What size is a #3 drill bit?

A #3 drill bit is 0.2130 in in diameter, which is 5.410 mm.

What thread does a #3 drill bit tap?

A #3 drill bit is the tap drill for 1/4"-28 UNF at 75% requested thread engagement, where it leaves 79.8% thread with a cut tap.

What is the metric equivalent of a #3 drill bit?

The closest metric drill to a #3 drill bit (5.410 mm) is 5.4 mm, which is 5.400 mm — 0.0004 in smaller.

What fraction is a #3 drill bit closest to?

A #3 drill bit (0.2130 in) sits closest to the 7/32" fractional drill at 0.2188 in, a difference of 0.0058 in.

Number drill · #3

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

https://shopmath.org/drills/number/3 · data verified August 1, 2026

A #3 drill bit is 0.2130 in (5.410 mm) in diameter. It is the tap drill for 1/4"-28 UNF at 75% thread engagement with a cut tap, where it leaves 79.8% thread. The nearest fractional size is 7/32" at 0.2188 in. Diameters are nominal.

Series: Number
Designation: #3
Inch: 0.2130
mm: 5.410
Standard: ANSI/ASME B94.11M

Number Drill · #3 · nominal diameter

0.2130 in

Millimetre: 5.410mm
Taps: 1/4"-28 UNF

Nominal diameter. As a tap drill it leaves 79.8% thread in 1/4"-28 UNF with a cut tap.

#3 Drill Bit Size in Inches and Millimetres

The nominal diameter of a #3 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.

#3 drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.2130 in
Diameter, millimetre	5.410 mm
Cross-sectional area	0.03563 in²

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

Threads a #3 Drill Bit Taps

Every thread in this reference whose tap drill is the #3 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 #3 drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
1/4"-28 UNF	Unified Fine (UNF)	75%default	79.8
1/4"-32 UNEF	Unified Extra Fine (UNEF)	85%	91.1
1/4"-28 UNF	Unified Fine (UNF)	80%	79.8
1/4"-20 UNC	Unified Coarse (UNC)	60%	57.0

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

#3 Drill Bit Equivalents in Every Other Drill Series

The closest size to the #3 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 #3 drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
A	Letter	0.2340	5.944	+0.0210
7/32"	Fractional	0.2188	5.556	+0.0058
5.4 mm	Metric	0.2126	5.400	−0.0004

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 #3 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 #3 drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
#4	Number	0.2090	5.309
5.4 mm	Metric	0.2126	5.400
#3	Number	0.2130	5.410
5.5 mm	Metric	0.2165	5.500
7/32"	Fractional	0.2188	5.556

Sources and Method for the #3 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.
Tap drill usage assumes cut taps. 1/4"-28 UNF at 75% requested engagement is what this drill is normally bought for.
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 #3 Drill Bit Reference

Citation

ShopMath. "#3 Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/number/3

BibTeX

@misc{shopmath-drills-number-3,
  title        = {#3 Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/number/3}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/number/3

Common Questions About the #3 Drill Bit

What size is a #3 drill bit?: A #3 drill bit is 0.2130 in in diameter, which is 5.410 mm.
What thread does a #3 drill bit tap?: A #3 drill bit is the tap drill for 1/4"-28 UNF at 75% requested thread engagement, where it leaves 79.8% thread with a cut tap.
What is the metric equivalent of a #3 drill bit?: The closest metric drill to a #3 drill bit (5.410 mm) is 5.4 mm, which is 5.400 mm — 0.0004 in smaller.
What fraction is a #3 drill bit closest to?: A #3 drill bit (0.2130 in) sits closest to the 7/32" fractional drill at 0.2188 in, a difference of 0.0058 in.

All number drill sizes · Complete drill size chart · Every drill series