What size is a 13/16" drill bit?

A 13/16" drill bit is 0.8125 in in diameter, which is 20.637 mm. It is the 13/16 in fractional size.

What thread does a 13/16" drill bit tap?

A 13/16" drill bit is the tap drill for 7/8"-14 UNF at 75% requested thread engagement, where it leaves 67.4% thread with a cut tap.

What is the metric equivalent of a 13/16" drill bit?

The closest metric drill to a 13/16" drill bit (20.637 mm) is 20.75 mm, which is 20.750 mm — 0.0044 in larger.

Fractional drill · 13/16

13/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/13-16 · data verified August 1, 2026

A 13/16" drill bit is 0.8125 in (20.637 mm) in diameter. It is the tap drill for 7/8"-14 UNF at 75% thread engagement with a cut tap, where it leaves 67.4% thread. Diameters are nominal.

Series: Fractional
Designation: 13/16
Inch: 0.8125
mm: 20.637
Standard: ANSI/ASME B94.11M

Fractional Drill · 13/16 · nominal diameter

0.8125 in

Millimetre: 20.637mm
Fraction: 13/16in
Taps: 7/8"-14 UNF

Nominal diameter. As a tap drill it leaves 67.4% thread in 7/8"-14 UNF with a cut tap.

13/16" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 13/16" 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.

13/16" drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.8125 in
Diameter, millimetre	20.637 mm
Fraction	13/16 in
Cross-sectional area	0.51849 in²

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

Threads a 13/16" Drill Bit Taps

Every thread in this reference whose tap drill is the 13/16" 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 13/16" drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
7/8"-14 UNF	Unified Fine (UNF)	75%default	67.4
7/8"-20 UNEF	Unified Extra Fine (UNEF)	85%	96.2
7/8"-14 UNF	Unified Fine (UNF)	60%	67.4
7/8"-14 UNF	Unified Fine (UNF)	65%	67.4
7/8"-14 UNF	Unified Fine (UNF)	70%	67.4

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

13/16" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 13/16" 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 13/16" drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.5845
Z	Letter	0.4130	10.490	−0.3995
20.75 mm	Metric	0.8169	20.750	+0.0044

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 13/16" 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 13/16" drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
20.25 mm	Metric	0.7972	20.250
20.5 mm	Metric	0.8071	20.500
13/16"	Fractional	0.8125	20.637
20.75 mm	Metric	0.8169	20.750
21 mm	Metric	0.8268	21.000

Sources and Method for the 13/16" 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.

Formula

Fractional drill diameter

Ø  =  13/16 in  =  0.8125 in

Fractional drills are exact fractions of an inch in 1/64 in steps; the decimal is the fraction evaluated, not a measurement.

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. 7/8"-14 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 13/16" Drill Bit Reference

Citation

ShopMath. "13/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/13-16

BibTeX

@misc{shopmath-drills-fractional-13-16,
  title        = {13/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/13-16}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/13-16

Common Questions About the 13/16" Drill Bit

What size is a 13/16" drill bit?: A 13/16" drill bit is 0.8125 in in diameter, which is 20.637 mm. It is the 13/16 in fractional size.
What thread does a 13/16" drill bit tap?: A 13/16" drill bit is the tap drill for 7/8"-14 UNF at 75% requested thread engagement, where it leaves 67.4% thread with a cut tap.
What is the metric equivalent of a 13/16" drill bit?: The closest metric drill to a 13/16" drill bit (20.637 mm) is 20.75 mm, which is 20.750 mm — 0.0044 in larger.

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