M18×2.5 Thread Dimensions and Tap Drill
M18×2.5 is the ISO metric coarse thread for 18 mm fasteners, which is why it is usually written simply as M18. It has a 60° profile with a pitch of 2.5 mm (10.2 threads per inch). ISO 261 lists M18 as a second-choice diameter. Fine-pitch versions of the same diameter are M18×2, M18×1.5 and M18×1.
- Tap drill15.5 mm77% thread
- Pitch2.5 mm10.2 TPI
- Major diameter18.000 mm0.7087 in
- Pitch diameter16.376 mm0.6447 in
- Minor diameter (nut)15.294 mm0.6021 in
- Tensile stress area192 mm²0.298 in²
- Clearance hole20 mmmedium fit, ISO 273
Thread profile
M18×2.5 basic dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 18.000 | 0.7087 |
| Pitch P | 2.500 | 0.0984 |
| Pitch diameter d2 = D2 | 16.376 | 0.6447 |
| Minor diameter, internal thread D1 | 15.294 | 0.6021 |
| Minor diameter, external thread d3 | 14.933 | 0.5879 |
| Height of fundamental triangle H | 2.165 | 0.0852 |
| Thread height, internal (5H/8) | 1.353 | 0.0533 |
| Thread height, external (17H/24) | 1.534 | 0.0604 |
| Crest flat of external thread (P/8) | 0.312 | 0.0123 |
| Crest flat of internal thread (P/4) | 0.625 | 0.0246 |
| Property | Value |
|---|---|
| Threads per inch | 10.16 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 2.78° |
| Tensile stress area As | 192 mm² (0.298 in²) |
Basic (nominal) sizes per ISO 724, before tolerances. d3 is the root diameter of a bolt with the rounded root that ISO 898-1 uses for the stress area.
M18×2.5 tolerance limits (6g bolt, 6H nut)
6g and 6H are the general-purpose tolerance classes for commercial screws and nuts. The 6g screw has a fundamental deviation of 42 µm, so even at its largest it sits 0.042 mm below the basic size and leaves room for a thin plating.
| External thread 6g | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Major diameter d | 17.958 | 17.623 | 0.335 |
| Pitch diameter d2 | 16.334 | 16.164 | 0.170 |
| Internal thread 6H | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Minor diameter D1 | 15.744 | 15.294 | 0.450 |
| Pitch diameter D2 | 16.600 | 16.376 | 0.224 |
Computed from the ISO 965-1 deviations and tolerances for normal length of engagement; the results match the limits tabulated in ISO 965-2.
M18×2.5 tap drill size
Tap drill for M18×2.5: 15.5 mm
This leaves about 77% thread in the tapped hole.
The rule for metric tap drills is diameter minus pitch: 18 − 2.5 = 15.5 mm, which is a stocked drill size.
The 6H minor diameter of the finished nut thread must lie between 15.294 and 15.744 mm. The recommended drill falls inside that band.
Other drills that work for M18×2.5
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 6H limits | Thread engagement |
|---|---|---|---|---|---|
| 15.25 mm | 15.25 | 0.6004 | 85% | no | |
| 39/64" (0.6094 in) | 15.48 | 0.6094 | 78% | yes | |
| 15.5 mmrecommended | 15.50 | 0.6102 | 77% | yes | |
| 15.75 mm | 15.75 | 0.6201 | 69% | no | |
| 16 mm | 16.00 | 0.6299 | 62% | no | |
| 16.25 mm | 16.25 | 0.6398 | 54% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for M18
The clearance hole depends only on the 18 mm diameter, not on the pitch. The medium series, 20 mm, is the usual choice for general assembly.
| Series | Hole Ø (mm) | Hole Ø (in) | Clearance on Ø (mm) |
|---|---|---|---|
| Fine (H12) | 19 | 0.7480 | 1 |
| Medium (H13) | 20 | 0.7874 | 2 |
| Coarse (H14) | 21 | 0.8268 | 3 |
Hole diameters from ISO 273.
M18×2.5 proof and breaking loads
Loads for steel bolts and screws by property class, from the tensile stress area of 192 mm². The proof load is the highest load the fastener must carry without permanent set.
| Property class | Proof stress (MPa) | Min tensile strength (MPa) | Proof load (kN) | Min breaking load (kN) |
|---|---|---|---|---|
| 4.6 | 225 | 400 | 43.2 | 76.8 |
| 4.8 | 310 | 420 | 59.5 | 80.6 |
| 5.6 | 280 | 500 | 53.8 | 96.0 |
| 5.8 | 380 | 520 | 73.0 | 99.8 |
| 6.8 | 440 | 600 | 84.5 | 115 |
| 8.8 | 600 | 830 | 115 | 159 |
| 10.9 | 830 | 1040 | 159 | 200 |
| 12.9 | 970 | 1220 | 186 | 234 |
Class 9.8 applies only up to 16 mm and is omitted. Stresses from ISO 898-1:2013 Table 3; As from ISO 898-1. Loads rounded to three significant figures.
M18 pitches compared
All standard pitches on the M18 diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
Closest inch thread
No inch thread is interchangeable with M18×2.5. The nearest is 3/4-10 UNC, which is 1.05 mm larger in diameter with a pitch of 2.540 mm against 2.500 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
| Thread | Major Ø (mm) | Pitch (mm) | TPI | Ø difference (mm) |
|---|---|---|---|---|
| M18×2.5 (this thread) | 18.000 | 2.500 | 10.16 | — |
| 3/4-10 UNC | 19.050 | 2.540 | 10.00 | +1.050 |
| 3/4-16 UNF | 19.050 | 1.587 | 16.00 | +1.050 |
| 11/16-24 UNEF | 17.462 | 1.058 | 24.00 | -0.538 |
How the M18×2.5 dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 2.5 mm
- H = 0.866025 × P = 2.165 mm
- d2 = d − 0.649519 × P = 18 − 1.624 = 16.376 mm
- D1 = d − 1.082532 × P = 18 − 2.706 = 15.294 mm
- d3 = d − 1.226869 × P = 18 − 3.067 = 14.933 mm (external root, 17H/24 deep)
- As = π/4 × ((d2 + d3) ÷ 2)² = π/4 × 15.655² = 192.47 mm²
- Lead angle = arctan(P ÷ (π × d2)) = 2.78°
Related sizes and tools
Sources: ISO 68-1 and ISO 724 (profile and basic dimensions), ISO 261 (diameter and pitch series), ISO 965-1 (tolerances), ISO 273 (clearance holes), ISO 898-1 (mechanical properties) and tap drill tables based on ISO 2306. How this site checks its data. Last reviewed 29 September 2026.