H8 hole tolerance (IT8)
Capital H8 is an ISO 286 hole tolerance class (IT8, EI = 0): at Ø10 mm the deviations are +0.022/0 mm, at Ø25 mm +0.033/0 mm (upper/lower). Looking for the h8 shaft?
Hole tolerance class H8 per ISO 286-1:2010: fundamental deviation EI = 0, grade IT8 (14 µm at ≤ 3 mm to 97 µm at 400–500 mm). For an exact size, open the calculator with H8 selected.
- Type
- Hole
- Grade
- IT8
- Tolerance
- 14 … 97 µm
- Fund. deviation
- EI = 0
H8 limits by size range
| Size (mm) | Upper (µm) | Lower (µm) | Ø max (mm) | Ø min (mm) | Upper (mm) | Lower (mm) | Dev. (in) |
|---|---|---|---|---|---|---|---|
| ≤ 3 | +14 | 0 | 3.014 | 3.000 | +0.014 | 0 | +0.00055 / 0 |
| 3 – 6 | +18 | 0 | 6.018 | 6.000 | +0.018 | 0 | +0.00071 / 0 |
| 6 – 10 | +22 | 0 | 10.022 | 10.000 | +0.022 | 0 | +0.00087 / 0 |
| 10 – 14 | +27 | 0 | 14.027 | 14.000 | +0.027 | 0 | +0.00106 / 0 |
| 14 – 18 | +27 | 0 | 18.027 | 18.000 | +0.027 | 0 | +0.00106 / 0 |
| 18 – 24 | +33 | 0 | 24.033 | 24.000 | +0.033 | 0 | +0.00130 / 0 |
| 24 – 30 | +33 | 0 | 30.033 | 30.000 | +0.033 | 0 | +0.00130 / 0 |
| 30 – 40 | +39 | 0 | 40.039 | 40.000 | +0.039 | 0 | +0.00154 / 0 |
| 40 – 50 | +39 | 0 | 50.039 | 50.000 | +0.039 | 0 | +0.00154 / 0 |
| 50 – 65 | +46 | 0 | 65.046 | 65.000 | +0.046 | 0 | +0.00181 / 0 |
| 65 – 80 | +46 | 0 | 80.046 | 80.000 | +0.046 | 0 | +0.00181 / 0 |
| 80 – 100 | +54 | 0 | 100.054 | 100.000 | +0.054 | 0 | +0.00213 / 0 |
| 100 – 120 | +54 | 0 | 120.054 | 120.000 | +0.054 | 0 | +0.00213 / 0 |
| 120 – 140 | +63 | 0 | 140.063 | 140.000 | +0.063 | 0 | +0.00248 / 0 |
| 140 – 160 | +63 | 0 | 160.063 | 160.000 | +0.063 | 0 | +0.00248 / 0 |
| 160 – 180 | +63 | 0 | 180.063 | 180.000 | +0.063 | 0 | +0.00248 / 0 |
| 180 – 200 | +72 | 0 | 200.072 | 200.000 | +0.072 | 0 | +0.00283 / 0 |
| 200 – 225 | +72 | 0 | 225.072 | 225.000 | +0.072 | 0 | +0.00283 / 0 |
| 225 – 250 | +72 | 0 | 250.072 | 250.000 | +0.072 | 0 | +0.00283 / 0 |
| 250 – 280 | +81 | 0 | 280.081 | 280.000 | +0.081 | 0 | +0.00319 / 0 |
| 280 – 315 | +81 | 0 | 315.081 | 315.000 | +0.081 | 0 | +0.00319 / 0 |
| 315 – 355 | +89 | 0 | 355.089 | 355.000 | +0.089 | 0 | +0.00350 / 0 |
| 355 – 400 | +89 | 0 | 400.089 | 400.000 | +0.089 | 0 | +0.00350 / 0 |
| 400 – 450 | +97 | 0 | 450.097 | 450.000 | +0.097 | 0 | +0.00382 / 0 |
| 450 – 500 | +97 | 0 | 500.097 | 500.000 | +0.097 | 0 | +0.00382 / 0 |
Deviations at the upper end of each range (Ø columns use that size); use the calculator for the exact size. — : not defined by ISO 286 for that size. Upper/Lower (mm) and inch columns are shown on wider screens.
About H8
H8 denotes a hole tolerance zone with zero line deviation in the ISO 286 system. It is used in H8/e8 (Plain bearings in gearboxes and pumps, sliding gears, easily assembled couplings.) and H8/f7 (Accurate machines and shafts in plain bearings at moderate speeds and pressures: machine tool spindles, gearbox shafts.).
At Ø25 mm H8 is +0.033 / 0 mm, i.e. 25.000 to 25.033 mm.
How the limits are calculated
H8 uses grade IT8 (a standard tolerance of 33 µm at 18–30 mm). Fundamental deviation: EI = 0: the tolerance zone sits entirely above the nominal size, and IT8 sets its width. See the fundamental deviation table for every letter. Values per ISO 286-1:2010.
H8 in practice
Where it's used
H8 is one grade coarser than H7: IT8 is about 1.6 times IT7, so the bore gets roughly 60 % more room, which often means one fewer finishing operation. It is the hole of running and locating fits where a few extra micrometres of play don't matter: H8/f7 for plain-bearing journals, H8/e8 for gearbox and pump bearings, H8/h8 and H8/h7 for parts slid on and fixed by a key or pin. Honed hydraulic cylinder tube is commonly supplied to H8.
How to check it
Plug gauges work as for H7: the GO end is the nominal size, the NO-GO end the upper limit. With instruments the 10:1 rule of thumb is easier to meet than at IT7; a dial bore gauge reading to 2 µm resolves an H8 band comfortably. Check roundness separately on honed or thin-walled bores: an oval bore can pass a two-point gauge and still bind the shaft.
Easily confused with
Designers often default to H7 out of habit when the function only needs H8. Check what the fit actually requires before paying for the tighter bore. h8 in lower case is the shaft class with its zone below the nominal size; together, H8/h8 is a locational clearance fit with zero minimum clearance.
Worked example
A Ø50 H8/f7 plain bearing: the bore measures 50.000–50.039 mm (+0.039/0 mm). With an f7 journal, the diametral clearance is 0.025–0.089 mm, enough room for an oil film at moderate speed.