What size is a #18 drill bit?

A #18 drill bit is 0.1695 in in diameter, which is 4.305 mm.

What is the metric equivalent of a #18 drill bit?

The closest metric drill to a #18 drill bit (4.305 mm) is 4.3 mm, which is 4.300 mm — 0.0002 in smaller.

What fraction is a #18 drill bit closest to?

A #18 drill bit (0.1695 in) sits closest to the 11/64" fractional drill at 0.1719 in, a difference of 0.0024 in.

Number drill · #18

#18 Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/number/18 · data verified August 1, 2026

A #18 drill bit is 0.1695 in (4.305 mm) in diameter. The nearest fractional size is 11/64" at 0.1719 in. Diameters are nominal.

Series: Number
Designation: #18
Inch: 0.1695
mm: 4.305
Standard: ANSI/ASME B94.11M

Number Drill · #18 · nominal diameter

0.1695 in

Millimetre: 4.305mm

Nominal diameter. A worn or run-out drill cuts oversize, which leaves less thread in a tapped hole than the table says.

#18 Drill Bit Size in Inches and Millimetres

The nominal diameter of a #18 drill bit. Drill sizes are nominal — a worn drill, run-out or a soft material that closes in behind the cut all move the finished hole.

#18 drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.1695 in
Diameter, millimetre	4.305 mm
Cross-sectional area	0.02256 in²

1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.

Threads a #18 Drill Bit Taps

Every thread in this reference whose tap drill is the #18 drill bit, and the thread engagement it leaves. The engagement asked for and the engagement the drill actually produces are different numbers — that gap is why both columns are here.

Threads tapped with a #18 drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
#10-32 UNF	Unified Fine (UNF)	50%	50.5
#12-24 UNC	Unified Coarse (UNC)	85%	85.9

Values are for cut taps. Rows marked default are the shop-default engagement for that thread.

#18 Drill Bit Equivalents in Every Other Drill Series

The closest size to the #18 drill bit in each of the other three indexes, with the difference in inches. None of them is the same drill — check the difference before substituting one into a tapped hole.

Nearest number, letter, fractional and metric equivalents to a #18 drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
A	Letter	0.2340	5.944	+0.0645
11/64"	Fractional	0.1719	4.366	+0.0024
4.3 mm	Metric	0.1693	4.300	−0.0002

A positive difference means the equivalent drill is larger, which leaves less thread in a tapped hole. Sizes without a link are stored for the nearest-drill calculation but do not carry their own page.

Drill Sizes Either Side of the #18 Drill Bit

Ordered by diameter across all four indexes, so the next size up may be a letter, a fraction or a metric drill rather than the next one in this series.

Standard drill sizes adjacent to a #18 drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
#19	Number	0.1660	4.216
4.3 mm	Metric	0.1693	4.300
#18	Number	0.1695	4.305
11/64"	Fractional	0.1719	4.366
#17	Number	0.1730	4.394

Sources and Method for the #18 Drill Bit Size

Drill diametercompiled

Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.

Verified against Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

Compiled from independent public sources that agree cell-for-cell.

Tap drill usagecomputed+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. Drill manufacturing tolerance, wear and run-out all cut oversize, and every one of those moves a tapped hole toward less thread 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 #18 Drill Bit Reference

Citation

ShopMath. "#18 Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/number/18

BibTeX

@misc{shopmath-drills-number-18,
  title        = {#18 Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/number/18}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/number/18

Common Questions About the #18 Drill Bit

What size is a #18 drill bit?: A #18 drill bit is 0.1695 in in diameter, which is 4.305 mm.
What is the metric equivalent of a #18 drill bit?: The closest metric drill to a #18 drill bit (4.305 mm) is 4.3 mm, which is 4.300 mm — 0.0002 in smaller.
What fraction is a #18 drill bit closest to?: A #18 drill bit (0.1695 in) sits closest to the 11/64" fractional drill at 0.1719 in, a difference of 0.0024 in.

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