iso fit calculator

What an ISO fit calculator does

An ISO fit calculator helps you determine whether a hole-and-shaft pair will assemble with clearance, transition, or interference. In mechanical design, this matters for everything from sliding bushings to press-fit gears. The fit designation, such as H7/g6, tells you where each tolerance zone sits and how wide it is.

In a typical hole-basis system, the hole is H, meaning the lower deviation is zero. You then choose a shaft letter and grade to set the assembly behavior:

• Clearance fit: shaft is always smaller than hole (easy assembly, rotation, sliding).
• Transition fit: can be either slight clearance or slight interference.
• Interference fit: shaft is always larger than hole (press or shrink assembly).

How this calculator works

1) Nominal size and IT grade

ISO tolerance grades (IT6, IT7, IT8, etc.) define tolerance width. The calculator estimates that width from the nominal diameter using the standard tolerance unit formula:

• i (µm) = 0.45 × ∛D + 0.001 × D
• Then, for example: IT6 = 10i, IT7 = 16i, IT8 = 25i, IT9 = 40i, IT11 = 100i

2) Fundamental deviation by letter

The letter (g, h, k, m, n, p, s...) positions the tolerance zone above or below nominal size. For shaft fits in this tool:

• f, g place the shaft below nominal (more clearance tendency).
• h touches nominal at the top of shaft tolerance (classic baseline shaft).
• k, m, n, p, s shift upward (toward transition/interference behavior).

3) Final limits and fit class

Once hole and shaft limits are found, the calculator computes minimum and maximum clearance:

• Min clearance = hole minimum − shaft maximum
• Max clearance = hole maximum − shaft minimum

If both values are positive, the fit is clearance. If both are negative, interference. If one is positive and one is negative, it is transition.

Choosing a fit for real applications

Common practical choices

• H7/g6: smooth location with predictable clearance for precision running parts.
• H7/h6: close location where zero-clearance edge cases are acceptable.
• H7/k6 or H7/m6: tighter location with potential light press behavior.
• H7/p6 or H7/s6: strong interference for permanent or semi-permanent joints.

Design tips before release

• Include coating, plating, and thermal expansion in your stack-up.
• Check surface finish and geometry (roundness, cylindricity), not just size.
• Confirm assembly process: hand push, arbor press, or thermal method.
• Validate with prototype measurements and capability data (Cp/Cpk).

Limitations and good engineering practice

This page is intentionally fast and practical, but it is not a substitute for a controlled drawing package. Use it to compare fit intent early in design, then lock final limits from authoritative ISO tables for your diameter range and revision-controlled specifications.

If your product is safety-critical, heavily loaded, or high-speed, involve manufacturing and quality engineering before freezing tolerances. A good fit is not only “mathematically correct”—it must also be manufacturable, inspectable, and reliable in service.