What Is a #30 Drill Bit? Size, Diameter, and Fraction Equivalent

A #30 drill bit measures 0.1285 inches in diameter — 3.264 mm — and its closest fractional match is 1/8 inch, though the #30 is actually the slightly larger of the two.

Reaching for a 1/8-inch bit when a job calls for a #30 is the quiet mistake that loosens a tapped hole or throws off clearance. The #30 belongs to the number drill series, a gauge system machinists developed for sizes that fall between the familiar fractions. Its decimal size sits just above 1/8 inch, and that tiny gap matters more than it looks on paper.

What Size Is a #30 Drill Bit In Decimals And Millimeters?

A #30 drill bit is 0.1285 inches in decimal form, which converts to 3.264 mm in the metric system. Some published charts list the metric figure as 3.2639 mm, a rounding difference of a few ten-thousandths.

That decimal spec is the number to trust. Fractions, wire gauges, and letter sizes all describe diameter in different ways, and the number drill series assigns each size a specific decimal diameter rather than a rounded fraction. Drill bit size charts list the #30 alongside its number-series siblings — not in the fractional column — which tells you it was never meant to be expressed as a simple fraction.

For most shop work, quoting “0.1285 inch” or “3.264 mm” settles the question instantly. Those are the exact figures a tap chart, clearance table, or engineering drawing will reference when it names a #30 by number.

What Fraction Is A #30 Drill Bit Closest To?

The closest common fractional size to a #30 drill bit is 1/8 inch, which measures 0.1250 inch or 3.175 mm. The #30 is larger by 0.0035 inch — roughly 0.089 mm.

No standard fractional-inch equivalent exists for a #30, which is the whole reason the number series exists. Number drills fill the gaps between fractions, and the #30 lands just above 1/8 inch instead of matching it.

Whether that difference matters depends entirely on the job:

• Precision work: a drawing, tap chart, or clearance spec that names #30 needs the exact bit — the 0.0035-inch gap changes the fit.
• Tapping and fastener engagement: hole size controls thread depth and grip; rounding down to 1/8 inch can bind a tap or leave a fastener loose.
• Rough work: for a pilot hole in softwood or a quick clearance hole, the difference is often invisible in practice.

A few charts label #30 as the 1/8-inch equivalent for convenience, but the same charts list it as slightly larger. Treat 1/8 inch as a nearby reference point, never as an exact swap.

Can You Substitute A 1/8-Inch Bit For A #30?

No — not when a specification calls for a #30 by name. The diameters differ by 0.0035 inch, and in drilling, that gap is real rather than cosmetic.

Substitution causes the most trouble in three situations. Tapping is the first: a tap chart pairs a specific drill size with each thread, and swapping in a smaller bit changes how much material the tap cuts. Clearance holes are the second — a fastener spec assumes a hole diameter, and a smaller hole can seize the shank. The third is any documented procedure, from an aircraft maintenance manual to a machine shop work order, where the named size is part of the record.

For everyday carpentry, a 1/8-inch bit and a #30 will drill nearly identical pilot holes in pine or plywood. The distinction only bites when fit, thread quality, or clearance is specified. If you want the genuine 3.264 mm cut and are stocking up, a roundup of tested 30 drill bit options worth buying saves the hunt for a single odd size.

The Wikipedia entry on drill bit sizes lays out how the number, letter, and fractional series overlap, which explains why a #30 lands just above 1/8 inch instead of on it.

Frequently Asked Questions

Is a #30 drill bit the same as 1/8 inch?

No. A #30 measures 0.1285 inches, while 1/8 inch equals 0.1250 inches. The #30 is larger by 0.0035 inch, or about 0.089 mm. They are close enough to look alike on a shelf but different enough to change a fit when the spec matters.

What is a #30 drill bit used for?

The #30 handles clearance and tapping work where a specific diameter is called out, and it is a common choice for pilot and clearance holes around small fasteners. It is also a frequent pick for tapping certain machine-screw threads, where the chart pairs it with a specific tap size rather than with 1/8 inch.

Why is a #30 drill bit not sold as a fraction?

Number drill sizes exist to fill the gaps between fractional sizes. The #30 falls between standard fractions with no exact match, so the number series — not the fractional series — gives it a precise decimal diameter instead of a rounded label.