ISO tolerance grades.
ISO 286 tolerance reference — IT grade values across nominal size ranges, plus common hole and shaft fits (H7, h6, k6, n6, etc.) with actual dimensional values. The system everyone in metric machining uses, demystified.
IT grade values by size range
Each "IT grade" defines the total tolerance band width for a given nominal size. Lower numbers = tighter tolerance. IT5 is precision-ground; IT7 is standard machining; IT11 is rough casting. All values in micrometers (μm).
| Nominal size | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 |
|---|---|---|---|---|---|---|---|
| ≤ 3 mm | 4 μm | 6 μm | 10 μm | 14 μm | 25 μm | 40 μm | 60 μm |
| 3–6 mm | 5 μm | 8 μm | 12 μm | 18 μm | 30 μm | 48 μm | 75 μm |
| 6–10 mm | 6 μm | 9 μm | 15 μm | 22 μm | 36 μm | 58 μm | 90 μm |
| 10–18 mm | 8 μm | 11 μm | 18 μm | 27 μm | 43 μm | 70 μm | 110 μm |
| 18–30 mm | 9 μm | 13 μm | 21 μm | 33 μm | 52 μm | 84 μm | 130 μm |
| 30–50 mm | 11 μm | 16 μm | 25 μm | 39 μm | 62 μm | 100 μm | 160 μm |
| 50–80 mm | 13 μm | 19 μm | 30 μm | 46 μm | 74 μm | 120 μm | 190 μm |
| 80–120 mm | 15 μm | 22 μm | 35 μm | 54 μm | 87 μm | 140 μm | 220 μm |
| 120–180 mm | 18 μm | 25 μm | 40 μm | 63 μm | 100 μm | 160 μm | 250 μm |
| 180–250 mm | 20 μm | 29 μm | 46 μm | 72 μm | 115 μm | 185 μm | 290 μm |
Common fits at Ø25 mm (worked example)
Same tolerance grade gives different actual dimensions depending on the nominal size. Below are the most-used hole and shaft tolerances worked out at Ø25 — copy the pattern and re-derive for your nominal size using the IT grade table above.
| Tolerance | Type | Upper Δ | Lower Δ | Actual size at Ø25 | Use |
|---|---|---|---|---|---|
| H7 | Hole | +21 | 0 | 25.000 to 25.021 | Standard ground hole. The reference for most fits. |
| H8 | Hole | +33 | 0 | 25.000 to 25.033 | Machined hole, looser tolerance. |
| H9 | Hole | +52 | 0 | 25.000 to 25.052 | Coarser machined hole, free fit. |
| H11 | Hole | +130 | 0 | 25.000 to 25.130 | Cast or rough hole. Drilled-only OK. |
| h6 | Shaft | 0 | −13 | 24.987 to 25.000 | Precision shaft, ground. |
| h7 | Shaft | 0 | −21 | 24.979 to 25.000 | Standard ground shaft. |
| h9 | Shaft | 0 | −52 | 24.948 to 25.000 | Turned shaft. |
| g6 | Shaft | −7 | −20 | 24.980 to 24.993 | Slight clearance. Used for sliding fit. |
| f7 | Shaft | −20 | −41 | 24.959 to 24.980 | Free running with lubrication. |
| k6 | Shaft | +15 | +2 | 25.002 to 25.015 | Transition fit, light press. |
| n6 | Shaft | +28 | +15 | 25.015 to 25.028 | Medium press fit, requires arbor press. |
| p6 | Shaft | +35 | +22 | 25.022 to 25.035 | Press fit, permanent assembly. |
| s6 | Shaft | +48 | +35 | 25.035 to 25.048 | Heavy press / shrink fit, requires heating. |
Reading the system. Each tolerance is two parts: a letter for the position of the tolerance band relative to nominal (uppercase H, K, N for holes; lowercase h, k, n for shafts) and a number for the IT grade (the band's width). So "H7" is a hole with the band starting at nominal, IT grade 7 wide. "h7" is a shaft with the band ending at nominal, IT grade 7 wide. "H7/h7" describes a clearance fit of 0 to 21 μm at Ø10–18 mm, or 0 to 21 μm at Ø18–30 mm — wider at larger nominal sizes.
Common fits and what they're for
| Fit designation | Type | Use case |
|---|---|---|
| H11/c11 | Loose running | Rough applications, large clearance, e.g. agricultural shafts |
| H9/d9 | Free running | Standard journal bearing fit |
| H8/f7 | Close running | Precision running fit with light load and lubrication |
| H7/g6 | Sliding | Easy to assemble, no significant force; locating slides |
| H7/h6 | Locational clearance | Easy assembly, can rotate freely; gauge blocks |
| H7/k6 | Transition (light press) | Press-on assembly, removable with arbor press |
| H7/n6 | Transition (medium press) | Light interference; small motor / pulley fit |
| H7/p6 | Press | Permanent assembly, requires substantial force |
| H7/s6 | Heavy press / shrink | Shaft heated for assembly; gear hub on shaft |
Common pitfalls
- Uppercase and lowercase aren't optional. H7 (hole) and h7 (shaft) are different tolerances. A drawing that says just "7" is ambiguous and wrong.
- The same fit gives different clearance at different sizes. H7/h7 at Ø10 is up to 18 μm of clearance; at Ø250 it's up to 92 μm. Don't memorize "H7/h7 = 21 μm" — that's only true at Ø18–30 mm.
- Hole-basis (H) is the standard. Industry convention is to use H as the hole tolerance and adjust the shaft. Hole tools (reamers, broaches) come in standard sizes; shafts are easier to grind to size.
- "Press fit" depends on materials and surface finish. A theoretically valid press fit (e.g. H7/p6) can still gall, score, or seize if the surface finish is rough or the materials are similar (steel-on-steel without lubrication). The tolerance band alone doesn't guarantee success.
- ISO 286 isn't the same as ANSI/ASME Y14.5 GD&T. ISO 286 is for size tolerances on cylindrical features. GD&T is the broader system for geometric tolerances (flatness, parallelism, true position). They're complementary, not competing.
Common questions
What does 'H7/g6' mean on an engineering drawing?
It's an ISO 286 hole/shaft fit. H7 is the hole tolerance — the upper-case letter (H) sets the position relative to nominal size (H = lower limit at nominal), the number (7) sets the precision class. g6 is the shaft tolerance — lower-case g positions the shaft slightly below nominal, grade 6 is tighter. Together H7/g6 creates a 'sliding fit' with small clearance.
What's the difference between IT6 and IT7?
IT (International Tolerance) grades define the size of the tolerance zone in micrometers. IT6 is tighter than IT7. For a 30 mm dimension, IT6 = 13 µm, IT7 = 21 µm. Lower IT numbers are tighter, achievable through better machining processes. IT5 and below typically need grinding; IT11+ can be done with rough turning.
How tight is 'press fit' vs 'sliding fit'?
Press fit (typical H7/p6) has interference — the shaft is slightly bigger than the hole, requiring force to assemble. Sliding fit (H7/g6) has small clearance — fits by hand, slides smoothly. Loose running fit (H11/c11) has large clearance for thermal expansion or rough manufacturing. Choose based on whether you need motion, location, or torque transmission.
Why are tolerances asymmetric — why not just ±0.05?
ISO basic-hole and basic-shaft systems use one-sided tolerances because most fits work by adjusting one part to match a standard. A H7 hole is always at nominal-or-larger, never smaller. Choosing matching shaft tolerances (g6, h6, k6, p6, etc.) controls whether the fit is clearance, transition, or interference. This is more rigorous than symmetric tolerances for assembly.
Can I just use ±0.05 instead of looking up ISO tolerances?
For prototypes or non-critical parts, yes. For production parts that need to interchange, ISO tolerances are how machinists, suppliers, and inspectors communicate. A drawing with ±0.05 doesn't specify which limit matters most — a shaft 30.05 may be unacceptable in one assembly but fine in another. ISO codes make intent unambiguous.
Sources
- IT grade values: ISO 286-1:2010 — Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits, Table 1.
- Fundamental deviations: ISO 286-1:2010, Tables 2–6 (shaft and hole deviations).
- Recommended fits: ISO 286-2:2010 — Tables of standard tolerance classes and limit deviations for holes and shafts.
- US equivalent: ANSI B4.2 (preferred metric limits and fits) uses the same ISO 286 system; ANSI B4.1 (older inch system) does not.
Disclaimer. Tolerance values shown are nominal per ISO 286-1:2010. For production drawings, always verify against the current standard and confirm with your manufacturer that the specified tolerance is achievable with their process.