📏 Machining & Metrology

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ISO 286 Fit Identification from a Measured Bore and Shaft

Every fits calculator on the internet asks what you intended: pick a nominal size, pick H7 and g6, and it prints the limits. That is the easy direction, and it is not the question an inspector has. The question on the floor is what did I actually achieve? You have a bore gauge reading and a micrometer reading and a drawing that says H7/g6, and you need to know whether the pair behaves as called, how much margin is left before it does not, and — if it does not — what it is instead. Enter the two measurements. This works backwards from them.

Your two measurements

Enter a bore and a shaft
Units
Size step
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Actual clearance
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Enter both measurements
Fit type
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Bore deviation
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Shaft deviation
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Bore best match
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Shaft best match
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Where your parts sit

The zero line is nominal. The called-out hole zone sits above it and the shaft zone below, both drawn to scale in micrometres. Your two measurements are the pins — drag either one and everything below re-resolves. A pin outside its band is a part outside its class.

Hole zone Shaft zone Inside its class Outside

What your bore could be called

Every hole class whose zone contains your measurement, finest first. The margin is the distance to the nearer limit — a small margin means a small measurement error would change the answer, which is worth knowing before you write it down.

What your shaft could be called

The same search over the shaft classes.

Was it measured at 20 °C?

ISO 1 fixes the reference temperature for all dimensional specification at 20 °C. If your part and your gauge were not both at 20 °C, the reading carries a thermal error — and on a long part in a warm shop it can be comparable to the tolerance you are trying to hold.

Both at 20 °C — no correction needed.

Why this runs backwards

ISO 286 is written forwards. You choose a nominal size, a letter and a grade, and the standard gives you two numbers. That is what a designer needs and it is what every calculator implements. An inspector has the opposite problem: two numbers came off the parts, and the question is what they mean.

Working backwards is not just the forward calculation rearranged, because the answer is not unique. A bore measuring 15 µm above nominal at 25 mm sits inside H7, and also inside H8, H9 and H11, and inside J8 and several others. All of those are true statements. What makes the answer useful is ordering them — finest class first, since the finest class that contains a measurement is the strongest claim you can make about it — and reporting how much room is left before each one stops being true.

What the two measurements give you directly

deviation = measured − nominal

clearance = bore − shaft

A class contains a measurement when

EI ≤ deviation ≤ ES  (hole)    ei ≤ deviation ≤ es  (shaft)

A positive clearance is a clearance fit, a negative one is an interference, and the sign is the whole distinction. Note that a single pair of parts cannot be a transition fit: transition is a property of two tolerance zones that overlap, meaning some pairs assemble with clearance and others with interference. Your specific pair landed on one side or the other. The tool says which, and separately says whether the classes it matched are a transition pairing.

Why this tool is a table and not a formula

ISO 286 does publish formulae. The standard tolerance factor is i = 0.45 × ∛D + 0.001D, with D the geometric mean of the size step rather than your actual size, and the grades are multiples of it — IT7 is 16i, IT8 is 25i, and so on. It is tempting to implement that and be done.

It does not work. Computing the grades from the formula and comparing cell by cell against the published table, 47 of 156 cells disagree, worst case 16.7%. The mismatches are not spread evenly either — they cluster at the small end, where 10 of 12 grades are wrong below 3 mm and 9 of 12 between 3 and 6 mm. That is precisely where a great deal of inspection work sits. No rounding rule rescues it; the current edition of ISO 286-1 does not even print deviation formulae any more, having become a table-only standard.

So the numbers here are transcribed, not computed. The formulae appear on this page as explanation of where the table came from historically, and nowhere else. The one piece of structure worth keeping is the standard's internal rule that each grade is ten times the grade five steps finer — that holds for 77 of 78 pairs in the published table, and it is used here as a consistency check on the transcription rather than as a source of values.

The corrections that catch people out

Hole deviations are mostly the mirror of the shaft letter of the same name: EI = −es for A through H, ES = −ei for K through ZC. Mostly. For holes K, M and N up to and including IT8, and P through ZC up to and including IT7, the standard adds a correction term Δ, the difference between the grade in use and the next finer grade. Its purpose is specific: it makes a shaft-basis fit deliver exactly the same clearance as the equivalent hole-basis fit, so that H7/p6 and P7/h6 are genuinely interchangeable. Skip it and your K7 hole is wrong by several microns.

A few more that a simplified implementation misses: the correction never applies at or below 3 mm; hole N is defined as exactly zero for IT9 and coarser; hole J exists only as J6, J7 and J8 and is asymmetric, so it cannot be derived at all; shaft t is undefined below 24 mm, which this tool reports rather than quietly treating as zero; and hole M6 in the 250–315 mm step is a hardcoded exception at −9 µm where the rule would give −11.

Margin matters more than the verdict

A pass/fail answer from a single measurement is a small lie, because the measurement has an uncertainty and the class boundary does not care. A bore 1 µm inside H7 and a bore 12 µm inside H7 are both "H7", and only one of them will still be H7 when a different inspector with a different gauge measures it on a different morning. So every candidate here carries its margin to the nearer limit, and the diagram shows the pin against the band rather than printing a verdict alone.

Where these numbers came from, and how they were checked

 

What this tool does not do

It identifies fits. It does not compute interference pressure, assembly or shrink-fit temperatures, or transmissible torque. Those look like the natural next step and they are not: a contact pressure without a hoop-stress check is a number that can burst a hub, and an assembly-temperature instruction is an instruction to heat something. If you need them, they belong with the design engineer who owns the joint and the material data, not with an inspection aid.

Reference tables

Everything this tool uses, with the size step and source for each group.

 

Frequently Asked Questions

I measured a bore and a shaft — what fit is it?

Enter both above with the nominal size. The clearance is simply bore minus shaft, and its sign tells you whether the pair went together with clearance or interference. What takes a table is the second half: which ISO 286 classes each measurement falls inside. A measurement usually satisfies several classes at once — a bore can be simultaneously H7, H8 and H9 — so the answer is a ranked list with the finest class first, not a single label. The finest class that contains your measurement is the strongest thing you can honestly say about that part.

Is my part still H7?

Type H7/g6 (or whatever the drawing calls) into the called-fit box and the tool checks your measurements against those exact limits, reporting how far inside or outside each one sits. Pay attention to the margin rather than the verdict: a bore one micron inside H7 is nominally passing, but a different gauge on a different day may well call it otherwise. If the margin is smaller than your measurement uncertainty, the honest answer is that you cannot tell.

What IT grade did I actually achieve?

Strictly, an IT grade is a property of a tolerance — a width — so it takes two limits or a batch to establish one, not a single part. What a single measurement supports is a narrower claim, and it is the one this tool makes: the finest grade whose zone, for the matched letter, still contains your reading. If you want the achieved grade properly, measure the spread across a run of parts and compare that spread against the IT width table in the reference section below.

Why does my K7 hole not match the simple mirror rule?

Because ISO 286 adds a correction there. For holes K, M and N up to IT8 and P through ZC up to IT7, the deviation is not the plain mirror of the same-letter shaft — a term Δ, the difference between the grade in use and the next finer grade, is added. The reason is that it makes H7/p6 and P7/h6 produce identical clearance, so hole-basis and shaft-basis designs interchange cleanly. Implementations that skip it are wrong by several microns on exactly those classes, and this is the single most common error in reverse-fit calculators.

Does temperature matter for this measurement?

ISO 1 sets 20 °C as the reference temperature for all dimensional specification, so every limit in this tool is a size at 20 °C. If the part and the gauge are at different temperatures or made of different materials, the reading carries an error proportional to the size and to the mismatch. On a 300 mm steel part measured 5 °C warm with a steel gauge at 20 °C the error is around 18 µm — larger than the whole IT7 tolerance at that size. Use the panel above to see the correction for your case.