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Limits and fits, ISO 286

A ⌀25 H7 isn't inspected against “H7”. Enter the diameter and the fit to get the two numbers you actually measure against.

Your dimension

Capital letter for a hole, lower case for a shaft.

Enter a diameter to see the limits

Deviations are taken from the tables of ISO 286-1, not recomputed from its formula: the standard publishes rounded values that don't always match what the formula returns. These are the same values CoteCote applies.

The most common fits

Deviations in microns, for the diameters most often met in mechanical work.

Fit⌀6⌀10⌀20⌀30⌀50⌀80⌀120
H7+12/0+15/0+21/0+21/0+25/0+30/0+35/0
H8+18/0+22/0+33/0+33/0+39/0+46/0+54/0
h60/-80/-90/-130/-130/-160/-190/-22
h70/-120/-150/-210/-210/-250/-300/-35
g6-4/-12-5/-14-7/-20-7/-20-9/-25-10/-29-12/-34
f7-10/-22-13/-28-20/-41-20/-41-25/-50-30/-60-36/-71
k6+9/+1+10/+1+15/+2+15/+2+18/+2+21/+2+25/+3
m6+12/+4+15/+6+21/+8+21/+8+25/+9+30/+11+35/+13
n6+16/+8+19/+10+28/+15+28/+15+33/+17+39/+20+45/+23
p6+20/+12+24/+15+35/+22+35/+22+42/+26+51/+32+59/+37

How to read it: the first value is the upper deviation, the second the lower deviation, both in microns.

What a fit encodes

A fit is written with a letter and a number, for example H7. These are two independent pieces of information. The letter says where the tolerance zone sits relative to the nominal size. The number, called the standard tolerance grade or IT grade, says how wide it is.

The way it's written carries a third piece of information: a capital letter denotes a hole, a female feature. A lower-case letter denotes a shaft, a male feature. That's why an assembly is written H7/g6, never H7/G6.

Why the tolerance changes with diameter

H7 isn't a fixed value. On a ⌀10 it gives 0 to +0.015. On a ⌀200 it gives 0 to +0.046. The standard assumes a large part is harder to hold than a small one, and widens the interval accordingly, following a formula based on the cube root of the diameter.

This is why a table of fits always carries a column of diameters. Reading “H7 = 21 microns” without stating the size means nothing: it's true between 18 and 30 mm, and wrong everywhere else.

The positions you'll actually meet

In practice a handful covers most mechanical work. H for holes, lower deviation at zero, the basis of the hole-basis system. h for shafts, upper deviation at zero. Then, from loosest to tightest on the shaft side: f and g leave clearance, k and m are neutral to slightly tight, n and p interfere properly.

Grades follow the same logic. IT5 and IT6 for bearing seats or precise guides, IT7 for the great majority of mechanical fits, IT8 to IT11 for less demanding parts.

One example, carried to a verdict

A dimension read as ⌀5.25 f7. Position f, for a nominal size between 3 and 6 mm, gives an upper deviation of −10 microns and a lower deviation of −22 microns. The limits are therefore 5.228 and 5.240. A reading of 5.235 passes; a reading of 5.245 doesn't — even though it's closer to the nominal size printed on the drawing.

Frequently asked questions

What exactly does H7 mean?
The letter sets where the tolerance zone sits relative to the nominal size; the number sets how wide it is. H puts the lower deviation at zero, so an H7 hole is always equal to or larger than its nominal size.
Why is the hole a capital letter and the shaft lower case?
That's the convention of the standard. A capital letter always denotes a female feature, a hole. A lower-case letter denotes a male feature, a shaft.
Does an H7 fit always give the same tolerance?
No, it depends on the diameter. H7 is +0.015 on a 10 mm bore and +0.046 on a 200 mm bore: the larger the part, the wider the interval.
Up to what diameter does this calculator work?
Up to 500 mm, the range covered by ISO 286-1 for grades IT5 to IT11.

This calculation, on every dimension of a drawing, in one pass

CoteCote reads the fits straight off the drawing and returns the limits, without you opening a table. It runs locally, on Windows.

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