Fits and Tolerances

The thirteen common ISO 286 hole-basis fits, from loose running to shrink, with what each one means and where it is used — plus how to read a designation.

Data verified 2026-09-29 · based on ISO 286-1:2010, ISO 286-2:2010

Quick Answer

Common ISO 286 Fits and Their Applications

These are hole-basis fits: the hole is held at H (its lower deviation is zero, so the hole is never smaller than nominal) and the shaft carries the allowance. Shaft-basis fits, where the shaft is held at h and the hole carries the allowance, exist for the same letters and are used where one shaft diameter must accept several different holes. Numerical tolerances are not on this page — the actual micrometres depend on the nominal size, and are given in the ISO 286 tolerance chart. This page is about which fit to choose; that page is about how big the tolerance is.

How to Read a Fit Designation

A designation such as H7/g6 or 50H7/g6 carries four pieces of information, and reading them in the wrong order is the usual mistake.

The letter is the fundamental deviation, and it sets where the tolerance zone sits relative to the nominal size. For a hole, H means the zone starts exactly at nominal and extends upward — so an H hole is never undersize. For a shaft, h means the zone starts at nominal and extends downward — so an h shaft is never oversize. The letters run from A (largest clearance) through H/h (line-to-line) to ZC (largest interference) for holes and from a to zc for shafts, with holes in capitals and shafts in lower case.

The number is the tolerance grade, IT01 to IT18. Lower numbers are tighter. IT7 for a medium size is a few tens of micrometres; IT11 is roughly four times that. The grade is set by how precisely the feature can be produced and measured, not by how precise the function demands — a turned diameter is realistically IT7 to IT9, a ground one IT5 to IT7.

So H7/g6 reads: a hole whose zone sits at nominal and upward with an IT7 tolerance, mated to a shaft whose zone sits below nominal with an IT6 tolerance. The result is always a small clearance, and the shaft is ground because IT6 is a grinding grade.

Choosing Between Clearance, Transition and Interference

The choice is driven by three questions: must the joint transmit torque, must it be disassembled, and how accurately must it locate?

Clearance fits assemble by hand and always leave a gap. They are the default wherever a part must move or be taken apart. Their weakness is location accuracy: a clearance fit locates only as well as the clearance allows, which for H8/f7 at 50 mm is tens of micrometres of possible eccentricity. Where accurate location matters and the joint does not move, H7/h6 gives zero minimum clearance and still assembles by hand — it is the best of both and the reason it is the most-used fit in general engineering.

Transition fits may come out as a small clearance or a small interference depending on where the actual sizes land inside their tolerance zones. They locate accurately and transmit a little torque by friction, which is enough to hold a keyed gear or pulley in place. Their weakness is that the outcome is not predictable for a given pair of parts: H7/k6 at the extremes can be 20 µm of clearance or 20 µm of interference.

Interference fits always interfere, and hold by the friction generated by the elastic squeeze. They transmit substantial torque without a key. The engineering content is in the stress they create: the hub is in tension and the shaft in compression, and the interference must be large enough to hold the load but small enough not to yield either part. At H7/u6 the assembly needs the outer part heated — typically to 100–200 °C for a steel hub — and the design must confirm that the resulting hoop stress stays below yield.

One caution that applies across all three: a fit designation alone does not specify surface finish, roundness, or straightness. Two parts can be perfectly within their size tolerances and still assemble badly because one is oval or tapered. Where a fit is critical, the drawing needs a geometrical tolerance as well as a size tolerance.

Frequently Asked Questions

What does H7 mean?

H7 describes a hole: H is the fundamental deviation and 7 is the tolerance grade. H means the tolerance zone starts exactly at the nominal size and extends upward, so an H hole is never smaller than nominal — it is the basis of the hole-basis fit system. 7 is an IT7 tolerance grade, which for a 50 mm diameter is 25 µm. A shaft is written with a lower-case letter, so h7 is a shaft that is never larger than nominal.

What is the difference between a clearance and an interference fit?

In a clearance fit the shaft is always smaller than the hole, so the parts assemble by hand and can move relative to each other. In an interference fit the shaft is always larger, so the parts must be pressed or shrunk together and hold by friction — no key is needed to transmit torque. Transition fits fall between: depending on where the actual sizes land inside their tolerance zones, the result may be either.

Which fit should I use for a bearing?

It depends on which ring rotates and how the load is applied. A rotating inner ring under a stationary load direction typically uses a transition or light interference fit such as k6 or m6 so the ring cannot creep — but not so tight that it expands the raceway. A non-rotating inner ring often uses a looser fit. Bearing manufacturers publish recommended shaft and housing fits for each catalogue series, and those tables should be followed rather than the general guidance here.

How do I calculate the actual clearance?

From the numerical deviations for the specific nominal size, which are tabulated against the IT grades rather than being fixed. For 50H7/g6 the hole is +0 to +25 µm and the shaft is −9 to −25 µm, giving a clearance between 9 and 50 µm depending on where each part actually measures. Use the ISO 286 tolerance chart for the grade values at your nominal size.

Why is H7/h6 so common?

Because it does exactly what most joints need: zero minimum clearance, so the part locates as accurately as a clearance fit can, and it still assembles by hand without a press. The H hole is easy to produce and inspect, and the h shaft is turned to nominal, so both parts are straightforward. It is the default for covers, spigots, bolted joints and anything located by the fit but held by fasteners.

Related

• ISO 286 Tolerance Chart (IT grades)
• Surface Finish Chart
• Keyway Sizes
• Metric Coarse Thread Sizes
• AISI 4140 Properties
• GD&T Symbols

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Fit designations and their applications checked against both ISO 286 practice and the equivalent ASME B4.1 fit names
• Application descriptions cross-checked against two machine-design references rather than taken from a single source
• Every fit on the page verified as internally consistent — the letter ranges match the stated type

This page explains which fit to choose, not how large the tolerance is — the numerical deviations depend on the nominal size and are in the ISO 286 tolerance chart. A fit designation does not control roundness, taper or surface finish, all of which affect whether a joint assembles and how it performs. For bearings, follow the bearing manufacturer's published shaft and housing fits rather than general guidance.

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