Use this chart to decode common ISO metric thread designations. Diameter and pitch together identify the nominal thread geometry; they do not specify a finished hole tolerance.
| Nominal thread | Coarse pitch (mm) | Selected fine pitch (mm) |
|---|---|---|
| M2 | 0.4 | 0.25 |
| M2.5 | 0.45 | 0.35 |
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1 | 0.75 |
| M8 | 1.25 | 1 |
| M10 | 1.5 | 1.25 |
| M12 | 1.75 | 1.5 |
| M16 | 2 | 1.5 |
| M20 | 2.5 | 1.5 |
| M24 | 3 | 2 |
Read the callout before selecting a tool
M10 × 1.5 means a nominal 10 mm diameter and 1.5 mm axial pitch. The M10 name is not the tapping drill diameter. A full drawing may also specify internal or external tolerance class, thread length, handedness and number of starts. Use those details when ordering gauges and tools.
The fine column shows one common option per diameter. It is not a statement that every supplier stocks that combination or that other fine pitches are invalid. Where a pitch is omitted under an applicable metric-thread convention, the coarse series may be intended; resolve any uncertainty from the drawing specification rather than guessing.
A practical identification example
If an approximately 10 mm external thread has ten pitch intervals spanning 15 mm, the measured pitch is 1.5 mm. If the same interval count spans 12.5 mm, it suggests 1.25 mm. Measure intervals rather than simply counting visible crests, and confirm with a pitch gauge. Diameter measurement alone cannot distinguish these threads.
For a single-start thread, one revolution advances by one pitch. A multi-start screw advances by lead, which equals pitch times number of starts. This distinction matters for motion screws and feed calculations.
Use a separate reference for tapping and fit
The quick approximation nominal diameter minus pitch is only a starting point for some cut-tap drills. Form taps use different pre-hole requirements, and thread percentage, material and tolerance change the selection. Use the linked tap drill chart and actual tap documentation. Never force together parts simply because their nominal diameters match.
Additional coarse sizes and selected fine choices
Use these rows with the earlier M2–M24 table. Coarse pitches are checked against TR Fastenings’ metric-coarse geometry table; the fine examples are listed in its metric-fine table. An omitted fine pitch is not necessarily invalid.
| Nominal diameter | Coarse pitch, mm | Selected fine pitch, mm |
|---|---|---|
| M14 | 2.00 | 1.50 |
| M18 | 2.50 | 1.50 |
| M22 | 2.50 | 1.50 |
| M27 | 3.00 | 2.00 |
| M30 | 3.50 | 2.00 |
| M36 | 4.00 | 3.00 |
Check ten intervals, not ten crests
For an M14×2 thread, ten complete pitch intervals span 20 mm. For M14×1.5, the same interval count spans 15 mm. Counting ten crests from the first to the tenth includes only nine intervals: 18 mm and 13.5 mm respectively. This counting distinction can otherwise lead to a false pitch estimate.
Diameter and pitch identify a nominal thread, but a purchasing or inspection callout also needs the relevant tolerance class, internal/external form, length and handedness. M14×1.5 and M14×2 do not become interchangeable because the outside diameters are similar. Use a pitch gauge and the documented mating specification; do not force parts together to identify them.
For single-start motion, one revolution advances one pitch. A deliberately specified two-start 1.5 mm-pitch thread would advance 3 mm per revolution; the pitch gauge still measures 1.5 mm. Use the pitch and lead calculator to keep those two quantities separate. That example explains geometry, not a standard fastener substitution.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.
References and verification
Original explanations and worked examples by MfgWorkbench. Sources below support the stated concepts or selected reference data; they do not certify every example or a production application.
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