it’s not exact. The true conversion produces 3.175 mm, which means a 3.2 mm bit runs about 0.025 mm oversized. For many woodworking and general drilling tasks, that difference is irrelevant. For machining tolerances or tight clearance fits, it absolutely matters.

When a project calls for a specific metric hole, treat 3.2 mm as a practical substitute, not a precise match. When your drawing specifies 1/8 inch, you want the bit that cuts 0.125 inch. Checking the decimal equivalent against your fit requirement is the reliable way to decide, not assuming the nominal fraction tells the whole story.

What Is a 1/8 Drill Bit Used For?

The 1/8 bit’s most common job is pilot holes for screws and fasteners. Because pilot-hole sizing depends on the screw’s thread design and how much thread engagement you want, a tap-drill chart for your specific fastener is the right reference—a one-size-fits-all approach will strip threads or snap screws. Charts often list sizes for 75% thread depth, and the recommended fractional bit shifts from fastener to fastener.

In metalworking and machining, 1/8 serves as a starting hole for larger bits and as a final size for small dowel and pin fits. The bit also appears constantly in crafts, model building, and electronics enclosures where plastics and soft metals need a clean, predictable hole. Matched against a metric drawing, it’s safer to confirm the specified metric dimension rather than assume 1/8 was intended.

Before you buy, know which screw and material you’re working with, then consult the tap-drill chart for that exact fastener rather than leaping to one fractional size.

For a tested roundup of quality 1/8-inch bits that hold up in real use, our best 1/8 inch drill bit recommendations cover the top performers. That list compares brands across price, durability, and cutting speed so you can skip the trial and error.

Common Mistakes With 1/8 Drill Bits

Three errors account for nearly all 1/8-bit frustration. First, treating 3.2 mm as the exact equivalent of 1/8 inch when precision matters. Second, confusing cutting diameter with shank size or assuming the bit matches a wire gauge—it doesn’t, because 1/8 is a fractional designation, not a numbered size. Third, assuming one 1/8 bit fits every pilot-hole application when the correct size depends on the screw, its threads, and the material you’re drilling into. The fix for all three is the same: check the decimal equivalent, confirm the shank fits your chuck, and reference the tap-drill chart for your specific fastener.