FAQ / ISO 286 Limits and Fits reference

This page is the ISO 286 Limits and Fits reference used by engineers to figure out how tight or loose a shaft will fit into a hole (a bearing bore, a housing, etc.). Here's how to actually read and use it.

The basic idea

Every shaft and hole has a nominal size (say, 25 mm) plus tiny allowed variations from that nominal size, called tolerances. The tables give you those variations in micrometres (µm), as a pair of numbers: the maximum and minimum deviation from nominal.

A "fit" is just a combination of one shaft tolerance class + one hole tolerance class. The gap or overlap between them determines whether the part slides freely, needs to be pressed in, etc.

Step 1 — Find your nominal size row

Both tables (shaft and hole) are split into size bands: "over 3 to 6mm", "over 6 to 10mm", "over 30 to 40mm", etc. Find the column matching your actual dimension. E.g., a 35 mm shaft falls in the "over 30 – inc. 40" column.

Step 2 — Find your tolerance class row

Down the left side you'll see codes like h6, g6, k6, H7, F7. This is the ISO tolerance class:

• Lowercase letters (a, d, e, f, g, h, j, k, m, n, p, r) = shaft tolerances
• Uppercase letters (E, F, G, H, J, K, M, N, P, R) = hole tolerances
• The number (5, 6, 7, 8...) = the IT grade, which controls how tight the tolerance band is (lower number = tighter/more precise)

Step 3 — Read the two numbers

At the intersection of your size column and tolerance row, you get two numbers, e.g. for a 30–40mm shaft at g6: -9 / -25. That means:

• Maximum deviation: shaft can be up to 9 µm smaller than nominal
• Minimum deviation: shaft can be up to 25 µm smaller than nominal

So a nominal 35mm shaft toleranced g6 will actually measure between 34.991 mm and 34.975 mm.

Do the same lookup on the hole table for your chosen hole class (e.g. H7: +25 / 0 for that same size band) — meaning the hole can be nominal to +25 µm oversize.

Step 4 — Work out the fit

Compare the shaft's allowed range against the hole's allowed range:

• If the shaft range is always smaller than the hole range → clearance fit (always a gap, parts move freely)
• If the shaft range is always larger → interference fit (press fit, parts must be forced together)
• If the ranges overlap → transition fit (could go either way depending on where each part lands within tolerance)

Step 5 — Use the "Clearance Descriptions" table as a shortcut

Rather than working it out from scratch, that bottom table gives you common pre-picked combinations by name:

Practical example: you're mounting a bearing's inner ring on a shaft and want it to stay put without excessive force. You'd look up "Slight interference" → hole H7, shaft k6. Then go to the shaft table, find your shaft's size band, read the k6 row for your exact µm tolerance, and machine to that spec.

ISO Shaft Tolerances

ISO Hole Tolerances

Clearance Descriptions