What size is a H drill bit?

A H drill bit is 0.2660 in in diameter, which is 6.756 mm.

What is the metric equivalent of a H drill bit?

The closest metric drill to a H drill bit (6.756 mm) is 6.8 mm, which is 6.800 mm — 0.0017 in larger.

What fraction is a H drill bit closest to?

A H drill bit (0.2660 in) sits closest to the 17/64" fractional drill at 0.2656 in, a difference of 0.0004 in.

What bolt fits through a H drill bit hole?

A hole drilled with a H drill bit is the standard close clearance hole for 1/4"-32 UNEF, and a clearance hole for 2 other fastener sizes listed on this page.

Letter drill · H

H Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/letter/h · data verified August 1, 2026

A H drill bit is 0.2660 in (6.756 mm) in diameter. The nearest fractional size is 17/64" at 0.2656 in. Diameters are nominal.

Series: Letter
Designation: H
Inch: 0.2660
mm: 6.756
Standard: ANSI/ASME B94.11M

Letter Drill · H · nominal diameter

0.2660 in

Millimetre: 6.756mm

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

H Drill Bit Size in Inches and Millimetres

The nominal diameter of a H 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.

H drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.2660 in
Diameter, millimetre	6.756 mm
Cross-sectional area	0.05557 in²

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

Threads a H Drill Bit Taps

Every thread in this reference whose tap drill is the H 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 H drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
5/16"-24 UNF	Unified Fine (UNF)	85%	85.9

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

Fasteners That Pass Through a H Drill Bit Hole

A hole drilled with a H drill bit is the standard clearance hole for these fasteners — the through-hole the bolt passes freely, not the tapped hole.

Fasteners whose clearance hole is drilled with a H drill bit
Fastener	Series	Clearance class
1/4"-32 UNEF	Unified Extra Fine (UNEF)	close
1/4"-28 UNF	Unified Fine (UNF)	close
1/4"-20 UNC	Unified Coarse (UNC)	close

Clearance classes per ISO 273 (metric) and ASME B18.2.8 (inch): close for located joints, normal for general assembly, loose for slotted or adjustable joints.

H Drill Bit Equivalents in Every Other Drill Series

The closest size to the H 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 H drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.0380
17/64"	Fractional	0.2656	6.747	−0.0004
6.8 mm	Metric	0.2677	6.800	+0.0017

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 H 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 H drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
6.7 mm	Metric	0.2638	6.700
17/64"	Fractional	0.2656	6.747
H	Letter	0.2660	6.756
6.8 mm	Metric	0.2677	6.800
6.9 mm	Metric	0.2717	6.900

Sources and Method for the H 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 H Drill Bit Reference

Citation

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

BibTeX

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

Permanent URL

https://shopmath.org/drills/letter/h

Common Questions About the H Drill Bit

What size is a H drill bit?: A H drill bit is 0.2660 in in diameter, which is 6.756 mm.
What is the metric equivalent of a H drill bit?: The closest metric drill to a H drill bit (6.756 mm) is 6.8 mm, which is 6.800 mm — 0.0017 in larger.
What fraction is a H drill bit closest to?: A H drill bit (0.2660 in) sits closest to the 17/64" fractional drill at 0.2656 in, a difference of 0.0004 in.
What bolt fits through a H drill bit hole?: A hole drilled with a H drill bit is the standard close clearance hole for 1/4"-32 UNEF, and a clearance hole for 2 other fastener sizes listed on this page.

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