BIZOMAT MAKEOS · Threads
ISO Metric Thread Limits of Size (6g, 6H and Others)
What are the limits of size for this metric thread in the chosen tolerance class?
Major, pitch and minor diameter limits of external and internal ISO metric threads for common tolerance classes (4h, 6h, 6g, 6f, 6e, 8g and 4H, 5H, 6H, 7H, 6G) from the ISO 965-1 tolerance formulas.
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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 | Thread pitch. |
| External class | choice | Tolerance class of the external thread (6g is the usual medium class). |
| Internal class | choice | Tolerance class of the internal thread (6H is the usual medium class). |
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 M in the designation, such as 10 for M10.
- Units and format
- Enter a number in mm: millimetres. The form accepts 1 mm to 600 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the nominal size, not a measured diameter; tolerances depend on the ISO diameter range it falls in.
What this means: Pitch
- What it is
- The distance between adjacent thread crests along the axis. Coarse threads often omit it from the designation; fine threads state it, as in M10 × 1.25.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.2 mm to 8 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger pitch gives larger fundamental deviations and wider tolerances.
- Get it right
- For coarse threads look up the pitch in the ISO coarse series, as it is not written in the designation.
What this means: External class
- What it is
- The tolerance class of the external (bolt or screw) thread: the number is the tolerance grade and the lower-case letter is the position (fundamental deviation).
- Units and format
- Choose one of 6 options from the list: 4h; 6h; 6g; 6f; 6e; 8g.
- Example
- The form opens with “6g” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- A higher grade number widens the tolerances; e, f and g move the limits below basic size.
- Get it right
- Letters e, f and g leave a gap below the basic size for coatings; h has none.
What this means: Internal class
- What it is
- The tolerance class of the internal (nut or tapped hole) thread: the number is the tolerance grade and the upper-case letter is the position (fundamental deviation).
- Units and format
- Choose one of 5 options from the list: 4H; 5H; 6H; 7H; 6G.
- Example
- The form opens with “6H” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- A higher grade number widens the tolerances; G moves the limits above basic size.
- Get it right
- H sits at the basic size; G is moved above it to leave room for coatings.
Method and formulas
- Fundamental deviations (µm): e = −(50 + 11P), f = −(30 + 11P), g = −(15 + 11P), h = 0; G = +(15 + 11P), H = 0.
- Grade 6 tolerances: Td = 180P^(2/3) − 3.15P^(−1/2); Td2 = 90P^0.4 d^0.1 (d = geometric mean of the ISO diameter range); TD2 = 1.32 Td2; TD1 = 433P − 190P^1.22 (P < 1) or 230P^0.7. Grades 4, 5, 7, 8 multiply by 0.63, 0.8, 1.25, 1.6.
- Tolerances are rounded to the R40 preferred-number series and deviations to whole micrometres, as ISO 965 does. Limits: d max = d + es, d2 max = d2 + es, minus the tolerances; internal D2 min = d2 + EI, D1 min = D1 + EI, plus the tolerances.
- Example: M10 × 1.5-6g has pitch diameter 8.994 / 8.862 mm; 6H has 9.026 / 9.196 mm.
Limitations
- ISO 965 tabulates values rounded to a preferred series; formula values can differ by a few micrometres. Use the ISO 965-2/-3 tables for acceptance.
- Root contour and minor diameter of external threads are not limited here.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find inspection limits for special metric threads
- Set thread turning and milling sizes
- Compare tolerance classes
Who it is for
- Quality inspectors
- Machinists
- Design engineers
What you get
- External major and pitch diameter limits
- Internal minor and pitch diameter limits
- Fundamental deviations and tolerances
Interpreting the result
- Use these values to set up machining; inspect with gauges or the tabulated limits.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1867–1912. Topic reference; no source tables or recommended settings are reproduced.
- ISO 965-1 ISO general purpose metric screw threads — Tolerances
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Open the ISO Metric Thread Limits of Size (6g, 6H and Others) calculator