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ISO 286 Hole-Basis Fits Explained (H7/g6, H7/h6, H7/k6, H7/p6)
Guide · Updated October 28, 2026
A callout like Ø25 H7/g6 packs four limits into six characters. The capital letter and number describe the hole, the lowercase pair describes the shaft. The letter sets where the tolerance zone sits relative to the nominal size (the fundamental deviation) and the number sets how wide it is (the IT grade). In the hole-basis system the hole is always H, so its lower limit sits exactly on nominal, and the shaft letter chooses the fit: clearance, transition or interference.
Set your bore and mic checks to the real limits
Once you have the limits, set a micrometer, bore gage or snap gage on a stack at each one. The free Gage Block Calculator builds the gage block stack for any size you need to set.
Step by step
- Find the size band: ISO 286 groups sizes into bands. 25 mm falls in the over 18 to 30 mm band, and every value in that row applies.
- Look up the hole: For H7 at 18–30 mm the deviations are 0 and +21 µm, so the hole is 25.000 to 25.021 mm.
- Look up the shaft: For g6 at 18–30 mm the deviations are −7 and −20 µm, so the shaft is 24.980 to 24.993 mm.
- Work out the fit: Maximum clearance = largest hole − smallest shaft = 25.021 − 24.980 = 0.041 mm. Minimum clearance = smallest hole − largest shaft = 25.000 − 24.993 = 0.007 mm.
- Convert if needed: Divide by 25.4: the hole is 0.98425 to 0.98508 in and the clearance is 0.0003 to 0.0016 in.
- Set your gages: Set the bore gage at the hole limits and the micrometer or snap gage at the shaft limits using gage blocks.
- Measure at temperature: Check parts at or near 20 °C (68 °F), the ISO reference temperature, especially at the tighter grades.
Common hole-basis fits by size
| Nominal size (mm) | H7 hole | g6 shaft | h6 shaft | k6 shaft | p6 shaft |
|---|---|---|---|---|---|
| over 6 to 10 | 0 / +15 | −5 / −14 | 0 / −9 | +1 / +10 | +15 / +24 |
| over 10 to 18 | 0 / +18 | −6 / −17 | 0 / −11 | +1 / +12 | +18 / +29 |
| over 18 to 30 | 0 / +21 | −7 / −20 | 0 / −13 | +2 / +15 | +22 / +35 |
| over 30 to 50 | 0 / +25 | −9 / −25 | 0 / −16 | +2 / +18 | +26 / +42 |
| over 50 to 80 | 0 / +30 | −10 / −29 | 0 / −19 | +2 / +21 | +32 / +51 |
Deviations in micrometers (µm; 1 µm = 0.001 mm ≈ 0.00004 in) from ISO 286-2. Check the standard or your drawing note for sizes and grades not shown.
What each fit is for
H7/g6 is a close sliding or locating clearance fit, used for parts that must move with little play, such as sliding gears and spigots. H7/h6 is a locating clearance fit with zero minimum clearance: parts slide together by hand when lubricated but have very little play. H7/k6 is a transition fit: depending on actual sizes it may have a little clearance or a little interference, and it is common for locating parts that are assembled with a mallet or light press. H7/p6 is a locating interference fit: parts are pressed together or shrunk, and they transmit light loads without keys.
Worked example: all four fits at 25 mm
With the hole at 25.000 to 25.021 mm: H7/g6 gives 0.007 to 0.041 mm clearance. H7/h6 (shaft 24.987 to 25.000 mm) gives 0 to 0.034 mm clearance. H7/k6 (shaft 25.002 to 25.015 mm) ranges from 0.019 mm clearance to 0.015 mm interference. H7/p6 (shaft 25.022 to 25.035 mm) gives 0.001 to 0.035 mm interference. In inches, 0.035 mm is about 0.0014 in, which is why p6 parts need a press or heat to assemble.
How to check the hole and shaft
An H7 bore at 25 mm has only 21 µm (about 0.0008 in) of tolerance, which is too tight for a caliper. Use a dial bore gage set on a ring or on gage blocks in a holder, or an inside micrometer with 0.001 mm resolution. For the shaft, a 0.001 mm or 0.0001 in micrometer checked on a 25 mm gage block is the usual tool. Measure in at least two directions 90° apart and at two depths, because out-of-roundness and taper eat into a tight tolerance.
Shaft-basis fits
Some drawings use the shaft-basis system instead, with the shaft at h and the hole letter changing, for example G7/h6 or P7/h6. The idea is the same: the h shaft has its upper limit on nominal, and the hole letter chooses the fit. Shaft-basis is common when one ground shaft carries several parts with different fits. Always look up the hole and shaft separately rather than assuming the deviations mirror each other exactly.
Tips
- Bores: small hole gages for bores below about 13 mm.
- Inch work: inch–metric conversion.
- Temperature: gage block thermal soak.
FAQ
What does H7 mean?
A hole whose lower limit is the nominal size, with an IT7 tolerance width. At 25 mm that is 0 to +0.021 mm.
Is H7/g6 a press fit?
No. It is a close clearance fit; at 25 mm it has 0.007 to 0.041 mm clearance.
What fit is H7/k6?
A transition fit. At 25 mm it ranges from 0.019 mm clearance to 0.015 mm interference.
What fit is H7/p6?
A locating interference fit. At 25 mm it has 0.001 to 0.035 mm interference.
Where do the numbers come from?
ISO 286-1 and ISO 286-2, the international standard for limits and fits.
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