Drill Bit Size Chart: Fractional, Number, Letter and Metric Drill Sizes

The 63 fractional sizes, from 0.0156 in (1/64) up to 0.9844 in (63/64), ordered by diameter. Millimetres are the decimal inch value multiplied by 25.4 and shown to four decimal places; the closest metric drill in the next column is computed from the metric series, not copied across.

The 80 number sizes, from 0.0135 in (#80, the smallest) up to 0.2280 in (#1, the largest). The designator runs the opposite way to the diameter, which is the most common misreading of a drill size chart. Millimetres are the decimal inch value multiplied by 25.4, and the closest metric drill is computed rather than copied.

The 26 letter sizes, from A at 0.2340 in up to Z at 0.4130 in, ordered by diameter. Letter and number drills are two separate catalogues that overlap in diameter range, so the letter is a name and not a shorthand for a number size. Millimetres are the decimal inch value multiplied by 25.4.

Metric drill sizes: 169 sizes, from 0.35 to 25.00 mm

The 169 metric sizes published as commonly obtainable, from 0.35 mm to 25.00 mm, ordered by diameter. Decimal inches are the millimetre diameter divided by 25.4, and the closest inch drill in the next column is computed from the inch series and reported as a match, not as an equivalent.

Conversion used in every table on this page: millimetres = decimal inches × 25.4, and decimal inches = millimetres ÷ 25.4, each to four decimal places. 1 in = 25.4 mm.

Basis, sources and known gaps

• The four tables hold 63 fractional, 80 number and 26 letter sizes (together 169 inch sizes) and 169 metric sizes; each diameter is generated from a single data source (utils/pilotHoleCalc.ts) and can be recomputed size by size.
• Inch series: 39 rows are printed directly in the FED-STD-H28/2B drill table (a published US federal standard); the remaining 130 rest on the long-established number, letter and fractional drill-size constants.
• The letter drill K has only one published printed value to confirm it and is flagged in the table (1 row in total).
• Metric series, 169 sizes: 163 are confirmed by two independent printed values, and 6 have a single readable printed value (6.20, 6.70, 6.80, 16.25, 22.00, 24.00 mm) — each is flagged individually.

For 6 metric sizes only one published value is available, or the printed values conflict; these rows are individually flagged in the table with the basis used (the reproducible arithmetic value in case of conflict) — never silently averaged.

The numbers in this table are diameters, not thread sizes. Number drill designators are names, not diameters: a #7 drill is 0.2010 in, not 7 anything. Where a letter drill and a fractional drill happen to share one diameter (the letter E and 1/4), both are kept as separate catalogue entries and neither is treated as a substitute for the other.

The range this page covers, and where it stops

• Frozen coverage of this edition: number drills #80 to #1 (80 sizes, stopping at #80 on the small side), letter drills A to Z (26 sizes), fractional drills 1/64 in to 63/64 in (63 sizes) and metric drills 0.35 to 25.00 mm (169 sizes).
• Some published drill charts go further, for example small wire-gauge number sizes above #80 and longer metric series. Those sizes are not listed here and are not inferred from the sizes that are: an absent size stays absent.
• The page therefore makes no exhaustiveness claim. It lists the sizes this site carries, with the basis of each row printed in the row itself.

What the metric series does and does not cover

• The metric series covers 0.35–25.00 mm — the commonly obtainable sub-set as published by the source, not a series defined by any standard; the formal ISO 235 / DIN 338 / BS 328 series are not used on this page.
• Above 25.00 mm the source describes taper-shank metric drills (up to 80 mm) without giving a size-by-size table.

Wood screw thread root diameter: two estimates, kept apart

• The wood screw lead-hole table lists measured practice values. The load-design rule behind a lead hole scales with the thread root diameter, and no published standard defines that root diameter — so two engineering estimates are given side by side for you to pick by conservatism: the manufacturing estimate root = 0.67 × body diameter, and the carpentry practice of simply drilling to the source-table lead hole.
• The second is the habit the 1926 National Bureau of Standards paper records and recommends discontinuing; neither is a design value.

Boundary: the diameters above are reference values for selecting a drill, not a fit or an acceptance basis. A drill that matches a hole to four decimal places on paper still drills oversize, so unless the fit is critical, take the nearest drill and test on a scrap piece. For load-carrying joints, follow the material supplier's data and the design requirement.

How far apart neighbouring sizes are

Sizes taken from the three inch series with the next size down and the next size up in the same series, and the step to each. The steps are not constant: in the number series they narrow towards the small end, which is why no fixed substitution rule picks the right drill.

Worked example: reading one row in each direction

Two rows taken from the tables above and recomputed step by step, so the conversion can be checked by hand.

• Inch side. The row for the fractional size 13/64 reads 0.2031 in and 5.1587 mm. The millimetre value is the inch value multiplied by 25.4: 0.2031 × 25.4 = 5.1587, which is the figure printed in the row.
• The closest metric size to 0.2031 in in the metric table is 5.20 mm, which is 0.0406 mm away. Rows are matched, never merged: 13/64 keeps its own inch diameter and 5.20 mm keeps its own metric diameter.
• Metric side. The row for 6.20 mm reads 0.2441 in, which is 6.20 ÷ 25.4 rounded to four decimal places. The closest inch size in the tables is D at 0.2460 in, 0.0019 in away.
• The two directions use the same published values, and neither series is a rounding of the other: if a job needs the exact nominal diameter, order the drill by the catalogue the drawing uses.

Where this table ends and the neighbouring pages begin

This page lists drill sizes. Three neighbouring pages answer a different question, and the two are kept apart so that a search for one does not land on the other.

• Drill sizes for threaded holescross-reference

  That page starts from a thread size rather than a drill size and gives the rule value plus the nearest standard drill for each thread series. Useful when the hole has to be drilled before a thread is cut.

• Bolt size chart: metric hex bolt dimensionsapplication

  Shank, head and nut dimensions, for when a bolt has to pass through the hole and the clearance diameter matters more than the nominal drill size.

• Wood screw lead and clearance holesapplication

  The screw table lower on this page: per screw size, the body diameter, the lead hole and the clearance hole, with a link from each size to its own page.

• Pilot hole calculatorcalculation

  Enter a screw size and a material and it returns the lead hole, the clearance hole and the closest drills in both systems, on the same basis as the tables above.

Wood screw pilot holes and clearance holes

The same data in the other direction: for each wood screw size, the body diameter, the lead hole the threads bite into, the clearance hole the screw passes through, and the closest drill in each system. The screw size links to its own page with the full basis. A screw number and a drill number are different catalogues: a #8 screw does not take a #8 drill.

The source table lists no lead hole for the smallest screw size it covers, so that row is left blank rather than computed; the clearance hole column still carries its value.

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