Example Data Table
| Nominal Size | Hole Zone | Common Use | Expected Reading |
|---|---|---|---|
| 10 mm | H7 | Dowel location hole | Lower limit starts at nominal |
| 25 mm | H8 | General machined bore | Wider production tolerance |
| 50 mm | JS7 | Centered tolerance zone | Limits split around nominal |
| 100 mm | G7 | Allowance above nominal | Positive lower deviation |
Formula Used
For sizes up to 500 mm, the standard tolerance unit is: i = 0.45 × D1/3 + 0.001 × D. Here, D is the geometric mean of the nominal diameter step. The standard tolerance is: IT = grade multiplier × i.
The lower hole deviation is EI. The upper deviation is: ES = EI + IT. The lower limit is: nominal + EI. The upper limit is: nominal + ES. This calculator also adjusts for coating, target offset, and thermal growth.
For H holes, EI is zero. For JS holes, the zone is centered around nominal. Other zone letters use built-in reference offsets. For strict inspection work, enter the exact table deviation using the custom EI option.
How To Use This Calculator
Enter the nominal hole diameter first. Select millimeters or inches. Choose the hole position letter and tolerance grade. Use H7 for many common hole basis fits. Add a measured actual size if you want a pass or fail check. Add optional shaft limits to estimate clearance, transition, or interference fit behavior.
Use coating thickness when plating or coating reduces the finished bore. Use target offset when the process intentionally aims above or below the nominal value. Enter the temperature fields when the part is checked away from the reference temperature. Then press the calculate button.
ISO Hole Tolerance Guide
Why Hole Tolerance Matters
Hole tolerance controls the accepted size range of a finished bore. It protects assembly function, inspection consistency, and machining cost. A hole can look simple, yet its limit values decide whether a shaft, pin, bearing, or fastener will fit correctly. Small errors may create loose movement, high press force, scrap, or rework.
Understanding The Hole Basis System
Many designs use a hole basis system. In this system, the H hole is very common because its lower deviation is zero. The minimum H hole equals the nominal size before extra process adjustments. The shaft tolerance is then selected to create clearance, transition, or interference.
Using Grades Correctly
IT grades describe tolerance width. Lower grade numbers are tighter. Higher grade numbers are wider. Tight grades usually need better machines, stable temperature, careful tooling, and stronger inspection. Wide grades are easier to make, but they may reduce fit control.
Advanced Shop Adjustments
Real parts often need more than a basic table value. Coating can reduce the usable bore. Temperature can change measured size. A target offset may be added for process capability. This tool includes those fields so planners can test practical conditions before release.
Inspection Use
Enter the measured hole size to check the part against the calculated limits. The result shows whether the feature is low, high, or inside tolerance. When shaft limits are entered, the calculator also reports fit type and clearance range.
Best Practice
Use standard tables for final drawings and audits. Use this calculator for planning, review, and quick checks. When a zone letter uses a special table value, choose custom EI and enter the exact deviation from your approved specification.
FAQs
What is an ISO hole tolerance?
It is the permitted size range for a hole. It uses a nominal diameter, a position letter, and an IT grade to define lower and upper limits.
What does H7 mean?
H7 means the hole uses H position with IT7 tolerance width. For an H hole, the lower deviation is zero before added process adjustments.
What is EI?
EI is the lower deviation of the hole from nominal size. It tells how far the minimum hole limit sits from the basic size.
What is ES?
ES is the upper deviation of the hole from nominal size. It equals the lower deviation plus the selected standard tolerance width.
Why add coating thickness?
Coating or plating can reduce the final usable bore. The calculator subtracts twice the per-side coating thickness from the hole limits.
Why include temperature?
Metal size changes with temperature. The thermal field helps estimate the size shift when measuring away from the reference temperature.
Can I check a shaft fit?
Yes. Enter shaft minimum and maximum sizes. The calculator estimates minimum clearance, maximum clearance, and the likely fit type.
Should custom EI be used?
Use custom EI when your drawing, standard table, or quality system gives an exact lower deviation. This improves traceability for final checks.