ISO 286 Fits Explained: H7/g6 and Hole-Basis
ISO 286 is the international limits-and-fits system: one letter, one number, and a shaft/hole pair acquires a guaranteed relationship — slip, tap, or press — before either part is made. It is the vocabulary underneath almost every rotating or locating joint in mechanical design, and it repays an hour of understanding many times over.
What the callout is saying
A designation like Ø40 H7/g6 packs four facts into two codes. Each code is a letter plus a grade:
- Letter — position of the tolerance zone. For shafts, letters a–h sit below nominal (progressively less clearance), j–n straddle or start just above it, and p onward guarantee interference. For holes the letters are capitalized; H is the special case whose lower deviation is exactly zero, so an H hole never goes undersize.
- Number — width of the tolerance zone. This is the IT grade: IT5 is fine, IT11 is coarse. The grade number alone never tells you where the zone sits — that is the letter's job — only how wide it is.
- Case — hole or shaft. Uppercase applies to the hole (internal feature), lowercase to the shaft (external feature). H7 is a hole; g6 is a shaft.
So H7/g6 reads: a hole sized nominal-to-plus-IT7, against a shaft that starts below nominal by the g deviation and spans IT6. The result is always clearance — the only question is how much.
The IT grades are computed, not arbitrary
IT grades come from a standard tolerance unit that scales with size: i = 0.45·√³D + 0.001·D in micrometres, with D the geometric mean of the diameter step in millimetres. Grades are fixed multiples — IT6 = 10i, IT7 = 16i, IT8 = 25i, and so on — evaluated on stepped size ranges. The ranges designers use most:
| Nominal step (mm) | IT6 (µm) | IT7 (µm) | IT8 (µm) | IT9 (µm) | IT11 (µm) |
|---|---|---|---|---|---|
| >18–30 | 13 | 21 | 33 | 52 | 130 |
| >30–50 | 16 | 25 | 39 | 62 | 160 |
| >50–80 | 19 | 30 | 46 | 74 | 190 |
| >80–120 | 22 | 35 | 54 | 87 | 220 |
That is why the same fit code means different micrometres at different diameters — the table lookup is size-step dependent, and a few letters (c and the deep-interference letters from t onward) additionally change between the 30–40 and 40–50 sub-steps.
Hole-basis vs shaft-basis
Two conventions anchor the system. Hole-basis keeps the hole at H (lower deviation zero) and picks the shaft letter to set the fit. It is the default choice because holes are made with fixed-size tooling — drills, reamers, boring bars — so standardizing the hole and varying the turned or ground shaft is cheaper. Shaft-basis is the mirror image (h shaft, chosen hole letter) and survives where the shaft is already fixed: bought-in ground bar, standard bearings pressed onto a mandrel, or one shaft carrying several different fits.
The fits you will actually use
Limit clearances below are computed for the >30–40 mm step (hole always H7 = +25/0 µm; positive = clearance, negative = interference):
| Fit | Class | Limits (µm) | Typical use |
|---|---|---|---|
| H11/c11 | clearance | +120 / +440 | loose running: hinges, exposed shafts, dirty environments |
| H9/d9 | clearance | +80 / +204 | free running: general shafts with generous lubrication |
| H8/f7 | clearance | +25 / +89 | normal running: journal boxes, moderate-speed rotation |
| H7/g6 | clearance | +9 / +50 | precision sliding: accurate location that still moves freely |
| H7/h6 | clearance | 0 / +41 | locational clearance: spigots, hand-assembled parts, no perceptible play |
| H7/js6 | transition | +33 / −8 | slight clearance bias: dowels, precise location with easy assembly |
| H7/j6 | transition | +30 / −11 | similar to js6, marginally more likely to clear |
| H7/k6 | transition | +23 / −18 | true transition: gear hubs, couplings — snug, assembled with light taps |
| H7/m6 | transition | +16 / −25 | tight transition: accurate location, press possible |
| H7/n6 | transition | +8 / −33 | mostly interference: semi-permanent assembly |
| H7/p6 | interference | −1 / −42 | light press: bearings, bushings; can just touch at worst corner |
| H7/r6 | interference | −9 / −50 | medium press: gears and couplings transmitting torque |
| H7/s6 | interference | −18 / −59 | heavy press: permanent joints, significant assembly force |
| H7/t6 | interference | −23 / −64 | very heavy press or shrink fit territory |
| H7/u7 | interference | −35 / −85 | shrink-fit class: maximum transmissible torque without keys |
Check the sub-step. In the >40–50 mm sub-step the c, t and u shaft deviations shift (c to −130, t to +54, u to +70 µm), moving those three rows to roughly +130/+450, −29/−70 and −45/−95 µm. Every other row shown is identical across the whole 30–50 step. For any diameter outside 30–40 mm, recompute from the standard tables — do not extrapolate.
How to choose
- Start from function, not habit. Needs to move? Clearance — pick from how freely (h6→c11 is an increasing-play ladder). Needs to locate without fasteners? Transition. Needs to transmit load on its own? Interference — and then check the hub stresses, because a heavy press fit can crack a thin boss.
- Default to hole-basis H7. It is the cheapest accurate hole and every shaft letter pairs with it cleanly.
- Spend grades where they pay. Tightening a hole from IT8 to IT7 or a shaft from IT7 to IT6 is a process step change in cost. Put the fine grade on the feature that controls the fit; leave the other coarse.
- Remember the assembly. A press fit is not just a fit — it is a dimension in a stack-up that can close gaps elsewhere. Fits chosen in isolation surprise you at assembly.
For a worked walk through the preferred fits — which to pick and why — see the fit-selection guide.
Common mistakes
- Reading the grade as position: 6 is not "tighter than 7 in the same place" — f7 and f6 start at the same deviation and differ only in width.
- Specifying H7/n6 where you meant "definitely presses" — n6 can still clear by up to 8 µm at this size. If assembly must never slide, move to p6 or beyond.
- Copying a fit from a different diameter without re-reading the tables.
- Ignoring temperature: aluminium housing, steel shaft, hot service — differential expansion can eat a designed clearance whole.
These tables are what SuperNX applies automatically: fits declared on the model are read as the published ISO 286 zones rather than re-keyed numbers, so the analysis chain inherits the same values your supplier will measure against.
Next steps: check a press-fit interference with the press-fit calculator, or look up hole limits in the H7 tolerance chart.
Video chapters and transcript
- 0:00 Intro
- 0:20 Decoding the code
- 1:15 The two zones
- 1:53 Calculating clearance
- 2:29 Bigger diameters
- 3:13 Neighbouring fits
- 4:07 Typical uses
- 4:52 How to choose
- 5:37 Doing it automatically
Read the full transcript
0:00 Intro
You open a drawing and see Ø25 H7 slash g6. Two letters, two numbers, and a slash. So what does that actually allow? In the next few minutes we'll decode every character, work out the clearance, and finish with a simple way to choose a fit.
0:20 Decoding the code
Start with the slash. The part before it is the hole, and the part after it is the shaft. A capital letter is a hole. A lowercase letter is a shaft. The letter itself sets the position of the tolerance zone relative to the nominal size. For H, the zone starts at exactly zero and grows upward. That is the hole-basis system: you keep one reamer or boring size, and you vary the shaft to get the fit you want. The number is the IT grade, and it sets the width of the zone. At twenty-five millimetres, IT6 is thirteen microns and IT7 is twenty-one microns. The shaft is usually one grade finer than the hole, because shafts are easier to grind precisely.
1:15 The two zones
Here are the two zones at Ø25. The H7 hole runs from zero to plus twenty-one microns, so from twenty-five point zero zero zero to twenty-five point zero two one millimetres. The g6 shaft sits just below the zero line, from minus twenty to minus seven microns. That is twenty-four point nine eight zero to twenty-four point nine nine three. The zones never overlap, so every pair has clearance.
1:53 Calculating clearance
Now the clearance. The tightest case is the smallest hole with the largest shaft. So minimum clearance is the hole's lower limit minus the shaft's upper limit. Zero minus minus seven is seven microns. The loosest case is the largest hole with the smallest shaft. So maximum clearance is the hole's upper limit minus the shaft's lower limit. Twenty-one minus minus twenty is forty-one microns. So H7 g6 at twenty-five millimetres is a clearance fit of seven to forty-one microns.
2:29 Bigger diameters
The code stays the same when the diameter changes, but the numbers scale with size. At Ø40, the H7 hole runs from zero to plus twenty-five microns, and g6 runs from minus nine to minus twenty-five. Minimum clearance is zero minus minus nine: nine microns. Maximum clearance is twenty-five minus minus twenty-five: fifty microns. So the same H7 g6 gives seven to forty-one microns at Ø25, and nine to fifty at Ø40. Read the numbers for your size range.
3:13 Neighbouring fits
Hole basis means the hole stays the same and only the shaft changes. Here is H7 again with four shafts. The hole stays fixed. g6 gives clearance of seven to forty-one microns. h6 touches zero, so clearance is zero to thirty-four. This is the H7 over h6 you will often compare with H7 g6, and with a little more play. k6 straddles the zone: nineteen microns of clearance at best, fifteen of interference at worst. A transition fit. And p6 is always larger than the hole: interference of one to thirty-five microns. A press fit. Moving the shaft letter up the chart takes you from play to press.
4:07 Typical uses
Here is the family as a table. H7 g6 is the precision running or sliding fit: pins, guided shafts, a revolute joint with little play. Note that H7 over h6 is not a typo. A capital H is the hole and a lowercase h is the shaft, same letter, opposite roles. It is the locating, slip fit. H8 f7 is a running fit with lubrication and more play, twenty to seventy-four microns. H7 k6 is a transition fit with a light tap, and H7 p6 is a light press fit. These are common practice, so check the values for your diameter.
4:52 How to choose
So how do you choose? Start from function, not from a chart. First: does it move? If the shaft rotates or slides, you need clearance. H7 g6 when you want little play, H8 f7 when there is lubrication and you can afford more. Second: does it only locate? Then H7 h6 gives a slip fit, and H7 k6 if you want a light tap. Third: does it transmit torque or hold itself in place? Then you need interference, such as H7 p6. Check the assembly force. Whatever the answer, pick the loosest fit that does the job. Tighter costs more to make.
5:37 Doing it automatically
On a real assembly, that decision repeats for every joint. SuperNX reads your Siemens NX model, and assigns a preferred ISO 286 fit to each interface based on how the joint actually moves. Then it carries those fits into the worst-case and RSS analysis for every key characteristic. To check any fit yourself, use the free ISO 286 fit calculator at nxtolerance.com slash standards. No sign-up needed. Links are in the description.