Fareed Mohmed · Engineering
Tolerance Selector by Application Scenario
Select · Justify · Apply
Pick the kind of feature you're dimensioning — a general fabrication dimension, a bearing seat, a press fit — and get a starting-point tolerance and fit class, with the reasoning behind it.
Find Tolerance by Scenario
Tolerance Spectrum
Every scenario placed by relative tightness, loosest on the left to most precise on the right. Click any scenario to look it up above.
Very Coarse
Coarse
Medium
Fine
Precision
Quick-Scan Table
| Scenario | Band | Typical Fit / Class | Recommended Tolerance |
|---|---|---|---|
| General Non-Mating Dimension | Medium | ISO 2768-m | ±0.2 mm (ISO 2768 class m, medium) |
| Large or Cost-Sensitive Fabrication | Very Coarse | ISO 2768-c | ±0.5 mm (ISO 2768 class c, coarse) |
| Welded / Fabricated Frame Dimension | Very Coarse | ISO 13920 (medium class typical) | ±1.0 to ±2.0 mm (ISO 13920, size-dependent) |
| Bolt Clearance Hole | Coarse | ISO 273 medium series | Medium clearance fit, hole ≈ +0.2/–0 mm over bolt diameter |
| Sheet Metal Bracket (Formed) | Coarse | ISO 2768-m to -c | ±0.2 to ±0.5 mm depending on form complexity |
| Tapped Hole Position (Bolt Pattern) | Medium | General ± positional tolerance | ±0.1 to ±0.2 mm on hole position |
| Dowel Pin Locating Hole | Precision | H7 | H7 reamed hole (≈ +0.021/0 mm for a 30 mm bore, example) |
| Bearing Housing Bore (Outer Race Seat) | Fine | H7 | H7 (≈ +0.025/0 mm for a 50 mm bore, example) |
| Bearing Shaft Seat (Inner Race) | Fine | k6 | k6 (≈ +0.018/+0.002 mm for a 30 mm shaft, example) |
| Sliding Fit / Guide Rail Running Clearance | Medium | H7/g6 | H7/g6 clearance fit (≈ 0.009–0.041 mm diametral clearance for a 30 mm shaft, example) |
| Press Fit / Permanent Interference Assembly | Precision | H7/p6 | H7/p6 interference fit (≈ 0.001–0.032 mm interference for a 30 mm shaft, example) |
| Keyway Width (Shaft/Hub Key Fit) | Fine | N9 | N9 on the hub keyway width (±0.02 mm typical) |
| Precision Machined Mating Face | Precision | General ± tolerance + GD&T flatness | ±0.02 to ±0.05 mm, with flatness called out separately |
| Gasket / Seal Groove Dimension | Fine | General ± tolerance | ±0.05 to ±0.1 mm on groove width/depth |
Explanatory Notes
These are starting points, not finished callouts
Every recommendation here is a reasonable default for a typical case — the actual tolerance on your drawing should account for the real stack-up, the specific bearing or component manufacturer's fit recommendation, and how the part will actually be inspected. Treat this as where to start thinking, not where to stop.
Size tolerance is not the same as geometric tolerance
Everything here is a linear (size) dimensional tolerance. Flatness, parallelism, perpendicularity, position and runout are separate GD&T callouts that a size tolerance alone doesn't control — a hole can be perfectly on-size and still be badly out of position. Precision mating faces and bearing bores usually need both.
Check the tolerance against what the process can actually hold
Specifying H7 on a sand-cast surface, or ±0.02 mm on an as-welded dimension, sets up a feature that has to be machined afterward regardless of what the base process claims to achieve. Cross-check your chosen tolerance against the Manufacturing Process vs. Achievable Tolerance tool before it goes on a drawing.
Tighter isn't safer — it's just more expensive
A common instinct is to tighten every tolerance "to be safe." In practice this raises cost and rejection rate without necessarily improving the assembly, since an unnecessarily tight tolerance on a non-critical feature buys nothing functionally. Reserve tight tolerances for dimensions that genuinely drive fit, function or interchangeability.
Related tools
For clearance and tap-drill sizes by bolt diameter, see Bolt Clearance & Tap Drill Sizes. For what a chosen manufacturing process can realistically hold, see Manufacturing Process vs. Achievable Tolerance.