Turn an ISO fit callout into real numbers — the hole and shaft limits, the clearance or interference range, and the fit type — with the tolerance zones drawn to scale so you can see the gap or overlap that decides whether a part slides, locates, or presses.
ISO size step: —
Deviations always shown in µm per ISO 286. Limit diameters shown in the chosen unit.
The dashed line is the nominal (zero) size. The blue band is the hole tolerance zone, the brass band the shaft. A gap between them (hole above shaft) is clearance; an overlap (shaft above hole) is interference. Deviations in µm.
The callout. An ISO 286 fit like H7/g6 names a hole and a shaft. The letter is the fundamental deviation — where the tolerance zone sits relative to nominal (uppercase = hole, lowercase = shaft). The number is the IT grade — how wide the zone is (lower = tighter). The elegance of the system: H7/g6 gives the same character of fit at 10 mm or 100 mm, even though the actual micron values scale with size.
IT grade width. The tolerance zone width comes from the standard tolerance factor i = 0.45·∛D + 0.001·D (µm, D in mm as the geometric mean of the size step), times a grade multiplier (IT6 = 10i, IT7 = 16i, IT8 = 25i, and so on in an R5 progression).
Fundamental deviation. For hole basis, H means the hole's lower deviation is zero — it starts at nominal and grows up. The shaft letter sets its upper deviation: g and f sit below nominal (clearance), h touches nominal, k and n straddle it (transition), p, s and beyond sit above (interference). This tool carries the common shaft positions and computes limits from them.
Clearance vs interference. Maximum clearance = largest hole − smallest shaft (loosest). Minimum clearance = smallest hole − largest shaft (tightest). If the tightest case is negative, that's interference — the shaft is bigger than the hole and must be pressed. When the range spans zero, it's a transition fit: could go either way depending where each part lands.
Fit families. Clearance (H7/g6 slide, H8/f7 run, H11/c11 loose) for motion; transition (H7/k6, H7/n6) for accurate location with easy assembly; interference (H7/p6 press, H7/s6 heavy press) for permanent joints without fasteners.
This tool computes IT grades from the ISO tolerance-factor formula and uses common fundamental-deviation values. It's a planning aid — verify critical fits against the licensed ISO 286-1/286-2 tables, which include sub-band and delta refinements this does not.