ISO 286 Limits & Fits Calculator
Hole and shaft limits, clearance or interference and the fit type for any ISO tolerance class, 1–500 mm
A fit is nothing more than two tolerance zones compared: the hole’s and the shaft’s. ISO 286 writes each zone as a letter (where the zone sits relative to the nominal size — H is "starts at zero and goes up", g is "sits a little below zero") and a number (how wide it is — IT7 on a 25 mm size is 21 µm). Put a hole class and a shaft class on the same nominal size and the extreme limits tell you whether the parts will slide, tap together or need a press.
This calculator does the table lookup for you: enter the nominal size, pick a preferred fit or set the hole and shaft classes yourself, and it returns the upper and lower deviations, the maximum and minimum diameters to put on the drawing, the largest and smallest clearance or interference the pair can produce, and the fit type. The tolerance-zone diagram shows both zones against the zero line at the true proportions.
Behind it are the standard’s own tables: IT grades 1–16 and the fundamental deviations of the letters a–z for all 25 size ranges up to 500 mm, with the hole deviations derived by the ISO mirror rule and the Δ correction that fine-grade K, M, N and P–ZC holes require. Hole-basis fits (H hole, varied shaft) cover most machine design because a hole is made with a fixed-size tool; shaft-basis fits (h shaft, varied hole) suit long ground shafts that carry several parts.
Frequently Asked Questions
What does H7/g6 mean?
A hole with tolerance class H7 and a shaft with g6, on the same nominal size. H means the hole tolerance zone starts at the nominal size and goes up (EI = 0); 7 is the grade, 21 µm wide at 25 mm. g means the shaft zone sits slightly below the nominal size (es = −7 µm at 25 mm); 6 is the grade, 13 µm wide. Together: Ø25 H7/g6 gives a clearance between 7 and 41 µm — a sliding fit.
What is the difference between a clearance, transition and interference fit?
Clearance: the shaft is always smaller than the hole (H7/g6, H8/f7, H11/c11); parts slide or run. Interference: the shaft is always larger (H7/p6, H7/s6, H7/u6); parts are pressed or shrunk together and transmit torque by friction. Transition: depending on where the actual sizes fall the pair may have a small clearance or a small interference (H7/k6, H7/n6); used for accurate location of parts that are assembled with a light press and can be taken apart.
How much clearance does H7/h6 have?
H7/h6 is the locational clearance fit: the hole zone is +IT7 above zero and the shaft zone is −IT6 below it, so the minimum clearance is exactly 0 and the maximum is IT7 + IT6. At Ø25 that is 0 to 34 µm; at Ø100, 0 to 57 µm. It assembles by hand and locates accurately, but it will not run without lubrication and clearance for heat.
Which fit for a bearing on a shaft?
Rolling-bearing manufacturers specify it, not ISO 286 alone — as a rule the rotating ring gets an interference or transition fit and the stationary ring a clearance or transition fit. A typical shaft for a rotating inner ring under normal load is k5 or k6 (m6 for heavier loads); a housing for a stationary outer ring is H7 (J7 / K7 when the outer ring rotates). Check the bearing catalog for the load case and size.
Hole basis or shaft basis — which should I use?
Hole basis (H hole, shaft varied) is the default: holes are made with reamers and drills of fixed size, while a shaft is turned or ground to any diameter, so varying the shaft is cheaper. Shaft basis (h shaft, hole varied) makes sense when one long shaft of standard bright bar carries several parts with different fits, or when using standard drawn shafting without machining.