Drill Bit Size Converter

Fractional inch, number (1–80), letter (A–Z) and metric drill sizes, all converted through diameter and all four directions at once — plus the nearest bit you actually own in each system, the tap drill for a metric or Unified thread at any thread percentage, and the clearance hole for the same screw.

One diameter, four names, and two hole rules

The four systems, and what each is for

Common metric sizes and the nearest bit in each inch system

Tap drills for Unified threads, at 75% of thread

Tap drills for metric threads

Clearance holes, ISO 273

Four drill scales, and the one question that matters

A drill size is a diameter wearing a label, and there are four labelling systems in use in the same range of diameters. Fractional drills are their fraction: a 1/4 in drill is 0.2500 in, which is 6.35 mm exactly. Number drills run from #1 at 0.2280 in down to #80 at 0.0135 in, and the numbering runs backwards, so #80 is the smallest bit in the set. Letter drills pick up where the numbers stop and run A at 0.234 in to Z at 0.413 in, with E landing exactly on a quarter of an inch. Metric drills are their diameter. The number and letter series are defined by ANSI/ASME B94.11M, which is copyrighted, so the diameters on this page come from two independent published charts that agree at all 106 sizes, with a third confirming the forty-five values it prints.

The question people actually have is not a conversion. It is: I need 3.4 mm and I own an inch set — what is closest, and how far out is it? So the page answers that in all four systems at once, with the signed error on each, because whether the nearest bit is over or under matters completely differently for a tapped hole (under is fine, over is scrap) and for a clearance hole (over is fine, under does not go together).

Now the part that earns the page: tap drills. A tap drill is not looked up, it is computed. The convention every chart uses is hole = major diameter − 1.299 × pitch × percentage of thread, written in inch shops as major diameter minus 0.0130 times the percentage divided by the threads per inch. Those are the same constant. At 75% and 1/4-20 UNC it gives 0.2013 in, and the published tap drill is a #7 at 0.2010 in. Run that over every UNC and UNF thread that could be cross-checked and the rule picks the published drill eleven times out of eleven. On the metric coarse series the same formula at 77% simplifies to the shortcut everyone already knows — 1.299 × 0.77 = 1.00, so the hole is the major diameter less the pitch, and M6 × 1 takes a 5.0 mm drill.

Two percentages, and they never agree. The percentage-of-thread convention counts a full thread depth of 0.6495 of the pitch per flank, the truncated vee of the American National and Unified forms. ISO 261 defines the internal thread’s own basic height as 0.54127 of the pitch per flank, so the minor diameter is the major less 1.0825 pitches. Measured that way, the “77%” hole from the shortcut is cutting 92% of the basic form. Neither number is wrong; they count different depths, and this page gives both and names the convention for each, because a tap drill argument is nearly always two people using different definitions of the same word.

And the clearance hole, since it is asked in the same breath. ISO 273 gives three series — fine, medium and coarse — and for the sixteen metric sizes two publishers agree on, this page uses the tabulated value. Those values imply ratios of about 1.06, 1.11 and 1.19 times the screw diameter, and those ratios are what is applied to inch threads, with the nearest stocked drill at or above the result, because no inch clearance table could be verified against a second source. A medium fit on an M6 is 6.6 mm; a free fit is 7.0 mm and lets the screw wander a third of a millimetre each way, which is either exactly what you wanted or a visible defect. For the other trade gauge that overlaps this range, see the wire gauge converter, and for the screw sizes that go into these holes the wood screw size converter. This page does the arithmetic. Where the answer carries a consequence — a dose, a structural load, a legal area, a medical measure — confirm it against the authority that governs it before relying on it.

Frequently asked questions

Related calculators

References

1. ANSI/ASME B94.11M, Twist Drills, the standard that defines the number (1 to 80) and letter (A to Z) drill series. It is copyrighted and its tables are not reproduced here; the diameters used were read from the two independent published charts named below, which agree at every one of the 106 sizes.
2. Wikipedia, List of drill and tap sizes and Drill bit sizes (read 26 September 2026): the number and letter drill diameters in decimal inches, and the working rules for tap drills — major diameter less the pitch for a metric coarse thread, about 85 per cent of the major diameter for an inch coarse thread.
3. Carbide Depot, Drill Size Chart, carbidedepot.com/formulas-drillsize.htm (read 26 September 2026) — the second, independent publisher of the number and letter series. It agrees with the table above at all 80 number drills and all 26 letter drills, to the fourth decimal place of an inch.
4. University of Wisconsin–Madison Physics Instrument Shop, Drill Chart (read 26 September 2026) — a third source, printing 31 of the number drills and 14 of the letters. It agrees with both of the above at every value it gives, and it is what settled #32 at 0.1160 in when one reading of the first source came back with #30’s figure instead.
5. 6G Tools, Tap Drill Calculator & Thread Engagement Calculator (read 26 September 2026), for the percentage-of-thread formulas and their constants: percentage = (major diameter − hole) ÷ (1.299 × pitch) × 100 in metric, and the identical inch form using 0.0130 per thread per inch. The 1.299 is twice the 0.6495-pitch thread depth of the American National and Unified truncated vee, which is the depth that convention counts.
6. ISO 261 and ISO 262, ISO general purpose metric screw threads — the coarse and fine pitch series, and the selected sizes. Copyrighted; cited by number. The pitches used here are part of the thread designation itself (M6×1 names its own pitch) and were taken from the table in Wikipedia’s ISO metric screw thread article, which also gives the published geometry this page uses: the minor diameter of the internal thread is the major diameter less 1.0825 times the pitch.
7. ISO 273, Fasteners — Holes for bolts and screws — Clearance holes, the source of the fine, medium and coarse clearance hole series. Copyrighted; the values used here were read from Mech Codex’s Metric Clearance Hole Sizes (ISO 273) reference and checked row by row against Xiaoman Tools’ metric clearance hole chart (both read 26 September 2026). The two agree at every size from M2 to M24.
8. ASME B1.1, Unified Inch Screw Threads (UN, UNR and UNJ Thread Form), which defines the UNC and UNF series. Copyrighted; cited by number. The threads per inch used here are part of the thread’s own designation, and the number-size major diameter follows the published American screw gauge rule, 0.060 in plus 0.013 in per gauge number.