What size is a 16.25 mm drill bit?

A 16.25 mm drill bit is 0.6398 in in diameter, which is 16.250 mm.

What fraction is a 16.25 mm drill bit closest to?

A 16.25 mm drill bit (0.6398 in) sits closest to the 41/64" fractional drill at 0.6406 in, a difference of 0.0009 in.

Metric drill · 16.25 mm

16.25 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/metric/16-25mm · data verified August 1, 2026

A 16.25 mm drill bit is 0.6398 in (16.250 mm) in diameter. The nearest fractional size is 41/64" at 0.6406 in. Diameters are nominal.

Series: Metric
Designation: 16.25 mm
Inch: 0.6398
mm: 16.250
Standard: ANSI/ASME B94.11M

Metric Drill · 16.25 mm · nominal diameter

0.6398 in

Millimetre: 16.250mm

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

16.25 mm Drill Bit Size in Inches and Millimetres

The nominal diameter of a 16.25 mm 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.

16.25 mm drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.6398 in
Diameter, millimetre	16.250 mm
Cross-sectional area	0.32146 in²

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

Threads a 16.25 mm Drill Bit Taps

Every thread in this reference whose tap drill is the 16.25 mm 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 16.25 mm drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
M18×1.5	ISO Metric Fine	85%	89.8
M18×2	ISO Metric Fine	65%	67.4
M18×2	ISO Metric Fine	70%	67.4

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

16.25 mm Drill Bit Equivalents in Every Other Drill Series

The closest size to the 16.25 mm 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 16.25 mm drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.4118
Z	Letter	0.4130	10.490	−0.2268
41/64"	Fractional	0.6406	16.272	+0.0009

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 16.25 mm 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 16.25 mm drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
5/8"	Fractional	0.6250	15.875
16 mm	Metric	0.6299	16.000
16.25 mm	Metric	0.6398	16.250
41/64"	Fractional	0.6406	16.272
16.5 mm	Metric	0.6496	16.500

Sources and Method for the 16.25 mm Drill Bit Size

Drill diametercomputed

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

Computed from the standard's formulas, then checked against published tables.

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 16.25 mm Drill Bit Reference

Citation

ShopMath. "16.25 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/metric/16-25mm

BibTeX

@misc{shopmath-drills-metric-16-25mm,
  title        = {16.25 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/metric/16-25mm}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/metric/16-25mm

Common Questions About the 16.25 mm Drill Bit

What size is a 16.25 mm drill bit?: A 16.25 mm drill bit is 0.6398 in in diameter, which is 16.250 mm.
What fraction is a 16.25 mm drill bit closest to?: A 16.25 mm drill bit (0.6398 in) sits closest to the 41/64" fractional drill at 0.6406 in, a difference of 0.0009 in.

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