Metric Thread Tolerances
Metric thread tolerance classes (6g, 6H) and fit for bolts and nuts—when to use which.
Tolerance class basics
ISO metric threads use a tolerance grade (number) and position (letter). External threads (bolts) often use 6g; internal threads (nuts) use 6H. The number is the tolerance grade (lower = tighter); the letter sets the limit position. For most fasteners, 6g/6H gives a good balance of fit and cost. Use our thread identifier and tap drill calculator for standard sizes.
Common tolerance classes
| Class | Application | Fit |
|---|---|---|
| 6g (external) | General bolts | Medium clearance |
| 6H (internal) | General nuts | Medium |
| 6e | External, more clearance | Looser |
| 5g / 5H | Precision | Tighter |
Why it matters
Tighter tolerances reduce play and improve alignment; looser tolerances ease assembly and allow for coating or debris. For tap drill size, use the same tap drill chart and tap drill calculator—tolerance mainly affects the tap and gauge, not the hole size for standard threads.
FAQ
What do metric thread tolerance classes mean?
The number is tolerance grade (e.g. 6); the letter is position (g, H). 6g = bolts, 6H = nuts; together they give a medium fit.
What is 6g vs 6H thread?
6g is for external threads (bolts) with allowance; 6H is for internal (nuts) with no allowance. Standard pairing for general use.
When do I need tighter thread tolerance?
For precision, positioning, or minimal backlash. Standard 6g/6H is fine for most fasteners.