M18 thread pitch and dimensions
The M18 thread pitch is 2.5 mm (ISO 261 coarse), about 10.2 threads per inch. Its basic pitch diameter is 16.376 mm and its minor diameter is 15.294 mm in the nut and 14.933 mm on the bolt (ISO 724 / ISO 898-1).
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| Dimension | Formula | mm | inch |
|---|---|---|---|
| Major diameter d | nominal | 18.000 | 0.7087 |
| Pitch P | ISO 261 | 2.500 | 0.0984 |
| Pitch diameter d2 | d − 0.649519 P | 16.376 | 0.6447 |
| Minor diameter D1 (nut) | d − 1.082532 P | 15.294 | 0.6021 |
| Minor diameter d3 (bolt) | d − 1.226869 P | 14.933 | 0.5879 |
| Fundamental triangle H | 0.866025 P | 2.165 | 0.0852 |
| Thread overlap H1 | 0.541266 P | 1.353 | 0.0533 |
| Thread | Pitch | Series | Pitch Ø d2 | Stress area As | Tap drill |
|---|---|---|---|---|---|
| M18 x 2.5 (this page) | 2.5 mm | coarse | 16.376 mm | 192 mm² | 15.5 mm |
| M18 x 1.5 | 1.5 mm | fine | 17.026 mm | 216 mm² | 16.5 mm |
| Thread | Pitch | Minor Ø nut D1 | Stress area As | Hex AF |
|---|---|---|---|---|
| M14 x 2.0 | 2.0 mm | 11.835 mm | 115 mm² | 21.0 mm |
| M16 x 2.0 | 2.0 mm | 13.835 mm | 157 mm² | 24.0 mm |
| M18 x 2.5 (this page) | 2.5 mm | 15.294 mm | 192 mm² | 27.0 mm |
| M20 x 2.5 | 2.5 mm | 17.294 mm | 245 mm² | 30.0 mm |
| M22 x 2.5 | 2.5 mm | 19.294 mm | 303 mm² | 34.0 mm |
Practical notes
Metric threads are named by major diameter and pitch: M18 x 2.5 means a 18 mm nominal diameter with 2.5 mm between crests. When no pitch is written (just "M18"), the ISO 261 coarse pitch is meant.
The pitch diameter and minor diameters above are basic (zero-tolerance) sizes. Real nuts and bolts are made to a tolerance class, usually 6H for the nut and 6g for the bolt (ISO 965), so a measured bolt is slightly under these values.
To identify a thread, measure the major diameter with calipers and the pitch with a thread gauge, or count crests over 10 mm. M18 x 2.5 has about 10.2 threads per inch, which is close to, but not the same as, any Unified thread; an inch nut will not run on it.
Tensile stress area As is what bolt strength is calculated on (proof load = As × proof stress for the property class in ISO 898-1). For M18 x 2.5 it is 192 mm².
FAQ
What is the pitch of an M18 thread?
The standard (coarse) pitch of M18 is 2.5 mm per ISO 261. Fine pitches also exist: 1.5 mm.
What is the minor diameter of M18 x 2.5?
The basic minor diameter is 15.294 mm for the internal thread (nut, D1 = d − 1.082532 P) and 14.933 mm for the external thread (bolt, d3 = d − 1.226869 P).
What is the tensile stress area of M18 x 2.5?
As = π/4 × ((d2 + d3)/2)² = 192 mm² for M18 x 2.5 (ISO 898-1).
What tap drill does M18 x 2.5 need?
15.5 mm for a cut tap (major diameter minus pitch, ISO 2306), which gives about 77% thread.
What spanner size fits an M18 bolt?
ISO 4032 nuts and ISO 4014/4017 bolts in M18 are 27.0 mm across flats. Older DIN 934/933 parts can differ at M10, M12, M14 and M22.
Sources: ISO 261 — ISO general purpose metric screw threads, general plan (pitch series) · ISO 724 — ISO general purpose metric screw threads, basic dimensions · ISO 898-1 — mechanical properties of fasteners (minor diameter d3, tensile stress area As) · ISO 4032 / ISO 4014 — hexagon nuts and bolts (width across flats)
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