ISO 286 Tolerance Calculator

How Does the ISO 286 Tolerance System Work?

ISO 286 defines a system of limits and fits for cylindrical parts using two components: a fundamental deviation (position letter) and an IT grade (tolerance magnitude). The deviation letter (a-z for shafts, A-Z for holes) sets how far the tolerance zone is offset from nominal, while the IT grade (IT01 to IT18) sets the zone width.

Fits are classified as clearance (shaft always smaller than hole, e.g., H7/f6), transition (may have clearance or interference, e.g., H7/k6), or interference (shaft always larger, e.g., H7/p6). The 'hole basis' system (H-hole with varying shaft letters) is most common because standard reamer sizes produce H-tolerance holes.

IT grades follow a geometric progression based on the 'standard tolerance factor' i = 0.45∛D + 0.001D. IT7 (general machining) is about 16i, corresponding to 21 μm for a 25 mm shaft. IT6 is used for precision fits, IT5 for gauge-quality work.

In modern CNC manufacturing, achieving IT7 tolerance is routine for turning and grinding operations, while IT6 requires careful process control and IT5 approaches the limits of conventional machining. For sub-micron tolerances (IT4 and below), specialized processes like jig grinding, lapping, or honing are required.

The economic impact of tolerance selection is significant: manufacturing cost roughly doubles for each IT grade tighter than IT10. A systematic tolerance analysis (worst-case or statistical) during design ensures that assembly function is guaranteed without over-specifying individual part tolerances.

Formula: i = 0.45 × ∛D + 0.001 × D (μm, D in mm) IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i Max Size = Nominal + Upper Deviation Min Size = Nominal + Lower Deviation

Example Calculation

For a Ø25 H7 hole: D = 25 mm, i = 0.45 × ∛25 + 0.001 × 25 = 1.341 μm. IT7 = 16 × 1.341 ≈ 21 μm. H fundamental deviation = 0. So: lower deviation = 0, upper deviation = +21 μm. Hole size: 25.000 to 25.021 mm.

When to Use This Calculator

• Designing a shaft-bearing interface and needing to determine the exact min/max dimensions for an H7/k6 transition fit
• Reviewing a machinist's inspection report and verifying whether measured dimensions fall within the specified ISO tolerance band
• Converting a fit specification from one nominal size to another — tolerance band widths change with nominal diameter
• Communicating with international suppliers who use ISO 286 designations to ensure consistent part interchangeability

Common Mistakes to Avoid

• Confusing uppercase (hole) and lowercase (shaft) deviation letters — H7 is a hole tolerance, h7 is a shaft tolerance, and they produce very different dimensions
• Assuming the same IT grade gives the same tolerance width across all sizes — IT7 is 21 μm at Ø25 but 52 μm at Ø120
• Specifying tighter tolerances than needed — IT7 costs roughly 3× more to manufacture than IT11; always choose the loosest tolerance that meets functional requirements
• Forgetting that deviation values are in micrometers (μm), not millimeters — a common transcription error that produces parts 1000× out of tolerance

How to Interpret Results

• Positive deviations on a shaft mean the shaft is oversize relative to nominal — it will create tighter (more interference) fits
• An H-hole always has zero lower deviation — the tolerance band extends only above nominal
• If the shaft max size exceeds the hole min size, interference exists in at least part of the tolerance range (transition or interference fit)
• Tolerance band width (upper minus lower deviation) determines manufacturing difficulty — wider bands are cheaper to produce

Related Standards & References

• ISO 286-1:2010 — Geometrical product specifications — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits
• ISO 286-2:2010 — Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts
• ASME B4.2 — Preferred metric limits and fits (US adoption of ISO 286)
• ISO 1829 — Selection of tolerance zones for general purposes

H7 — Limit deviations by nominal size

Every nominal size range ISO 286 covers, calculated by this tool for the class shown. Deviations in micrometres.

Frequently Asked Questions

What is the most commonly used fit in mechanical engineering?

H7/h6 (sliding fit) is the most universal fit for locating parts that need to slide or be assembled by hand. H7/f6 gives a free-running clearance fit for bearings. H7/p6 gives a medium press fit for permanent assemblies. These three cover roughly 80% of typical engineering fit requirements.

What is the difference between hole basis and shaft basis systems?

In hole basis (most common), the hole is always 'H' and different shaft letters create the fit type. This is practical because standard reamers and broaches produce H-tolerance holes. In shaft basis, the shaft is always 'h' and different hole letters are varied. Shaft basis is used when a single shaft must mate with multiple features, such as a stepped shaft with different fits along its length.