A 1/8 drill bit measures 0.1250 inches, which equals exactly 3.175 mm in diameter.
For the full breakdown, see our best 1 1/8 Inch Drill Bit guide.
Whether you are picking out a bit for a pilot hole or replacing a worn one from a set, knowing the precise size matters. A 1/8 drill bit is a standard fractional-inch size in US retail and DIY work. Its diameter is the key spec, and it converts cleanly to metric units for anyone working with metric fasteners or a digital caliper set to millimeters.
The Exact Diameter Of A 1/8 Drill Bit
A 1/8 drill bit has a nominal diameter of 0.1250 inches. This is the measurement across the widest part of the cutting flutes, which determines the size of the hole it drills.
The metric equivalent is 3.175 mm. To calculate this yourself, multiply the inch measurement by 25.4, the number of millimeters per inch: 0.125 × 25.4 = 3.175 mm.
Many conversion charts round this figure to 3.18 mm for simplicity. The value of 3.175 mm is the exact conversion, while 3.18 mm is a common rounding used on product listings and quick-reference charts. Either way, the bit is the same physical size—the difference is only in how the decimal is displayed.
What Size Is A 1/8 Drill Bit In Context
In US usage, fractional-inch drill sizes remain the standard for general maintenance, woodworking, and DIY projects. McMaster-Carr, a major industrial supplier, lists 1/8-inch bits as a standard fractional size across its product catalog. In that catalog, a 1/8 drill bit is its own distinct size listing, not a model number or a generic descriptive term. When you order a 1/8 bit, the supplier ships a cutting tool with a measured cutting diameter of 0.1250 inches.
Here is what that size looks like against a few common nearby bits:
| Bit Size | Inches | Millimeters |
|---|---|---|
| 7/64 inch | 0.1094 in | 2.778 mm |
| 1/8 inch | 0.1250 in | 3.175 mm |
| #8 drill size | 0.1990 in | 5.055 mm |
| 5/32 inch | 0.1563 in | 3.969 mm |
| 1-1/8 inch | 1.1250 in | 28.575 mm |
The distinction between 1/8 inch and the #8 wire-gauge drill size is a common mix-up. A #8 drill bit measures 0.1990 inches, which is notably wider than 1/8 inch and not interchangeable with it. If a project calls for a #8 bit and you grab a 1/8 bit instead, your hole will come out undersized.
How A 1/8 Drill Bit Is Used
A 1/8-inch bit is a go-to for pilot holes and small-diameter drilling in wood, plastic, and soft metals. The size is also common for installing #6 or #8 screws, where a pilot hole slightly narrower than the screw body prevents splitting. Drill-bit sizing determines the nominal hole diameter, but several variables can affect the finished product: material hardness, bit wear, flute design, and spindle speed all play a part. A chart provides the intended size; the actual hole can measure a few thousandths of an inch larger or smaller depending on these conditions.
It also pays to keep the shank separate from the cutting diameter. A 1/8 shank is common on router bits and step drills, but the “1/8” label on a bit’s package always points to the cutting diameter unless the listing explicitly says otherwise. McMaster-Carr, for instance, lists 1/8-inch drill bits and 1/8-inch shank options as separate catalog filters.
When you are set on the right size, the practical next step is finding one built for your material and budget.
FAQs
Is a 1/8 drill bit the same as a #8 drill bit?
No. A 1/8 drill bit measures 0.1250 inches, while a #8 drill bit measures 0.1990 inches. The #8 designation refers to a wire-gauge number, not a fractional measurement. They are entirely different sizes, and mixing them up will leave you with a hole that is either too small or too large for your fastener.
What screw size needs a 1/8 pilot hole?
A 1/8-inch bit is commonly used for pilot holes before driving #6 or #8 wood screws, though the exact recommendation depends on your wood’s density. For hardwood, a slightly larger pilot hole reduces splitting risk. For softwood, a 1/8 bit often works well. The goal is a hole slightly smaller than the screw’s core diameter so the threads still bite the wood.
Why does my chart say 3.18 mm instead of 3.175 mm?
Both figures describe the same bit. The exact metric conversion of 0.1250 inches is 3.175 mm. Many manufacturers and reference charts round that to 3.18 mm for simplicity, and drill bits this small can vary by a couple of thousandths depending on the tolerance. The rounding is a display choice, not a different bit size.
References & Sources
- McMaster-Carr. “Drill Bits with 1/8″ Drill Bit Size.” Confirms 1/8 as a standard fractional drill bit size listing.
