What are the four standard drill bit size systems?

Number gauges (#1 largest to #80 smallest), letter gauges (A smallest to Z largest), fractional inch sizes in 1/64 in steps, and metric sizes in millimetres. This reference carries 377 sizes across all four, interleaved by diameter so a metric drill sits between the inch sizes it falls between.

How do I convert a drill size to millimetres?

Every drill page and the chart give both units on the same row, because 1 in = 25.4 mm exactly — the millimetre value is the same diameter in a different unit, not a separate measurement or a rounded conversion.

Which drill size do I need for a tap?

94 of these sizes are the shop-default tap drill for at least one standard thread. Each drill page lists the threads it taps and the thread engagement it actually leaves, and the tap drill chart gives the whole matrix from 50% to 85% engagement.

Reference · drill sizes

Drill Size Reference — Number, Letter, Fractional and Metric

https://shopmath.org/drills · data verified August 1, 2026

377 standard drill bit sizes across four indexes, each with its decimal inch and millimetre diameter, its nearest equivalent in the other three indexes, and the threads it taps. 397 sizes are stored in total — the smallest number gauges are kept for the nearest-drill calculation without carrying their own page.

Sizes: 377
Series: 4
Tap drills: 94
Verified: August 1, 2026

Every Standard Drill Bit Size Series

Pick a series for its full decimal-equivalent chart, or go straight to a size for its equivalents in the other three indexes and the threads it taps.

Standard drill bit size series covered by this reference
Series	Sizes	Smallest	Largest	What it is
Fractional Drill Sizes	64	1/64" (0.0156 in)	1" (1.0000 in)	Inch fractional drills in 1/64 in steps from 1/64 to 1 in.
Letter Drill Sizes	26	A (0.2340 in)	Z (0.4130 in)	Letter-gauge twist drills, A (smallest) through Z (largest), 0.234 to 0.413 in.
Metric Drill Sizes	227	0.2 mm (0.0079 in)	25 mm (0.9843 in)	Metric twist drills, 0.2 mm to 25 mm on the BS 328 increment pattern.
Number Drill Sizes	60	#60 (0.0400 in)	#1 (0.2280 in)	Gauge-numbered twist drills, #1 (largest) through #80 (smallest).

A series appears here only when it carries at least three verified sizes.

Most Looked-Up Drill Bit Sizes

Common Questions About Drill Bit Sizes

What are the four standard drill bit size systems?: Number gauges (#1 largest to #80 smallest), letter gauges (A smallest to Z largest), fractional inch sizes in 1/64 in steps, and metric sizes in millimetres. This reference carries 377 sizes across all four, interleaved by diameter so a metric drill sits between the inch sizes it falls between.
How do I convert a drill size to millimetres?: Every drill page and the chart give both units on the same row, because 1 in = 25.4 mm exactly — the millimetre value is the same diameter in a different unit, not a separate measurement or a rounded conversion.
Which drill size do I need for a tap?: 94 of these sizes are the shop-default tap drill for at least one standard thread. Each drill page lists the threads it taps and the thread engagement it actually leaves, and the tap drill chart gives the whole matrix from 50% to 85% engagement.

Sources and Method for the Drill Size Data

Standard drill index — number, letter, fractional, metriccomputed+compiled

bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns

Verified against drills: 31 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Assumptions

Diameters are nominal. Manufacturing tolerance, wear and run-out all cut oversize, which leaves less thread in a tapped hole than the table says.
Dataset scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Drill Size Reference

Citation

ShopMath. "Drill Size Reference — Number, Letter, Fractional and Metric." ShopMath, verified August 1, 2026, https://shopmath.org/drills

BibTeX

@misc{shopmath-drills,
  title        = {Drill Size Reference — Number, Letter, Fractional and Metric},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills