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Check ISO fit

ISO Tolerance Calculator

Calculate maximum size, minimum size and tolerance from nominal size and deviations, or pick common ISO fits such as H7, g6, f7 and h6 with a visual tolerance diagram.

Tolerance diagram

nominal

H7

Horizontal line is nominal size. The bar is the tolerance zone (not to millimetre scale — height is relative to the larger deviation).

Maximum size

20.021 mm

Minimum size

20 mm

Tolerance

21 µm

Upper deviation

21 µm

Lower deviation

0 µm

Actual deviation span

21 µm

Reading an ISO fit

ISO 286 writes a hole or shaft as a letter plus an IT grade. Upper-case letters are holes; lower-case are shafts. H means the hole’s lower deviation is zero — it never goes under nominal. h means the shaft never goes over nominal. The number (6, 7, 8, 9…) is the standard tolerance grade: smaller is tighter.

Formula

Max = nominal + ES (hole) or es (shaft)

Min = nominal + EI or ei

IT7 ≈ 16i, where i = 0.45∛D + 0.001D (µm, D in mm)

Ø20 H7

IT7 for 18–30 mm is 21 µm. H sets EI = 0, so ES = +21 µm. Limits are 20.021 / 20.000 mm.

Common pairings

  • H7/g6 — close running clearance (bearings that must spin by hand)
  • H7/h6 — locational sliding
  • H7/k6 — transition, may need a light press
  • H7/p6 or H7/s6 — interference / press fits
  • H8/f7 — easy running, more play

Convert millimetres to inches with the unit converter, or check a tapped hole with the thread calculator.

Frequently asked questions

What does an H7 hole mean?+

The capital letter is the fundamental deviation for a hole; H means the lower deviation is zero (the hole never goes below nominal). The number is the IT grade — IT7 is a precision finishing grade used for bearings, bushings and locational fits.

How do I read a fit like H7/g6?+

H7 is the hole, g6 is the shaft. Because g is a small negative shaft deviation, H7/g6 is a close running (clearance) fit: the shaft is slightly under nominal and the hole is slightly over, so parts assemble by hand with little play.

Are the values from ISO 286?+

Standard tolerances (IT grades) and fundamental deviations follow ISO 286-1/2 tables for the usual nominal ranges up to 500 mm. Always confirm critical fits against the current standard and your drawing notes.

Can I enter custom deviations?+

Yes. Switch to custom mode and type upper and lower deviations (in mm or µm). The calculator returns maximum size, minimum size, tolerance width and a diagram of the zone relative to nominal.

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