What is the tap drill size for a #6-40 thread?

The listed cutting-tap starting size is #33 (0.1130 in / 2.8702 mm). Supplier lookup in the ICS wire-gauge column. Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.

What is the major diameter of a #6-40 thread?

The nominal major diameter is 0.1380 in (3.5052 mm). On an external thread this is the outside diameter of the thread; on an internal thread it is the theoretical largest diameter of the thread form.

What is the pitch diameter of a #6-40 thread?

The basic pitch diameter is 0.1218 in (3.0928 mm). This is the diameter at which the thread width and the groove width are equal, and it is the dimension that actually controls whether the thread assembles — it is what a thread micrometer or thread comparator measures.

How many threads per inch does a #6-40 have?

40 threads per inch, giving a pitch of 0.0250 in (0.635 mm). Threads per inch defines the pitch; it does not change with the nominal diameter within the same size designation.

What is the difference between major, pitch and minor diameter?

Major diameter is the largest diameter of the thread — the outside of an external thread. Pitch diameter is measured where the thread and groove widths are equal, and is the dimension that controls fit. Minor diameter is the smallest — the root of an external thread or the bore of an internal thread. Pitch diameter is the critical one: a thread can meet its major and minor limits and still not assemble if the pitch diameter is wrong.

#6-40 Thread Dimensions

Dimensions for the #6-40 UNF (Unified Fine) thread — pitch, pitch diameter, external and internal minor diameters, tap drill size and tensile stress area.

Page last changed 2026-10-19 · based on ASME B1.1-2019

Quick Answer

#6-40 has a pitch of 0.0250 in. The cutting-tap starting size listed here is #33 (0.1130 in). Supplier lookup in the ICS wire-gauge column. Confirm the selected tap’s hole range.

Thread Calculator

Major diameter (inch)

Threads per inch

Basic Dimensions

Dimension^[1]	Symbol^[1]	Inch^[2]	Millimetre^[3]
Major diameter (external) #	`d`	0.1380	3.5052
Pitch diameter (basic) #	`d2`	0.1218	3.0928
Minor diameter, external thread #	`d1`	0.1073	2.7261
Minor diameter, internal thread #	`D1`	0.1109	2.8178
Pitch #	`P`	0.0250	0.635
Thread height, external #	`h`	0.0153	0.3895
Thread height, internal #	`hn`	0.0135	0.3437
Crest flat #	`f`	0.0031	0.0794
Tensile stress area #	`As`	0.0101	6.5439

Tap Drill Size

For a #6-40 cutting tap, the listed starting drill is #33 (0.1130 in / 2.8702 mm).

The geometric estimate is major diameter − pitch = 0.1380 − 0.0250 = 0.1130 in. Selection basis: Supplier lookup in the ICS wire-gauge column. This does not specify an exact thread percentage or axial engagement length. Confirm the tap maker’s hole range, material and required thread; forming taps require different holes. See the tap drill size chart.

How to Read the Designation

#6-40 encodes three things: the nominal major diameter #6, the thread pitch as 40 threads per inch, and the series — UNF (Unified Fine). The series letter sets the pitch for that diameter; the same nominal diameter exists in UNC, UNF and UNEF at different pitches.

Numbers below 1/4 in are written as machine screw sizes (#0 through #12) whose major diameter follows d = 0.060 + 0.013 × N. A #6-40 has a nominal major diameter of 0.1380 in (3.5052 mm).

Class of Fit

The dimensions on this page are the basic dimensions — the theoretical profile with no allowance or tolerance applied. Production threads are specified by class:

Class 1A / 1B — loosest, for quick assembly and threads that may be dirty or damaged. Rarely specified.
Class 2A / 2B — the normal commercial fit, and the default for bolts, screws and nuts. Has a slight allowance on the external thread to prevent binding.
Class 3A / 3B — tight fit with no allowance, for close-tolerance work and high-strength fasteners. Requires better process control.

A and B denote external and internal threads respectively: 2A is an external bolt thread, 2B is an internal nut thread. The basic dimensions below are the reference from which the class limits are derived.

← #5-44 All UNF sizes #8-36 →

Frequently Asked Questions

What is the tap drill size for a #6-40 thread?: The listed cutting-tap starting size is #33 (0.1130 in / 2.8702 mm). Supplier lookup in the ICS wire-gauge column. Cutting taps only. The selected diameter is a starting reference, not a tolerance limit. Confirm the tap maker’s hole range, material, required thread and inspection method. Forming taps require different holes.
What is the major diameter of a #6-40 thread?: The nominal major diameter is 0.1380 in (3.5052 mm). On an external thread this is the outside diameter of the thread; on an internal thread it is the theoretical largest diameter of the thread form.
What is the pitch diameter of a #6-40 thread?: The basic pitch diameter is 0.1218 in (3.0928 mm). This is the diameter at which the thread width and the groove width are equal, and it is the dimension that actually controls whether the thread assembles — it is what a thread micrometer or thread comparator measures.
How many threads per inch does a #6-40 have?: 40 threads per inch, giving a pitch of 0.0250 in (0.635 mm). Threads per inch defines the pitch; it does not change with the nominal diameter within the same size designation.
What is the difference between major, pitch and minor diameter?: Major diameter is the largest diameter of the thread — the outside of an external thread. Pitch diameter is measured where the thread and groove widths are equal, and is the dimension that controls fit. Minor diameter is the smallest — the root of an external thread or the bore of an internal thread. Pitch diameter is the critical one: a thread can meet its major and minor limits and still not assemble if the pitch diameter is wrong.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	reference	ASME B1.1-2019 — source
[2]	Basic dimensions computed from the ASME B1.1 basic profile	derived	Pitch P = 1/n. Fundamental triangle height H = √3/2 × P. Pitch diameter d2 = d − 0.75H. External minor diameter d1 = d − 2(0.61343P). Internal minor diameter D1 = d − 2(0.54127P). Tensile stress area As = π/4 × (d − 0.9743/n)².
[3]	Millimetre conversion	derived	Inch value × 25.4.
[4]	Standard twist drill size series (number, letter and fractional)	reference	n/a — industry standard drill series — source
[5]	MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	reference	MIL-S-7742D (free public-domain mirror at everyspec.com) — source
[6]	ICS Cutting Tools — tap drill chart	reference	n/a — undated supplier chart, selected rows checked 2026-10-19 — source

Data Sources

Standard	Revision	What it covers on this page
ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	ASME B1.1-2019	thread form geometry, the designation system and class-of-fit definitions
MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	MIL-S-7742D (free public-domain mirror at everyspec.com)	the selection of nominal diameters and pitches in the standard series

Verification scope

2026-10-19 — #6-40: the listed #33 cutting-tap size matches the selected ICS chart column. This check does not cover the full thread geometry or tolerance standard. Evidence

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
All inch dimensions and the tensile stress area	Computed from the ASME B1.1 basic profile formulas, not transcribed from a table — see the Value Sources table above for the exact expressions.
Millimetre values	Inch value × 25.4, rounded to 4 decimal places.

Every dimension on this page is generated from the standard's own formulas, so the values are self-consistent by construction. Basic dimensions do not include class-of-fit allowances or tolerances — for a threaded assembly, specify the class and use the limit dimensions from ASME B1.1.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "#6-40 Thread Dimensions." https://refmanual.com/threads/unf/6-40/ (accessed 2026-10-19). Data from ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019.

BibTeX

@misc{unf-6-40,
  title        = {#6-40 Thread Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/threads/unf/6-40/}},
  note         = {Accessed: 2026-10-19},
  source       = {ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - #6-40 Thread Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/threads/unf/6-40/
PY  - 2026
DA  - 2026-10-19
N1  - Source: ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.