How it works
ISO 286 builds every tolerance from a single standard tolerance
unit that grows slowly with size:
i = 0.45 · ∛D + 0.001 · D
(in micrometres, with D the geometric-mean of the standard size
range). An IT grade just scales that unit — IT6 ≈ 10·i, IT7 ≈
16·i, IT8 ≈ 25·i — so the tolerance band widens predictably as the grade number
rises. The grade is the number in the fit, e.g. the 7 in H7.
The letter places that band. In a hole-basis system the hole is
always H, whose lower deviation is EI = 0: the hole runs
from the nominal size up to nominal + IT. The shaft letter sets its
fundamental (upper) deviation es — for clearance
shafts (h, g, f, e, d) this is zero or negative, pushing the shaft below the
nominal size — and the lower deviation is ei = es − IT. The
clearance is then just the difference of the limits: the maximum
clearance is hole max − shaft min and the minimum clearance is
hole min − shaft max. Because EI = 0, the minimum clearance
equals −es.
Worked example
A 25 mm H7/g6 sliding fit. 25 mm falls in the 18–30 mm range
(geometric mean 23.238 mm), giving a tolerance unit i ≈ 1.307 µm, so
IT7 = 21 µm and IT6 = 13 µm. The hole H7 is
25.000–25.021 mm (EI = 0, ES = +21 µm). The shaft g6 has
es = −7 µm and ei = −20 µm, so it is
24.980–24.993 mm. The clearance therefore runs from
0.007 mm (hole min − shaft max) to 0.041 mm (hole max −
shaft min) — a true clearance fit that locates closely yet slides by hand. The
calculator returns exactly these numbers.
Frequently asked questions
- How do I read an ISO 286 fit like H7/g6?
- The first part is the hole (H7) and the second is the shaft (g6): a capital letter is the hole, a lower-case letter is the shaft, and the number after each is its IT tolerance grade. H7 is the hole tolerance, g6 the shaft tolerance. The letter sets where the tolerance band sits relative to the nominal size; the number sets how wide it is.
- What is the difference between a clearance, transition and interference fit?
- A clearance fit always leaves a gap — the shaft is never larger than the hole (e.g. H7/g6, H8/f7). An interference (press) fit always overlaps — the shaft is always larger than the hole, so it must be pressed or shrunk on. A transition fit sits in between: depending on where each part lands in its tolerance band you may get a small clearance or a small interference. This calculator covers hole-basis clearance and locational-clearance fits.
- What do H7/g6, H8/f7 and H9/d9 mean in practice?
- They name how the parts move relative to each other. H7/g6 is a sliding fit (locates accurately, slides by hand, minimal play); H8/f7 is a close running fit (rotates freely with a film of oil); H9/d9 is a free running fit (generous clearance for higher speeds, heat or coarser tolerances). H7/h6 is a locational clearance fit (just-touching, easy assembly) and H8/e8 a loose running fit.
- Where is the hole tolerance in a hole-basis fit?
- In a hole-basis fit the hole letter is always H, which means its lower deviation EI = 0 — the hole tolerance starts exactly at the nominal size and runs upward by the IT grade (so an H7 hole is nominal to nominal + IT7). All the variation between fits comes from the shaft. That is why the hole minimum here equals the nominal size.
- What about press fits or interference fits?
- This tool is for clearance and locational-clearance fits only, where the shaft is always smaller than the hole. For interference (press / shrink) fits — contact pressure, press-in force, torque capacity and hub stress — use the press fit / interference calculator.
- Does this work in metric and imperial?
- ISO 286 is a metric standard, so the deviations are defined in micrometres and the size ranges in millimetres. Toggle to imperial to read the resulting limits and clearances in inches; the deviation math runs in fixed internal units, so the toggle never changes the answer.
Method & assumptions
- Hole-basis system: the hole letter is H, so its lower deviation EI = 0 and the hole starts at the nominal size.
- Clearance and locational-clearance fits only (shaft letters h, g, f, e, d). The shaft is always at or below the nominal size, so the assembly always has a gap.
- Deviations are computed from the ISO 286 standard tolerance unit and rounded to the nearest micrometre, which reproduces the published table values; verify against the ISO 286 tables for critical work.
- Interference (press / shrink) fits are out of scope — use the press fit / interference calculator for contact pressure, press-in force and hub stress.
Related calculators
- True Position Calculator — GD&T true position from X/Y deviation with MMC bonus tolerance and a pass/fail check.
- MMC Bonus Tolerance Calculator — MMC/LMC bonus tolerance from a feature’s departure from its material limit.
- Composite Position Tolerance Calculator — Two-tier PLTZF/FRTZF position check from X/Y deviation.
- Flatness Tolerance Calculator — Flatness (surface form) from measured points, with a pass/fail check.
- Press Fit / Interference Calculator — Interference-fit contact pressure, assembly force, torque capacity and hub stress (Lamé).
- Cylindricity & Runout Calculator — Runout (TIR) and cylindricity zone from indicator and diameter readings.