What is ISO 286?
ISO 286 defines the system of limits and fits for holes and shafts. If you’ve ever seen a callout like H7/g6 on a drawing and wondered what it actually means, this is your guide.
The metric system uses a code-based approach instead of explicit ± tolerances. Once you understand the logic, it’s faster and more powerful than specifying every tolerance individually.
How the Code Works
- Capital letters (H, G, K, N, P, S) → always refer to the HOLE
- Lowercase letters (g, h, k, n, p, s) → always refer to the SHAFT
- The letter sets the position of the tolerance zone relative to nominal (how far above or below the line)
- The number (IT grade) sets the width of the tolerance band. Lower = tighter = more expensive.
H is special: it means the hole starts at nominal (zero deviation on the low side). That’s why most fits are “H-something” — you make the hole a standard size and adjust the shaft to achieve the fit you want. This is called the Hole Basis System, and it’s the standard approach for 95% of engineering work.
Common Fits — The Cheat Sheet
You don’t need to memorize the entire ISO 286 standard. These are the fits that cover the vast majority of real-world applications:
Clearance Fits (Parts Move Freely)
| ISO Code | Fit Name | Use Case |
|---|---|---|
| H11/c11 | Loose Running | Dirty environments, agricultural equipment, pivots with low precision requirements |
| H9/d9 | Free Running | High-speed rotation, low loads. Idler pulleys, free-spinning bushings. |
| H8/f7 | Close Running | Good general-purpose bearing fit. Moderate speed, moderate load. |
| H7/g6 | Sliding / Precision | The gold standard for moving parts. Minimal play but slides freely. Spigots, precision guides, instrument bearings. |
| H7/h6 | Location Clearance | Snug location fit. No perceptible play, but still assembles by hand. Stationary parts that need precise alignment. |
Transition Fits (Could Be Tight or Loose)
| ISO Code | Fit Name | Use Case |
|---|---|---|
| H7/k6 | Locational Transition | Line-to-line fit. Light tap with a rubber mallet. The go-to for locating dowel pins. |
| H7/n6 | Locational Interference | Heavier press or hammer. Still disassemblable without damage. Tight dowels, precision locators. |
Interference Fits (Pressed Together)
| ISO Code | Fit Name | Use Case |
|---|---|---|
| H7/p6 | Light Press | Arbor press fit. Bearing outer races into aluminum housings. Semi-permanent. |
| H7/s6 | Medium Press | Hydraulic press required. Bearing races into steel housings. Permanent assembly. |
| H7/u6 | Heavy Press / Shrink | Shrink fit (heat the housing or freeze the shaft). Maximum grip. Gear hubs, coupling hubs. |
IT Grade Reference
The number in the code (H7, g6) is the IT grade — it controls how wide the tolerance band is. Here’s what each grade means in practice:
| IT Grade | Typical Tolerance (for 25mm) | Application |
|---|---|---|
| IT5 | 0.009 mm (0.00035″) | Precision grinding. Gauge blocks, instrument parts. |
| IT6 | 0.013 mm (0.0005″) | Fine machining. Most shaft fits. |
| IT7 | 0.021 mm (0.0008″) | Standard machining. Most hole fits (H7). |
| IT8 | 0.033 mm (0.0013″) | General machining. Bearings, bushings. |
| IT9 | 0.052 mm (0.002″) | Milling, boring. Free-running fits. |
| IT10 | 0.084 mm (0.0033″) | Rough machining. |
| IT11 | 0.130 mm (0.005″) | Stamping, rough turning. Loose fits. |
Note: Tolerance width varies with nominal size. The values above are for a 25mm nominal diameter. Larger parts get wider tolerances; smaller parts get tighter ones.
Practical Tips
The 80/20 Rule
If you only remember three fits, make it these:
- H7/g6 — things that need to slide (precision clearance)
- H7/k6 — things that need to stay put but come apart (dowel pins)
- H7/p6 — things that need to be pressed in (bearings)
Hole Basis vs. Shaft Basis
Almost always use Hole Basis (capital H for the hole). Why? Holes are harder to adjust — you drill or ream to a standard size, then turn the shaft to match. Shaft Basis (lowercase h for the shaft) is only used when you have a standard shaft size you can’t change (like off-the-shelf shafting).
Converting to US-Style Tolerances
If you need to put actual numbers on a drawing for a US shop:
- Look up the nominal size in the ISO 286 tolerance table
- Find the upper and lower deviations for both the hole and shaft codes
- Convert to your preferred format (bilateral ±, unilateral, or limit dimensions)
Or use our Engineering Unit Converter for quick mm↔inch conversions.
When NOT to Use ISO 286
- Sheet metal — tolerances are governed by bend allowance, not fits
- Castings/forgings — use ISO 8062 (casting tolerances) instead
- Plastic parts — thermal expansion and creep make tight fits unreliable; consult material data sheets
- Threads — thread tolerances use their own system (ISO 965 for metric, ASME B1.1 for unified)