DIMENSIONS AND FITS
Tolerance calculator.
ISO 286 size limits for round, hexagonal, square and flat dimensions. Shaft and hole tolerances, clearances, interferences and transition fits.
Dimension and tolerance
Quick entry by tolerance notation
Examples: 20 h9 · 25 H7/g6
Verified table coverage: 0 < size ≤ 3150 mm; class-specific limits apply. Lists change with dimension. A selected class is not a delivery tolerance commitment.
TOLERANCE CALCULATION
Size limits
Diameter
h9- Minimum size
- 19.948 mm
- Maximum size
- 20 mm
- Lower deviation
- -52 µm (-0.052 mm)
- Upper deviation
- 0 µm (0 mm)
- Tolerance width
- 52 µm (0.052 mm)
Check a measured size
This check compares numerical size limits only; it does not assess measurement uncertainty or form tolerances.
The result shows size limits for the selected ISO class. Ovality, straightness, surface roughness and temperature effects are outside this calculation.
What does the class beside a dimension mean?
20 mm is the nominal size of a feature; h9 defines the interval in which its size may lie. ISO tolerance calculation translates that letter-and-number designation into a minimum size, maximum size and limit deviations. A class appearing on a drawing or in product information can then be read as actual dimensions in millimetres.
The letter locates the tolerance interval relative to nominal size, while the number identifies the grade that determines its width. At the same size, h9 and h11 give intervals of different widths. The numerical value of a class can also change with nominal size: the class alone is not enough to calculate a result.
Tolerances for round, hexagonal, square and flat sections
For round bar, the calculation uses diameter. A hexagonal section uses the across-flats dimension between opposite flat faces, rather than the distance between corners. A square section uses its side dimension. The same ISO class may be selected for these sections, but the dimension refers to a different part of each shape.
Flat-bar width and thickness are separate dimensions. In a 50 × 10 mm section, the 50 mm width and 10 mm thickness are calculated in their respective size ranges. Even with the same class, their tolerance widths need not match. Selecting a section defines the dimension being considered; it does not automatically assign a tolerance class to a supplied product.
How many millimetres is an h9 tolerance?
For a 20 mm h9 external dimension, the upper deviation is zero and the lower deviation is −0.052 mm. The maximum size is 20.000 mm and the minimum is 19.948 mm. The difference is 0.052 mm, or 52 µm. This does not mean ±0.052 mm: the h tolerance interval lies below nominal size.
Size ranges in the tolerance table are part of the calculation. A range described as over 18 mm up to and including 30 mm contains 20 mm and exactly 30 mm; exactly 18 mm belongs to the preceding range. Using the next row at a boundary can therefore produce a different and incorrect result.
What does a hole-and-shaft fit calculation show?
A single-size calculation gives the limits of one feature. A fit calculation considers a hole and shaft sharing the same nominal size. In a designation such as H7/g6, H7 is the hole class and g6 is the shaft class. The smallest hole paired with the largest shaft gives the tightest combination; the largest hole with the smallest shaft gives the loosest.
A clearance fit leaves clearance throughout the permitted size combinations, possibly reaching zero at a limit. In an interference fit, the shaft is larger than the hole. A transition fit can produce either clearance or interference, depending on actual sizes. The calculated interval describes dimensional fit; it does not by itself determine suitable loading, rotational speed or assembly force.
Material grade and tolerance are different properties
Grades such as 303, 316L and 17-4 PH identify the material. A designation such as h9 or H7 defines dimensional limits. At the same nominal size and ISO class, the calculated tolerance does not change when stainless steel is replaced by another metal. Conversely, two products of the same material grade can have different dimensional tolerances.
Size tolerance does not fully describe surface roughness, straightness, roundness or corner geometry. The calculator shows the limits for the selected size and class. Material weight is a separate calculation: it requires density as well as section dimensions and length. Passing the same section dimensions to the weight tool makes it easier to examine both pieces of information.
Frequently asked questions
Is a material grade needed to calculate an ISO tolerance?
No. A nominal size and tolerance class are sufficient for the dimensional calculation. Material grade describes material properties; it does not change the numerical limits of an ISO 286 class.
What is the difference between h7 and H7?
Lowercase h is used for an external feature or shaft; uppercase H is used for an internal feature or hole. The upper deviation is zero for h, while the lower deviation is zero for H. At the same nominal size, h7 and H7 therefore lie on different sides of the zero line.
How do h9 and h11 tolerances differ?
At the same nominal size, h11 gives a wider permitted interval than h9. Both have their upper limit at nominal size; the difference appears at the lower limit. A narrower interval does not mean greater material strength or corrosion resistance.
What are the size limits for 20 mm h9?
For 20 mm h9, the limits are 19.948–20.000 mm. The lower deviation is −52 µm, the upper deviation is 0 µm and the tolerance width is 52 µm.
Does a hexagonal section use the corner-to-corner dimension?
No. Across flats is the distance between opposite flat faces of a hexagonal bar. The corner-to-corner distance is a different geometric dimension; substituting it applies the calculation to the wrong size.
Are flat-bar width and thickness calculated together?
Width and thickness are calculated independently, and each can have a different class. Even if the same class is selected, their numerical tolerances can differ when the dimensions fall into different standard size ranges.
What does an H7/g6 fit calculation mean?
H7/g6 pairs an H7 hole with a g6 shaft at the same nominal size. The tool calculates the size limits of both features and the smallest and largest clearance resulting from those limits.
Is a tolerance result always a plus-or-minus value?
No. An interval can lie below nominal size, above it or across both sides. Results are read as lower and upper deviations. The total tolerance width is not automatically a value that can be written with a ± sign.
How many millimetres is one micrometre?
1 µm equals 0.001 mm. For example, 52 µm = 0.052 mm. Displaying size limits in millimetres and small deviations in micrometres provides two scales for reading the same result.
Does the result confirm a product is supplied to that tolerance?
No. The calculation shows the dimensional meaning of the selected class. A particular product’s supplied tolerance depends on its product definition and applicable requirements. Calculating an h9 interval does not mean that every round bar is supplied to h9.
How do ISO 286 and ISO 2768 differ?
ISO 286 covers size tolerances and fits identified individually by classes such as h9 and H7. ISO 2768 addresses general tolerances for dimensions without individual tolerance indications. This tool calculates ISO 286 limits; it does not replace an ISO 2768 general-tolerance table.