BIZOMAT MAKEOS · Threads
ISO Metric Thread Basic Dimensions
What are the basic pitch, minor and root diameters and stress area of this metric thread?
Basic dimensions of an ISO metric (M) thread from nominal diameter and pitch: pitch and minor diameters, thread height, tensile stress area, lead angle and proof load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal diameter | mm | Nominal (basic major) diameter. |
| Pitch | mm | Pitch from the thread designation (coarse M12 is 1.75 mm). |
| Number of starts | starts | Number of starts; 1 for normal fastener threads. |
| Proof strength (0 = not needed) | MPa | Proof stress for the bolt property class (ISO 898-1), to give a proof load; 0 to skip. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Nominal diameter
- What it is
- The nominal (basic major) diameter of the ISO metric thread, the number after the M in the designation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the thread size, not the tap drill or clearance hole.
What this means: Pitch
- What it is
- The axial distance from one thread crest to the next, the number after the × in the designation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All basic diameters fall by fixed multiples of the pitch, so a coarser pitch reduces them and the stress area.
- Get it right
- A plain designation without a pitch means the coarse series; check the standard pitch for that size.
What this means: Number of starts
- What it is
- The number of separate thread helices wound side by side. The lead, the axial travel per turn, equals starts times pitch.
- Units and format
- Enter a number in starts: a number of thread starts. The form accepts 1 starts to 20 starts. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 starts as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More starts increase lead and lead angle in proportion; diameters and stress area do not change.
- Get it right
- Ordinary fastener threads are single start. Only the lead and lead angle use this value.
What this means: Proof strength (0 = not needed)
- What it is
- The proof stress of the bolt property class, used only to turn the tensile stress area into a proof load.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 skips the proof load. Take the value from ISO 898-1 or the supplier.
Method and formulas
- H = 0.866025P; d₂ = d − 0.649519P; D₁ = d − 1.082532P; d₃ = d − 1.226869P.
- Tensile stress area As = π/4 ((d₂ + d₃)/2)² = π/4 (d − 0.938194P)².
- Lead = starts × P; lead angle = atan(lead ÷ π d₂).
- Example: M12 × 1.75 has d₂ = 10.863 mm, D₁ = 10.106 mm and As = 84.267 mm².
Limitations
- Basic profile only; tolerance class limits (e.g. 6g/6H) come from ISO 965 tables.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check thread dimensions for design or inspection
- Find the stress area for bolt calculations
- Set CNC thread-milling and turning depths
Who it is for
- Design engineers
- CNC programmers
- Quality inspectors
What you get
- Basic pitch diameter
- Basic minor diameters (internal and external root)
- Fundamental triangle and thread heights
- Tensile stress area and proof load
- Lead and lead angle
Interpreting the result
- Use the stress area for bolt stress and the minor diameter for shear through the thread.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1867–1890. Topic reference; no source tables or recommended settings are reproduced.
- ISO 68-1 (basic profile) and ISO 724 (basic dimensions) — https://www.iso.org/standard/4166.html
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