How is the ISO 286 standard tolerance unit defined?

For nominal sizes up to 500 mm the tolerance unit is i = 0.45 x cbrt(D) + 0.001 x D, in micrometres, where D is the geometric mean of the diameter step the size falls into. Grade IT values are fixed multiples of this unit: IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i, IT9 = 40i, IT10 = 64i.

What does H7 mean on a drawing?

The letter is the fundamental deviation of the feature, and the number is the tolerance grade. An uppercase letter is a hole, a lowercase letter is a shaft. H is the hole whose lower limit is the nominal size, so an H7 hole measures from the nominal size up to nominal plus the IT7 value for that size step.

What is the difference between clearance, transition and interference fits?

A clearance fit always leaves a gap, so the parts assemble freely or slide. A transition fit may be a small clearance or a small interference, so location is accurate but assembly needs light force. An interference fit always has the shaft larger than the hole, so it transmits torque by friction.

Why do I need the standard tables if this calculator exists?

The calculator derives the tolerance-grade values from the ISO 286-1 formula, which is exact for IT grades. The fundamental deviations of g, f, k, n and p features come from published tables, not from a formula, so those limits still have to be read from ISO 286-2 or your CAD tolerance database.

ISO 286 Tolerance Grades & Fit Calculator

Enter a nominal size to get the standard tolerance unit and the IT grade values that follow from it, then see the exact limits for H and h features. The fit table below explains what each common designation is for.

Tolerance grade lookup

Nominal size (mm)

0.1 to 500 mm.

Grade to highlight

Computed in your browser from the ISO 286-1 tolerance unit formula. Fundamental deviations of g, f, k, n and p features are table values and are not derived here.

Result

Diameter step— Geometric mean D— Tolerance unit i— Selected grade value— H limits (hole)— h limits (shaft)—

i = 0.45 × ∛D + 0.001 × D (µm). IT values are fixed multiples of i. H is the hole whose lower limit is nominal; h is the shaft whose upper limit is nominal.

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Reference

Common fits and what they are for

Fit	Type	Typical use
H7/h6	Clearance (close)	Location with easy assembly; dowel pins and locating bushes where a light push fit is wanted
H7/g6	Clearance (sliding)	Sliding or spigot location; the standard running fit for shafts that move but must not rattle
H7/f7	Clearance (running)	Rotating shafts in bearings at moderate speed with oil clearance
H8/f7	Clearance (running)	Same intent as H7/f7 with a wider hole band, used where machining the hole to IT7 is not economical
H9/d9	Clearance (free)	Loose running where fit is not critical and generous clearance keeps cost down
H7/k6	Transition	Accurate location, light press with a mallet; gears and pulleys on shafts
H7/n6	Transition	Tighter transition; location where a light interference is wanted but the part must still be removable
H7/p6	Interference	Press fit transmitting light torque by friction; bushes and bearing outer rings
H7/s6	Interference	Heavy press fit for permanent assemblies and high torque transmission

Standard practice is to keep the hole on a single grade and vary the shaft. That reduces the number of reamers and gauges a shop has to hold, and it is why H7 holes appear far more often than H6 or H8.

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Basis & references: ISO 286-1 — ISO code system for tolerances on linear sizes · ISO 286-2 — Tables of standard tolerance classes and limit deviations · NIST — Dimensional metrology

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