For the ISO 286 hole-basis system, pick the basic size and tolerance class (such as H7/g6) and see the hole and shaft limit deviations, maximum and minimum clearance, and the fit type in real time. The tolerance-zone diagram shows how the hole and shaft zones line up.
Parameters
Basic size D
mm
Nominal size shared by the hole and shaft
Hole tolerance class
Hole basis (letter H, lower deviation = 0)
Shaft fundamental deviation
Zone position. d-g = clearance side, js-n = transition side
Shaft tolerance grade IT
Tolerance width. The smaller, the more precise
Results
—
Fit designation
—
Fit type
—
Max clearance (μm)
—
Min clearance (μm)
—
Hole tolerance (μm)
—
Shaft tolerance (μm)
—
Tolerance-zone diagram (zero line, hole, shaft)
The dashed centre line is the zero line (basic size). Blue is the hole tolerance zone, orange the shaft zone. Their relative position decides the fit type.
Clearance vs basic size D
Fit comparison across shaft classes (min clearance)
ES and EI are the upper and lower limit deviations. In the hole-basis system EI_hole = 0 and ES_hole = +IT_hole.
$$IT = k\cdot i, \qquad i = 0.45\sqrt[3]{D} + 0.001\,D \;\;[\mu m]$$
The IT tolerance grade. i is the tolerance unit and D the geometric mean of the size range. The IT values in this tool follow the ISO 286 standard table.
Examples of shaft fundamental deviations (ISO 286). Min clearance ≥ 0 = clearance fit, max clearance ≤ 0 = interference fit, in between = transition fit.
What is the ISO Fit Tolerance Selector?
🙋
On drawings I see things like "Ø50 H7" or "H7/g6". What do they mean?
🎓
Good question. A part always has dimensional scatter, so you can never make it exactly "Ø50". So you state "Ø50, with an allowed error from here to here" — that band is the tolerance. The letter in "H7" gives the position of the tolerance zone, and the number 7 gives the width (the IT grade). "H7/g6" means the hole is H7 and the shaft is g6. How they fit together when assembled is the "fit".
🙋
I see. So can I freely choose the tolerance for both the hole and the shaft?
🎓
The rule is the "hole-basis" system: the hole is fixed as H. A hole is machined with a dedicated reamer and gauge, so you do not want many variants. You fix the hole at, say, H7, and vary the easier-to-cut shaft as g6, k6, n6 to create the different fits. Move "Shaft fundamental deviation" on the left from d toward n — you will see the orange band in the tolerance diagram climb from the clearance side toward the tight side.
🙋
It really does — the orange band rises. g gives a clearance fit, n a transition fit. How do I choose between them?
🎓
It depends on the function. A rotating shaft in a plain bearing must always have clearance, so a clearance fit like g6 or f7. A locating pin or gear that should have no play yet still be removable uses a transition fit like k6 or js6. Something pressed in and never removed uses an interference fit (p6, s6 etc.). This tool decides the type automatically from whether the minimum clearance is positive or negative — a negative minimum clearance means interference appears somewhere.
🙋
A transition fit can end up as either clearance or interference? Isn't that unstable?
🎓
That is the essence of a transition fit. Because of part-to-part variation, one pairing gives a tiny clearance and another a tiny interference. So it is not suited to "must definitely move" applications. A transition fit prioritises precise location and handles assembly and disassembly with a light press or tap. If you need a guaranteed fixing use an interference fit; if you need guaranteed motion use a clearance fit — be decisive.
Frequently Asked Questions
In the hole-basis system the hole tolerance is fixed as the reference (letter H, lower deviation always 0), and the fit type is created by changing the shaft tolerance class. The hole can be machined with a single tool and gauge, while the shaft is easy to adjust on a lathe, so ISO and JIS use the hole-basis system as standard. For example H7/g6 is a clearance fit and H7/k6 is a transition fit.
If the minimum clearance is 0 or more it is a clearance fit (there is always clearance, allowing relative motion); if the maximum clearance is 0 or less it is an interference fit (there is always interference and a press fit is needed); in between (minimum negative, maximum positive) it is a transition fit (depending on part variation it can be either). This tool detects the type from the maximum and minimum clearance. For the stress in interference fits, see the dedicated press-fit / shrink-fit tools.
IT (International Tolerance grade) represents the width of the tolerance (the precision); the smaller the number, the higher the precision. For the same grade, the tolerance width widens as the basic size grows — IT7 is 25 μm at Ø50 and 46 μm at Ø200. General machine parts use roughly hole IT7 and shaft IT6, precision parts use IT5, and rough parts IT9-IT11.
H7/g6 means the hole is tolerance class H7 and the shaft is g6. The letter is the fundamental deviation (the position of the tolerance zone) and the number is the IT grade (the width of the tolerance). Holes use uppercase, shafts lowercase. H is the hole basis (lower deviation 0), and g means the shaft upper deviation is slightly negative (slightly smaller), giving a small clearance. H7/g6 is the classic clearance fit for a part that slides smoothly.
Real-World Applications
Rotating shafts in bearings and bushings: A shaft rotating inside a plain bearing or bushing needs a small clearance to keep an oil film. A clearance fit such as H7/f7 or H8/e8 is chosen, securing the clearance from the rotational speed and load. Too small a clearance causes seizure; too large causes runout and noise.
Locating pins, keys and gears: Parts that must not have play but should still be removable use transition fits (H7/k6, H7/js6, H7/n6) — gear hubs, couplings and dowel pins are typical. Assembly is by a light tap or press, with disassembly still possible if needed: a delicate balance.
Working with geometric tolerances and drawings: A fit tolerance is part of the dimensional tolerancing, and on a real drawing it is combined with geometric tolerances such as concentricity and cylindricity. The design flow is to fix the hole and shaft limit deviations with this tool, then add geometric tolerances as the function requires. Over-tight tolerances drive cost up sharply, so keep them to what the function needs.
Cost and manufacturability: Tightening the IT grade by one step raises machining cost considerably. Whether grinding is needed or turning suffices, and whether gauge inspection is required, is set by the IT grade. Changing the basic size in this tool shows that for the same grade the tolerance widens with larger parts. Choosing the loosest grade that meets the function is the basis of cost-optimal design.
Common Misconceptions and Pitfalls
A common mistake is to think a transition fit always fits "just right". A transition fit is a range that, depending on part variation, can be a tiny clearance or a tiny interference. The average may sit near the middle, but the worst pairings give a few μm of clearance or a few μm of interference. For applications that must definitely move or must definitely stay fixed, choose a clearance or interference fit clearly, not a transition fit.
Next, the belief that tighter tolerance is always better. Tightening the IT grade by a single step raises cost steeply through added grinding and stricter inspection. Demanding 5 μm precision when 50 μm of clearance is functionally enough is the designer's self-indulgence. The correct attitude is to choose the loosest grade that satisfies the required fit, and this tool helps you find that "just right" grade.
Finally, ignoring temperature. Fits are specified at room temperature (20 °C). Combining materials with different thermal expansion — a steel shaft in an aluminium hole — changes the clearance markedly at operating temperature. A bearing used hot may have a clearance fit at room temperature but tighten when hot, or vice versa. For critical locations, calculate the dimensional change at operating temperature separately and do not judge from the room-temperature fit designation alone. Note also that the contact pressure and stress of a true interference fit (press fit, shrink fit) are outside the scope of this tool — use a dedicated press-fit / shrink-fit tool alongside it.