ISO 286 limits and fits calculator
Enter a nominal size, pick a fit — the limit deviations, the limits of size and the kind of fit that results, per ISO 286-1.
Calculator
Work out a fit
Every value comes from the tables of ISO 286-1 and is recomputed for your nominal size the moment anything changes.
Over 0 up to and including 3150 mm
Hole
Fundamental deviation · Tolerance grade
Shaft
Fundamental deviation · Tolerance grade
On hole basis the hole stays at H and the shaft carries the fit; on shaft basis it is the other way round. Move both letters away and the pair is reported as mixed.
One click sets both classes; the nominal size stays where it is.
Clearance fit
Transition fit
Interference fit
Callout for the drawing
⌀25 H7/g6
25 H7
25.021
25.000
25 g6
24.993
24.980
| Upper deviation | Lower deviation | Maximum size | Minimum size | Tolerance | |
|---|---|---|---|---|---|
| Hole H7 | +21 µm | 0 µm | 25.021 mm | 25.000 mm | 21 µm |
| Shaft g6 | −7 µm | −20 µm | 24.993 mm | 24.980 mm | 13 µm |
Kind of fit
Clearance fit
Clearance 7 – 41 µm
H7/g6 at 25 mm always leaves clearance: between 7 µm and 41 µm, depending on where the two parts land inside their zones.
Guide pillar in its guide bush — located, and free to move.
The zones are drawn heavily enlarged so they stay legible; the figures beside them are the real ISO 286-1 values.
What are limits and fits?
No part is made to its nominal size. It is made to an interval: a maximum size and a minimum size the actual feature has to fall between. The width of that interval is the tolerance; where it sits relative to the nominal size is the tolerance zone.
A fit is what happens when two of those zones meet — a hole and a shaft. Whether the parts assemble, with how much force, and whether they stay free to move afterwards has nothing to do with the two nominal sizes and everything to do with how the two zones overlap.
That is why a drawing states a fit rather than the same size twice: the callout describes the relationship between two parts, not the parts themselves.
What does ISO 286 cover?
ISO 286 is the international system of limits and fits for linear sizes. Part 1 sets out the basis: the standard tolerance grades IT01 to IT18 and the 28 fundamental deviations, a to zc for shafts and A to ZC for holes. Part 2 carries the tables of values derived from it.
It covers nominal sizes over 0 up to and including 3150 mm. Within that range, every combination of a letter and a grade number is a tolerance class, and every tolerance class has exactly two numbers at a given nominal size: the upper and the lower limit deviation.
BS EN ISO 286 and DIN EN ISO 286 are national adoptions of the same standard, with identical values. ANSI B4.2 is the closest American equivalent and uses the same letter-and-grade notation.
What do ES, EI, es and ei mean?
They are the four limit deviations: the distance between the nominal size and each of the two limits of size, in micrometres and carrying a sign.
Capitals are the hole, lower case is the shaft; ES and es are the upper deviations, EI and ei the lower ones. So the maximum size is always the nominal size plus the upper deviation, and the minimum size the nominal size plus the lower deviation — even when the deviation is negative and the addition makes the feature smaller.
The distance between the upper and lower deviation is the standard tolerance grade IT, which depends only on the nominal size and the grade number. The letter moves the whole zone without changing how wide it is.
Reading a fit off the drawing
The callout says everything about the pairing without stating a single figure — nominal size, both fundamental deviations and both tolerance grades live in five characters.
⌀25 — The nominal size both parts are referred to.
H — The hole’s fundamental deviation: where its zone sits relative to the zero line. H means the lower deviation is zero.
7 — The hole’s tolerance grade — how wide its zone is. That width depends on the nominal size, not on the letter.
g — The shaft’s fundamental deviation, here entirely below the zero line, which is why some clearance always remains.
6 — The shaft’s tolerance grade: one step finer than the hole, as in most standardised pairings.
Hole basis and shaft basis
Both systems produce the same fits. All they decide is which of the two parts stays on the nominal size: on hole basis the hole is always H, its lower deviation is zero, and the shaft brings the clearance or the interference. On shaft basis it is exactly the other way round.
Both drawings show the same pairing: at one nominal size, H7/g6 and G7/h6 give exactly the same clearance.
Why is hole basis the default?
Because the hole is the expensive size. A hole is produced by a tool that brings its own size with it — a reamer, a broach, a roller burnisher — and every additional hole size means another tool and another gauge.
A shaft is turned or ground and can be taken to any size at all without a size-specific tool. So if one of the two parts has to vary, it is cheaper to let the shaft vary.
Shaft basis still earns its place when the shaft is already finished: drawn and ground bar stock, rolling-bearing inner rings, standard dowel pins. Then h is fixed and the bore carries the fit.
Tolerance grades and what they are for
The grade number decides how wide the zone is, never where it sits. The real width grows with the nominal size, which is why IT7 on a 6 mm bore means something entirely different from IT7 on a 600 mm one.
| Grades | Level of precision | Typical application |
|---|---|---|
| IT01 – IT4 | Gauges and masters | Gauge blocks, plug gauges, measuring equipment — not a class to put on a component drawing. |
| IT5 – IT6 | Finest fits | Rolling-bearing seats, guide pillars, dowel pins, heavily loaded spigots. |
| IT7 – IT8 | Ordinary fits | Bores in plates, bush seats, ejector holes — where most tooling drawings live. |
| IT9 – IT11 | General machine work | Turned and milled sizes with no fitting function, undercuts, clearance holes. |
| IT12 – IT14 | Raw and formed sizes | Forgings, castings and sheet-metal parts before finishing. |
| IT15 – IT18 | Coarse sizes | Blanks, flame-cut parts, stock lengths. |
Clearance, transition or interference — what is the difference?
In a clearance fit the shaft zone sits entirely below the hole zone. Even in the worst case some clearance is left: the parts always assemble and stay free to move.
In an interference fit the shaft zone sits entirely above the hole zone. The parts can only be assembled with force or a temperature difference, and afterwards they transmit torque through friction.
In a transition fit the two zones overlap. The same callout can then produce a little clearance or a little interference, depending on where the parts as actually made fall inside their own zones. That is the point of it: transition fits locate accurately and can still be taken apart.
Which fit for which job?
The question that comes before the fit is always: does this have to move, does it have to locate, or does it have to hold?
Move — H7/g6 for guided parts that must not bind, a guide pillar in its bush being the obvious one. H8/f7 for lubricated shafts turning continuously. H9/d9 and H11/c11 when heat, dirt or a rough prior operation need room.
Locate — H7/k6 for dowels and spigots that survive dismantling. H7/n6 when the joint should sit tighter and only rarely comes apart.
Hold — H7/p6 for a bush pressed into its plate and left there. H7/s6 for joints meant to be permanent, such as an ejector bush in its bore. The larger the interference, the higher both the assembly force and the hoop stress in the surrounding wall.
What the table does not tell you
The computed fit applies to the state both parts were measured in: room temperature, clean, deburred, unloaded. An interference fit that holds on paper can let go at working temperature if the two materials expand at different rates.
Nor does the table say anything about form and position. A feature inside its limits can still be out of round, tapered or off-centre; a fit assumes that roundness, cylindricity and surface texture suit it. On press fits the texture matters directly: the roughness peaks are flattened during assembly, and part of the calculated interference goes with them.
And finally, a tolerance grade is a requirement, not a promise. What a given process actually holds belongs in the quotation and the control plan.
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