What is the tap drill size for M18×2.5?

The tap drill for M18×2.5 is 15.5 mm — 0.6102 in (15.500 mm). That leaves 77.0% thread engagement with a cut tap.

What is the pitch diameter of M18×2.5?

The basic pitch diameter of M18×2.5 is 0.6447 in (16.376 mm).

What is the minor diameter of M18×2.5?

The basic minor diameter of M18×2.5 is 0.6021 in (15.294 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.

What is the thread pitch of M18×2.5?

M18×2.5 has a pitch of 2.500 mm, the distance from one thread crest to the next.

What size clearance hole does a M18×2.5 fastener need?

close fit 19 mm (0.7480 in (19.000 mm)), normal fit 19.75 mm (0.7874 in (20.000 mm)), loose fit 20.75 mm (0.8268 in (21.000 mm)). The normal fit is the general-assembly default.

What is the tensile stress area of M18×2.5?

The tensile stress area of M18×2.5 is 0.29833 in² (192.47 mm²). Multiply it by the fastener material's allowable stress to get a load.

ISO Metric Coarse · M18×2.5

M18×2.5 Thread — Dimensions, Tap Drill & Clearance

https://shopmath.org/threads/metric-coarse/m18x2.5 · data verified August 1, 2026

The tap drill for M18×2.5 is 15.5 mm — 0.6102 in (15.500 mm) — which leaves 77.0% thread engagement with a cut tap. Basic pitch diameter is 0.6447 in (16.376 mm) and basic minor diameter is 0.6021 in (15.294 mm). Tapped-hole figures on this page assume a class 6H thread; drill sizes for 50–85% engagement are in the tap drill table below.

Series: ISO Metric Coarse
Designation: M18×2.5
Pitch: 2.500 mm
Standard: ISO 68-1, ISO 261, ISO 262, ISO 965-1, ISO 898-1

Tap drill · 77.0% thread · cut tap

15.5 mm

Drill Ø: 0.6102in
Drill Ø: 15.500mm
Clearance (normal): 19.75 mm

Assumes 75% thread engagement with a cut tap and a class 6H hole. That drill actually leaves 77.0% thread.

Basic Thread Dimensions for M18×2.5

Basic thread dimensions — M18×2.5
Property	Inch	Millimetre
Basic major diameter	0.7087	18.000
Pitch	0.0984	2.500
Basic pitch diameter	0.6447	16.376
Basic minor diameter	0.6021	15.294
External thread root diameter	0.5879	14.933
Sharp V thread height (H)	0.0852	2.165

Basic dimensions are the theoretical values before any tolerance class is applied.

Tap Drill Sizes for M18×2.5

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 15.5 mm drill. Deeper engagement means more holding strength and more torque on the tap.

Tap drill sizes by thread engagement — M18×2.5, cut tap
Property	Target hole Ø		Nearest standard drill			Property
Thread %	in	mm	Drill	in	mm	Actual thread %
50%	0.6447	16.376	16.5 mm	0.6496	16.500	46.2
60%	0.6319	16.051	16 mm	0.6299	16.000	61.6
65%	0.6256	15.889	16 mm	0.6299	16.000	61.6
70%	0.6192	15.727	15.75 mm	0.6201	15.750	69.3
75%default	0.6128	15.564	15.5 mm	0.6102	15.500	77.0
80%	0.6064	15.402	15.5 mm	0.6102	15.500	77.0
85%	0.6000	15.240	15.25 mm	0.6004	15.250	84.7

Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces — the number that matters at the machine. Values are for cut taps. Tapped-hole figures assume a class 6H hole.

Clearance Hole Sizes for M18×2.5

The through-hole a M18×2.5 fastener passes freely, not the tapped hole.

Clearance hole sizes — M18×2.5
Property	Property	Hole Ø		Property
Fit	Drill	in	mm	Use
Close	19 mm	0.7480	19.000	Located joints, dowelled assemblies
Normal	19.75 mm	0.7874	20.000	General assembly — the default
Loose	20.75 mm	0.8268	21.000	Slotted or adjustable joints, weldments

Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch). The drill column is the nearest standard drill at or above the specified hole diameter.

Tensile Stress Area of M18×2.5

The effective cross-section used to size a M18×2.5 fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

Tensile stress area — M18×2.5
Property	in²	mm²
Tensile stress area	0.29833	192.47

The formula this value comes from is in the sources section below.

Other Thread Pitches in the Same Size as M18×2.5

A M18 fastener is not always M18×2.5. These are the other standard pitches cut on the same nominal size.

Other standard pitches at nominal size M18
Thread	Series	Pitch	Tap drill
M18×1.5	ISO Metric Fine	1.500 mm	16.5 mm
M18×2	ISO Metric Fine	2.000 mm	16 mm

Sources and Method for M18×2.5

Basic dimensionscomputed+compiled

ISO 68-1 basic profile and ISO 898-1 tensile stress area; verified in docs/research/data-sourcing.md §3.2; diameter/pitch pairs: ISO 261 / ISO 262 diameter-pitch pairs via Wikipedia (CC BY-SA 4.0); cross-checked against threadlib metric_thread.csv (BSD-3) and FreeCAD CAM metric-internal-6H.csv (LGPL-2.1+); clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)

Verified against threadlib metric_thread.csv (BSD-3-Clause) and FreeCAD CAM metric-internal-6H.csv (LGPL-2.1+), agreeing cell-for-cell — 215 checks passed at 0.001 mm

Part computed from the standard's formulas, part compiled from agreeing public sources.

Tap drill sizescomputed

Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4

Verified against D-P rule verified as exactly the 77.0% thread case

Computed from the standard's formulas, then checked against published tables.

Formula

Tap drill hole diameter, metric threads

hole Ø  =  D − (%thread ÷ 100) × 1.29904 × P

  D = basic major diameter (mm)
  P = pitch (mm)

1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° ISO profile.

Actual thread engagement of the chosen drill

%thread  =  (D − drill Ø) ÷ (1.29904 × P) × 100

The drill column rounds to a real drill; this is the engagement that drill actually leaves.

Tensile stress area, ISO metric threads

As  =  (π ÷ 4) × ((d₂ + d₃) ÷ 2)²

  d₂ = basic pitch diameter
  d₃ = external thread root diameter

ISO 898-1 form, evaluated on unrounded intermediates — recomputing it from the rounded diameters shown above lands within 0.01%.

Assumptions

The headline tap drill assumes 75% thread engagement, the general shop default. Every other engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
Tapped-hole figures assume a class 6H internal thread, the general-purpose fit.
Dataset scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This M18×2.5 Reference

Citation

ShopMath. "M18×2.5 Thread — Dimensions, Tap Drill & Clearance." ShopMath, verified August 1, 2026, https://shopmath.org/threads/metric-coarse/m18x2.5

BibTeX

@misc{shopmath-threads-metric-coarse-m18x2-5,
  title        = {M18×2.5 Thread — Dimensions, Tap Drill & Clearance},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads/metric-coarse/m18x2.5}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads/metric-coarse/m18x2.5

Common Questions About M18×2.5

What is the tap drill size for M18×2.5?: The tap drill for M18×2.5 is 15.5 mm — 0.6102 in (15.500 mm). That leaves 77.0% thread engagement with a cut tap.
What is the pitch diameter of M18×2.5?: The basic pitch diameter of M18×2.5 is 0.6447 in (16.376 mm).
What is the minor diameter of M18×2.5?: The basic minor diameter of M18×2.5 is 0.6021 in (15.294 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.
What is the thread pitch of M18×2.5?: M18×2.5 has a pitch of 2.500 mm, the distance from one thread crest to the next.
What size clearance hole does a M18×2.5 fastener need?: close fit 19 mm (0.7480 in (19.000 mm)), normal fit 19.75 mm (0.7874 in (20.000 mm)), loose fit 20.75 mm (0.8268 in (21.000 mm)). The normal fit is the general-assembly default.
What is the tensile stress area of M18×2.5?: The tensile stress area of M18×2.5 is 0.29833 in² (192.47 mm²). Multiply it by the fastener material's allowable stress to get a load.

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