What size is a 5/16" drill bit?

A 5/16" drill bit is 0.3125 in in diameter, which is 7.938 mm. It is the 5/16 in fractional size.

What thread does a 5/16" drill bit tap?

A 5/16" drill bit is the tap drill for 3/8"-16 UNC at 75% requested thread engagement, where it leaves 77.0% thread with a cut tap.

What is the metric equivalent of a 5/16" drill bit?

The closest metric drill to a 5/16" drill bit (7.938 mm) is 7.9 mm, which is 7.900 mm — 0.0015 in smaller.

Fractional drill · 5/16

5/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/5-16 · data verified August 1, 2026

A 5/16" drill bit is 0.3125 in (7.938 mm) in diameter. It is the tap drill for 3/8"-16 UNC at 75% thread engagement with a cut tap, where it leaves 77.0% thread. Diameters are nominal.

Series: Fractional
Designation: 5/16
Inch: 0.3125
mm: 7.938
Standard: ANSI/ASME B94.11M

Fractional Drill · 5/16 · nominal diameter

0.3125 in

Millimetre: 7.938mm
Fraction: 5/16in
Taps: 3/8"-16 UNC

Nominal diameter. As a tap drill it leaves 77.0% thread in 3/8"-16 UNC with a cut tap.

5/16" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 5/16" 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.

5/16" drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.3125 in
Diameter, millimetre	7.938 mm
Fraction	5/16 in
Cross-sectional area	0.07670 in²

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

Threads a 5/16" Drill Bit Taps

Every thread in this reference whose tap drill is the 5/16" 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 5/16" drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
3/8"-16 UNC	Unified Coarse (UNC)	75%default	77.0
3/8"-16 UNC	Unified Coarse (UNC)	80%	77.0

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

5/16" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 5/16" 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 5/16" drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.0845
O	Letter	0.3160	8.026	+0.0035
7.9 mm	Metric	0.3110	7.900	−0.0015

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 5/16" 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 5/16" drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
7.8 mm	Metric	0.3071	7.800
7.9 mm	Metric	0.3110	7.900
5/16"	Fractional	0.3125	7.938
8 mm	Metric	0.3150	8.000
O	Letter	0.3160	8.026

Sources and Method for the 5/16" 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  =  5/16 in  =  0.3125 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. 3/8"-16 UNC 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 5/16" Drill Bit Reference

Citation

ShopMath. "5/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/5-16

BibTeX

@misc{shopmath-drills-fractional-5-16,
  title        = {5/16" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/5-16}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/5-16

Common Questions About the 5/16" Drill Bit

What size is a 5/16" drill bit?: A 5/16" drill bit is 0.3125 in in diameter, which is 7.938 mm. It is the 5/16 in fractional size.
What thread does a 5/16" drill bit tap?: A 5/16" drill bit is the tap drill for 3/8"-16 UNC at 75% requested thread engagement, where it leaves 77.0% thread with a cut tap.
What is the metric equivalent of a 5/16" drill bit?: The closest metric drill to a 5/16" drill bit (7.938 mm) is 7.9 mm, which is 7.900 mm — 0.0015 in smaller.

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