What is the tap drill size for M18x2?

The listed cutting-tap starting size is 16 mm (0.6299 in / 16 mm). Nearest supported drill to major diameter minus pitch; larger size on a tie. 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 pitch diameter of M18x2?

16.701 mm. This is the diameter at which thread and groove widths are equal and is the dimension that controls assembly — a thread can meet its major and minor limits and still not fit if the pitch diameter is wrong.

What is the tensile stress area of M18x2?

204.18 mm², per ISO 898-1. It is calculated from the pitch diameter and the minor diameter rather than the nominal 18 mm, because the nominal diameter overstates the load-bearing section by roughly a third.

Are the dimensions on this page toleranced?

No. These are basic dimensions — the theoretical profile before any allowance is applied. For a threaded assembly, specify the tolerance class (6g/6H is the normal commercial fit) and use the limit dimensions from ISO 965.

M18x2 Thread Dimensions

Dimensions for the M18x2 Metric Fine Threads (ISO 261) thread — pitch, pitch diameter, external and internal minor diameters, tap drill and tensile stress area.

Page last changed 2026-10-19 · based on ISO 261:1998

Quick Answer

M18x2 has a pitch of 2 mm. The cutting-tap starting size listed here is 16 mm (16 mm). Nearest supported drill to major diameter minus pitch; larger size on a tie. Confirm the selected tap’s hole range.

Thread Calculator

Major diameter (mm)

Pitch (mm)

Basic Dimensions

Dimension^[4]	Symbol^[4]	Millimetre^[1]
Major diameter (external) #	`d`	18
Pitch diameter (basic) #	`d2`	16.701
Minor diameter, external thread #	`d1`	15.546
Minor diameter, internal thread #	`D1`	15.835
Pitch #	`P`	2.000
Thread height, external #	`h`	1.227
Thread height, internal #	`hn`	1.083
Crest flat #	`f`	0.250
Tensile stress area #	`As`	204.18²

Tap Drill Size

For a M18x2 cutting tap, the listed starting drill is 16 mm (0.6299 in / 16 mm).

The geometric estimate is major diameter − pitch = 18 − 2 = 16.000 mm. Selection basis: Nearest supported drill to major diameter minus pitch; larger size on a tie. 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

M18x2 gives the nominal major diameter and the pitch directly in millimetres: 18 mm diameter and 2 mm pitch. This is the Metric Fine Threads (ISO 261) series.

Writing the pitch explicitly distinguishes this size from the other pitches available at the same diameter. An unqualified M18 would mean the coarse pitch for that diameter.

Tolerance Class

The dimensions on this page are the basic profile — no tolerance class allowance is applied. Production metric threads are specified by a tolerance class such as 6g (external) or 6H (internal), which is the normal commercial fit and the default when none is stated.

Coarser classes (8g, 7H) ease assembly and tolerate plating; finer classes (4h, 5H) are used where a precise fit is needed. The basic dimensions below are the reference from which those limits are derived.

← M16x1.5 All METRIC-FINE sizes M18x1.5 →

Frequently Asked Questions

What is the tap drill size for M18x2?: The listed cutting-tap starting size is 16 mm (0.6299 in / 16 mm). Nearest supported drill to major diameter minus pitch; larger size on a tie. 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 pitch diameter of M18x2?: 16.701 mm. This is the diameter at which thread and groove widths are equal and is the dimension that controls assembly — a thread can meet its major and minor limits and still not fit if the pitch diameter is wrong.
What is the tensile stress area of M18x2?: 204.18 mm², per ISO 898-1. It is calculated from the pitch diameter and the minor diameter rather than the nominal 18 mm, because the nominal diameter overstates the load-bearing section by roughly a third.
Are the dimensions on this page toleranced?: No. These are basic dimensions — the theoretical profile before any allowance is applied. For a threaded assembly, specify the tolerance class (6g/6H is the normal commercial fit) and use the limit dimensions from ISO 965.

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]	Basic dimensions computed from the ISO 68-1 basic profile	derived	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 − 0.9382P)² per ISO 898-1.
[2]	Standard metric twist drill series	reference	n/a — industry standard series — source
[3]	ISO 261 — ISO general purpose metric screw threads — General plan	reference	ISO 261:1998 — source
[4]	ISO 68-1 — ISO general purpose screw threads — Basic profile	reference	ISO 68-1:2023 — source
[5]	ISO 898-1 — Mechanical properties of fasteners, Part 1: Bolts, screws and studs	reference	ISO 898-1:2013

Data Sources

Standard	Revision	What it covers on this page
ISO 261 — General plan for metric screw threads	ISO 261:1998	the nominal diameter and pitch of this size
ISO 68-1 — Basic profile	ISO 68-1:2023	the thread form geometry from which every dimension here is computed
ISO 898-1 — Mechanical properties of fasteners	ISO 898-1:2013	the tensile stress area formula

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

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

Value	How it is derived
Pitch diameter, minor diameters, thread heights, tensile stress area	Computed from the ISO 68-1 basic profile and the ISO 898-1 stress-area formula — see the Value Sources table above.

Basic dimensions only, with no tolerance class allowance. Specify a class (6g/6H is standard) for a threaded assembly and use ISO 965 limit dimensions.

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. "M18x2 Thread Dimensions." https://refmanual.com/threads/metric-fine/m18x2/ (accessed 2026-10-19). Data from ISO 261 — General plan for metric screw threads, ISO 261:1998.

BibTeX

@misc{m18x2,
  title        = {M18x2 Thread Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/threads/metric-fine/m18x2/}},
  note         = {Accessed: 2026-10-19},
  source       = {ISO 261 — General plan for metric screw threads, ISO 261:1998}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - M18x2 Thread Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/threads/metric-fine/m18x2/
PY  - 2026
DA  - 2026-10-19
N1  - Source: ISO 261 — General plan for metric screw threads, ISO 261:1998
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.