Hole basis vs shaft basis
ISO 286 gives every fit two equivalent forms: hole-basis (hole fixed at H, shaft varies) and shaft-basis (shaft fixed at h, hole varies) — both give the same clearance at the same size.
Same clearance, different reference
| Fit | Hole | Shaft | Min (mm) | Max (mm) |
|---|---|---|---|---|
| H7/g6 | +0.021 / 0 | −0.007 / −0.020 | +0.007 | +0.041 |
| G7/h6 | +0.028 / +0.007 | 0 / −0.013 | +0.007 | +0.041 |
| H7/k6 | +0.021 / 0 | +0.015 / +0.002 | −0.015 | +0.019 |
| K7/h6 | +0.006 / −0.015 | 0 / −0.013 | −0.015 | +0.019 |
At Ø25 mm. H7/g6 and G7/h6 give the same min/max clearance; so do H7/k6 and K7/h6 — only which part reads as the zero-deviation reference changes.
When to use shaft basis
- One shaft carries several mating parts with different fits — a single ground shaft (h6) is cheaper to make than reaming a different hole class into each part on it.
- The shaft is a standard purchased part (drawn bar, precision ground stock) already held to an h-tolerance, and only the mating bores are made to suit.
- Rolling-bearing outer rings: bearing standards fix the outer ring as the shaft-basis reference, so the housing bore is toleranced to it (e.g. K7, N7, P7 against h6).
When to use hole basis
The default in general engineering: holes are more expensive to resize than shafts (reaming or boring to a non-standard diameter costs more than turning or grinding a shaft to size), so most catalogues, this calculator's defaults, and most drawings fix the hole at H and vary the shaft letter instead.
Hole-basis fits
Shaft-basis fits
Compare either system for your own size in the calculator (pick any hole and shaft class), or use the fit finder to search by target clearance instead.