BIZOMAT MAKEOS · Quality
ISO Tolerance Grade (IT) Value
How large is the ISO IT tolerance grade for this nominal size?
Standard tolerance for ISO 286 grades IT5 to IT16 calculated from the standard tolerance unit for the size step that contains the nominal size.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal size | mm | Basic size of the feature from the drawing (up to 3150 mm). |
| Tolerance grade | choice | The IT grade specified or being considered. |
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 size
- What it is
- The basic size of the feature, such as a shaft or hole diameter, as shown on the drawing. It selects the ISO 286 size step used in the formula.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3150 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A size exactly on a step boundary belongs to the lower step: 18 mm falls in the over 10 to 18 mm step.
What this means: Tolerance grade
- What it is
- The international tolerance (IT) grade specified on the drawing or under consideration. A higher grade number means a wider tolerance band.
- Units and format
- Choose one of 12 options from the list: IT5; IT6; IT7; IT8; IT9; IT10; IT11; IT12; IT13; IT14; IT15; IT16.
- Example
- The form opens with “IT7” selected as a worked example. Choose the option that matches your case.
- Get it right
- The tool gives the formula value; drawings and inspection use the rounded ISO 286-1 table value.
Method and formulas
- D = geometric mean of the ISO 286 size step containing the size (√(lower × upper); the first step uses 1 and 3 mm).
- Up to 500 mm: i = 0.45∛D + 0.001D µm. Above 500 mm: I = 0.004D + 2.1 µm.
- IT = k × i, with k = 7, 10, 16, 25, 40, 64, 100, 160, 250, 400, 640, 1000 for IT5 to IT16.
- Example: 25 mm in IT7 lies in the 18–30 mm step; the formula gives 20.918 µm.
Limitations
- ISO 286-1 tabulates rounded values; use the table value on drawings. The formula value can differ by about 1 µm or by rounding steps for coarse grades.
- Grades finer than IT5 use different formulas and are not covered.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate the tolerance for a chosen IT grade
- Compare grades for a process capability discussion
- Check a fit tolerance quickly
Who it is for
- Design engineers
- Quality engineers
- Machinists
What you get
- Formula tolerance value
- Size step and its geometric mean
- Standard tolerance unit
Interpreting the result
- Use the result to compare grades; take limits for specific fits (e.g. H7/g6) from ISO 286-2.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 635–668. Topic reference; no source tables or recommended settings are reproduced.
- ISO 286-1:2010 Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — https://www.iso.org/standard/45975.html
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