ISO 286 tolerance codes look cryptic but follow logical rules. Knowing them prevents over-spec and saves money.
Two-Letter System
- Lowercase letter (h, j, k…) — Shaft (external feature) tolerance
- Uppercase letter (H, J, K…) — Hole (internal feature) tolerance
- Number (6, 7, 8…) — Tolerance grade (smaller = tighter)
Tolerance Grades (IT)
| Grade | 50mm dia tolerance | Typical Use |
|---|---|---|
| IT5 | ±5.5 µm | Precision instruments |
| IT6 | ±8 µm | High-precision shafts |
| IT7 | ±12.5 µm | General precision |
| IT8 | ±19.5 µm | Standard machining |
| IT9 | ±31 µm | Reamed holes (typical) |
| IT10 | ±50 µm | Drilled holes |
| IT11 | ±80 µm | Rough machining |
Common Hole Tolerances
- H7 — Standard precision hole (location on bearings, fits with h6 shaft for clearance)
- H8 — Standard machining (drilled + reamed, easier to achieve)
- H9 — General fit, easy to machine
- H11 — Loose fit, easy production
Common Shaft Tolerances
- h6 — Standard precision shaft (mates with H7 hole)
- h8 — General precision (mates with H9 hole)
- g6 — Slightly under nominal, sliding fit with H7
- n6 — Above nominal, light press fit with H7
- p6 — Press fit with H7
- r6 — Heavy press fit with H7
Common Fits
| Fit Type | Hole/Shaft | Use |
|---|---|---|
| Loose running | H11/c11 | Pulleys on shafts (heat differential) |
| Free running | H9/d9 | Wide variation acceptable |
| Close running | H8/f7 | Bearings, journals, sliding members |
| Sliding fit | H7/g6 | Precision sliding fits |
| Locational clearance | H7/h6 | Precision location, parts can move freely |
| Light press | H7/n6 | Bearings on light-load shafts |
| Press fit | H7/p6 | Standard interference for medium loads |
| Heavy press | H7/s6 | Permanent assemblies, high loads |
Cost Implications
| Tolerance | Relative Cost | Production Method |
|---|---|---|
| H11 | 1× | Drilled |
| H9 | 1.5× | Reamed |
| H8 | 2× | Reamed precision |
| H7 | 3-5× | Reamed + bored / honed |
| H6 | 10-20× | Honed/lapped |
Don’t Over-Spec
Many drawings call out H7 when H9 would work. Each grade tighter = exponentially more cost. Question every H6/H7 callout: does the application actually need it?
For H7 reaming, see our H7 reamer guide.
