ISO 286 tolerances define the permitted dimensional variation of holes and shafts used in engineering components. Designations such as H7, g6 and H7/g6 identify the position and width of the tolerance zones relative to the basic or nominal size.
This guide explains how to use the RoyMech ISO tolerance tables to determine hole and shaft limits and to calculate the resulting clearance or interference between mating components. Worked examples show how the tolerance band, fundamental deviation and direct ISO 286-2 tables relate to each other.
Tolerance values in the RoyMech tables are normally given in micrometres (µm), where 1 µm = 0.001 mm.
Consider a nominal 40 mm diameter hole and shaft specified as:
Ø40 H7/g6
| Part | Meaning |
| 40 | Basic or nominal diameter in millimetres |
| H | Position of the hole tolerance zone relative to the basic size |
| 7 | IT tolerance grade of the hole |
| g | Position of the shaft tolerance zone relative to the basic size |
| 6 | IT tolerance grade of the shaft |
Upper-case letters are used for holes and lower-case letters are used for shafts. Therefore H7 defines the hole tolerance, g6 defines the shaft tolerance, and H7/g6 defines the combination of the two tolerances used for the fit.
The basic size is represented by the zero line. All tolerance deviations are measured from this size. A positive deviation is above the basic size and a negative deviation is below it.
The letter in a tolerance designation determines the location of the tolerance zone relative to this zero line. The IT grade determines the width of the tolerance zone.
Different symbols are used for holes and shafts:
| Component | Upper Deviation | Lower Deviation |
| Hole | ES | EI |
| Shaft | es | ei |
The maximum and minimum permitted dimensions are obtained by adding the appropriate deviation to the basic size:
Maximum hole size = Basic size + ES
Minimum hole size = Basic size + EI
Maximum shaft size = Basic size + es
Minimum shaft size = Basic size + ei
Consider a 40 mm H7 hole. The number 7 specifies the IT tolerance grade.
First use the ISO 286 Tolerance Band Table – 1 to 315 mm. For a basic size of 40 mm, the IT7 tolerance width is:
IT7 = 25 µm = 0.025 mm
The tolerance band tells us the total permitted variation in size. It does not, by itself, tell us where that tolerance zone lies relative to the 40 mm basic size.
To determine the position of the tolerance zone we also need the fundamental deviation.
For the 40 H7 hole, use the ISO Hole Fundamental Deviations – 0 to 500 mm.
For an H hole the lower deviation EI is on the zero line:
EI = 0 µm
From the IT tolerance table:
IT7 = 25 µm
Therefore the upper deviation is:
ES = EI + IT7
ES = 0 + 25 = +25 µm
Converting the deviations to millimetres:
EI = 0.000 mm
ES = +0.025 mm
The permitted hole dimensions are therefore:
Minimum hole = 40.000 mm
Maximum hole = 40.025 mm
Now consider the mating 40 g6 shaft. Use the ISO Shaft Fundamental Deviations – 0 to 500 mm.
For a 40 mm g shaft the table gives the upper deviation:
es = -9 µm
The IT6 tolerance width for this size is:
IT6 = 16 µm
The lower deviation is therefore:
ei = es - IT6
ei = -9 - 16
ei = -25 µm
Converting to millimetres:
es = -0.009 mm
ei = -0.025 mm
The permitted shaft dimensions are:
Maximum shaft = 40 - 0.009 = 39.991 mm
Minimum shaft = 40 - 0.025 = 39.975 mm
Therefore:
Ø40 g6 = 39.975 mm to 39.991 mm
The previous examples show how a tolerance can be determined from the IT tolerance width and fundamental deviation tables. RoyMech also provides direct ISO 286-2 limit tables.
Using the ISO 286-2 Hole Tolerance Limits – 3 to 400 mm, locate the nominal size containing 40 mm and then locate the H7 row.
The table gives the deviations directly:
ES = +25 µm
EI = 0 µm
Therefore:
Ø40 H7 = 40.000 mm to 40.025 mm
This is the same result obtained by combining the IT7 tolerance band with the H fundamental deviation in the previous worked example.
The same shortcut can be used for the shaft. From the ISO 286-2 Shaft Tolerance Limits – 3 to 400 mm, the 40 g6 shaft has:
es = -9 µm
ei = -25 µm
Therefore:
Ø40 g6 = 39.975 mm to 39.991 mm
Again, this agrees with the result calculated using the shaft fundamental deviation and IT6 tolerance band.
We can now combine the hole and shaft limits to determine the actual range of clearance for the Ø40 H7/g6 fit.
| Component | Minimum Size | Maximum Size |
| 40 H7 hole | 40.000 mm | 40.025 mm |
| 40 g6 shaft | 39.975 mm | 39.991 mm |
Minimum clearance occurs with the smallest permitted hole and the largest permitted shaft.
Minimum clearance = 40.000 - 39.991
= 0.009 mm
Maximum clearance occurs with the largest permitted hole and the smallest permitted shaft.
Maximum clearance = 40.025 - 39.975
= 0.050 mm
Ø40 H7/g6 clearance = 0.009 mm to 0.050 mm
Because there is clearance under both worst-case conditions, H7/g6 is a clearance fit.
Once the maximum and minimum sizes of the hole and shaft have been calculated, the resulting fit can be classified as a clearance fit, transition fit or interference fit.
| Fit Type | Description |
| Clearance Fit | The shaft is always smaller than the hole. Clearance exists between the mating components under all tolerance conditions. |
| Transition Fit | Depending on the actual manufactured sizes, the assembly may have either a small clearance or a small interference. |
| Interference Fit | The shaft is larger than the hole under the specified tolerance conditions, producing interference between the mating components. |
Different ISO tolerance combinations are used to obtain these fit conditions. For examples of commonly used hole-basis fits, including running, sliding, location, transition, press and force fits, see the ISO Limits, Fits and Tolerances reference page.
In the hole-basis system the hole normally uses an H tolerance position. The lower deviation of an H hole is zero, so the minimum hole size is equal to the basic size.
Different types of fit can then be obtained by changing the shaft tolerance position while retaining the same basic hole tolerance.
This is convenient in manufacturing because standard hole-producing tools and processes can be retained while the shaft tolerance is selected to obtain the required fit.
For common combinations of hole and shaft tolerances, see the ISO Limits, Fits and Tolerances page.
The following example demonstrates a larger tolerance grade using the same method.
For a 110 mm H11 hole:
EI = 0 µm
IT11 = 220 µm
Therefore:
ES = 0 + 220 = +220 µm = +0.220 mm
Minimum hole = 110.000 mm
Maximum hole = 110.220 mm
For a 110 mm e9 shaft, the fundamental deviation nearest the zero line is:
es = -72 µm = -0.072 mm
The IT9 tolerance width is:
IT9 = 87 µm = 0.087 mm
Therefore:
ei = -72 - 87 = -159 µm = -0.159 mm
Maximum shaft = 110 - 0.072 = 109.928 mm
Minimum shaft = 110 - 0.159 = 109.841 mm
The permitted shaft size is therefore:
109.841 mm to 109.928 mm
Not every tolerance zone lies entirely above or below the basic size. Some tolerance positions can extend across the zero line. This makes it especially important to identify whether the value in the fundamental deviation table represents the upper or lower deviation.
Consider a 300 K7 hole.
The tolerance width is:
IT7 = 52 µm = 0.052 mm
For this K7 hole the upper deviation is:
ES = +16 µm = +0.016 mm
The lower deviation is therefore:
EI = ES - IT7
EI = 16 - 52
EI = -36 µm = -0.036 mm
The resulting hole limits are:
Minimum hole = 300 - 0.036 = 299.964 mm
Maximum hole = 300 + 0.016 = 300.016 mm
This example demonstrates why it is important not to assume that all hole tolerance zones are entirely above the zero line or that all shaft tolerance zones are entirely below it.
RoyMech provides separate tolerance tables for larger diameters. The method is the same as for the smaller diameter tables, but the correct size range must be selected.
Consider a nominal diameter of 1300 mm. This diameter is greater than 1250 mm and up to and including 1600 mm, so the >1250 to 1600 mm size band is used in the direct ISO 286-2 tables.
From the large-diameter ISO 286-2 hole table:
ES = +345 µm
EI = +220 µm
The tolerance width is therefore:
T = ES - EI
T = 345 - 220
T = 125 µm = 0.125 mm
The permitted hole dimensions are:
Minimum hole = 1300 + 0.220 = 1300.220 mm
Maximum hole = 1300 + 0.345 = 1300.345 mm
For the corresponding e7 shaft the tolerance zone lies below the basic size. The upper deviation is the deviation nearest the zero line:
es = -220 µm
Using the same IT7 tolerance width of 125 µm:
ei = es - IT7
ei = -220 - 125
ei = -345 µm
The shaft dimensions are therefore:
Maximum shaft = 1300 - 0.220 = 1299.780 mm
Minimum shaft = 1300 - 0.345 = 1299.655 mm
The correct order of the shaft deviations is therefore es = -220 µm followed by ei = -345 µm. The upper deviation represents the larger permitted shaft dimension.
Care is required when the basic size lies exactly on a boundary between two table ranges.
Where a table is headed Over and Up to and including, a dimension equal to the upper limit remains in that range.
| Basic Size | Applicable Range |
| 1250 mm | Over 1000 mm up to and including 1250 mm |
| 1300 mm | Over 1250 mm up to and including 1600 mm |
Selecting the wrong size band will result in incorrect tolerance values, even when the tolerance designation itself is correct.
The RoyMech tolerance section provides two ways of obtaining dimensional limits.
| Method | Procedure | Useful For |
| Fundamental deviation method | Find the IT tolerance width, find the fundamental deviation, calculate the second deviation, then calculate the dimensional limits. | Understanding how the ISO tolerance system works and determining tolerance positions not available in the direct tables. |
| Direct ISO 286-2 tables | Select the basic size range and tolerance designation and read the upper and lower deviations directly. | Quick engineering reference when the required tolerance designation is included in the table. |
| Table | Size Range | Purpose |
| ISO 286 IT Tolerance Grades | 1 to 315 mm | Determine the width of the IT tolerance band. |
| ISO 286 IT Tolerance Grades | 250 to 3150 mm | IT tolerance widths for larger basic sizes. |
| Hole Fundamental Deviations | 0 to 500 mm | Determine the position of a hole tolerance zone. |
| Hole Fundamental Deviations | 500 to 3150 mm | Hole fundamental deviations for larger sizes. |
| Shaft Fundamental Deviations | 0 to 500 mm | Determine the position of a shaft tolerance zone. |
| Shaft Fundamental Deviations | 500 to 3150 mm | Shaft fundamental deviations for larger sizes. |
| ISO 286-2 Hole Tolerance Limits | 3 to 400 mm | Direct lookup of upper and lower hole deviations. |
| ISO 286-2 Hole Tolerance Limits | 400 to 3150 mm | Direct hole tolerance lookup for larger sizes. |
| ISO 286-2 Shaft Tolerance Limits | 3 to 400 mm | Direct lookup of upper and lower shaft deviations. |
| ISO 286-2 Shaft Tolerance Limits | 400 to 3150 mm | Direct shaft tolerance lookup for larger sizes. |