M8 thread pitch and dimensions
The M8 thread pitch is 1.25 mm (ISO 261 coarse), about 20.3 threads per inch. Its basic pitch diameter is 7.188 mm and its minor diameter is 6.647 mm in the nut and 6.466 mm on the bolt (ISO 724 / ISO 898-1).
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| Dimension | Formula | mm | inch |
|---|---|---|---|
| Major diameter d | nominal | 8.000 | 0.3150 |
| Pitch P | ISO 261 | 1.250 | 0.0492 |
| Pitch diameter d2 | d − 0.649519 P | 7.188 | 0.2830 |
| Minor diameter D1 (nut) | d − 1.082532 P | 6.647 | 0.2617 |
| Minor diameter d3 (bolt) | d − 1.226869 P | 6.466 | 0.2546 |
| Fundamental triangle H | 0.866025 P | 1.083 | 0.0426 |
| Thread overlap H1 | 0.541266 P | 0.677 | 0.0266 |
| Thread | Pitch | Series | Pitch Ø d2 | Stress area As | Tap drill |
|---|---|---|---|---|---|
| M8 x 1.25 (this page) | 1.25 mm | coarse | 7.188 mm | 36.6 mm² | 6.75 mm |
| M8 x 1.0 | 1.0 mm | fine | 7.350 mm | 39.2 mm² | 7.0 mm |
| Thread | Pitch | Minor Ø nut D1 | Stress area As | Hex AF |
|---|---|---|---|---|
| M5 x 0.8 | 0.8 mm | 4.134 mm | 14.2 mm² | 8.0 mm |
| M6 x 1.0 | 1.0 mm | 4.917 mm | 20.1 mm² | 10.0 mm |
| M8 x 1.25 (this page) | 1.25 mm | 6.647 mm | 36.6 mm² | 13.0 mm |
| M10 x 1.5 | 1.5 mm | 8.376 mm | 58.0 mm² | 16.0 mm |
| M12 x 1.75 | 1.75 mm | 10.106 mm | 84.3 mm² | 18.0 mm |
Practical notes
Metric threads are named by major diameter and pitch: M8 x 1.25 means a 8 mm nominal diameter with 1.25 mm between crests. When no pitch is written (just "M8"), 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. M8 x 1.25 has about 20.3 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 M8 x 1.25 it is 36.6 mm².
FAQ
What is the pitch of an M8 thread?
The standard (coarse) pitch of M8 is 1.25 mm per ISO 261. Fine pitches also exist: 1.0 mm.
What is the minor diameter of M8 x 1.25?
The basic minor diameter is 6.647 mm for the internal thread (nut, D1 = d − 1.082532 P) and 6.466 mm for the external thread (bolt, d3 = d − 1.226869 P).
What is the tensile stress area of M8 x 1.25?
As = π/4 × ((d2 + d3)/2)² = 36.6 mm² for M8 x 1.25 (ISO 898-1).
What tap drill does M8 x 1.25 need?
6.75 mm for a cut tap (major diameter minus pitch, ISO 2306), which gives about 77% thread.
What spanner size fits an M8 bolt?
ISO 4032 nuts and ISO 4014/4017 bolts in M8 are 13.0 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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