Interactive Fits & Tolerances Calculator
Enter a basic (nominal) size and the upper/lower deviations for hole and shaft. The calculator returns size limits, clearance/interference range, and fit class.
Tip: For ISO-style examples, try H7/g6 around 25 mm using the default values above.
What is a fit in mechanical design?
A fit defines how tightly or loosely two mating parts assemble, typically a shaft inside a hole. Because no manufactured feature is perfectly exact, every size is controlled by a tolerance range. The overlap (or gap) between these ranges determines whether parts slide freely, align snugly, or require press force.
Key terms you should know
- Nominal size: The target design size (basic size).
- Upper deviation: Maximum allowed size offset from nominal.
- Lower deviation: Minimum allowed size offset from nominal.
- Clearance: Hole size minus shaft size. Positive value means gap.
- Interference: Shaft larger than hole in assembly condition.
How this calculator works
1) Convert deviations to size limits
The calculator first computes min/max sizes for both hole and shaft:
- Hole Min = Nominal + Hole Lower Deviation
- Hole Max = Nominal + Hole Upper Deviation
- Shaft Min = Nominal + Shaft Lower Deviation
- Shaft Max = Nominal + Shaft Upper Deviation
2) Compute fit range
Then it evaluates two assembly extremes:
- Minimum clearance (tightest case) = Hole Min - Shaft Max
- Maximum clearance (loosest case) = Hole Max - Shaft Min
3) Classify the fit
- Clearance fit: Minimum clearance is greater than zero.
- Interference fit: Maximum clearance is less than zero.
- Transition fit: Range crosses zero (sometimes clearance, sometimes interference).
Common fit categories and applications
Clearance fits
Used when parts must move relative to each other. Typical applications include rotating shafts in bearings, sliding mechanisms, and parts requiring easy assembly/disassembly.
Transition fits
Used when accurate location is important but extreme press force is not desired. Assembly may require light pressing or controlled alignment depending on actual produced sizes.
Interference fits
Used for permanent or semi-permanent joints requiring torque transfer and no slip. Examples include gears or hubs press-fit onto shafts.
Practical engineering tips
- Match fit class to function first, then optimize cost.
- Tighter tolerances increase machining and inspection cost.
- Consider temperature effects for dissimilar materials.
- Always include realistic process capability and inspection method.
- Document assumptions (surface finish, lubrication, assembly method).
Final thought
A good tolerance strategy balances manufacturability, performance, and reliability. Use this calculator early in design reviews to test fit behavior quickly and reduce surprises during prototyping and production.