ISO Fit Tolerance Calculator
Enter a nominal size and the hole and shaft classes (H7 and g6, say) to get both sets of limits in millimeters and inches, the loosest and tightest the pair can be, and whether it is a clearance, transition or interference fit. One click loads any of the ISO preferred fits, hole basis or shaft basis. Sizes up to 500 mm (19.685 in).
Hole and shaft fit
The shaft is always smaller than the hole.
| Feature | Deviations (µm) | Limits (mm) | Limits (in) | Tolerance (µm) |
|---|---|---|---|---|
| Hole H7 | +21 / 0 | 25.021 / 25.000 | 0.98508 / 0.98425 | 21 |
| Shaft g6 | -7 / -20 | 24.993 / 24.980 | 0.98398 / 0.98346 | 13 |
How the limits are worked out
- Size step: ISO 286 groups sizes into steps (over 18 up to and including 30 mm, for example). D is the geometric mean of the step, so every size in it gets the same limits relative to nominal.
- Tolerance width: i = 0.45 x cube root of D + 0.001 x D micrometers. IT5 to IT11 are 7i, 10i, 16i, 25i, 40i, 64i and 100i, and every five grades after that the width grows by ten (IT12 = 10 x IT7).
- Zone position: each letter has its own formula, for example g at -2.5 x D^0.34 and u at IT7 + D. H and h sit on nominal.
- Holes mirror shafts: EI = -es for C to H and ES = -ei for K to U, plus a small correction (delta, the step from one grade to the next) for K, M and N up to IT8 and P to U up to IT7. That is why G7/h6 matches H7/g6 exactly.
- Fit: maximum clearance = largest hole - smallest shaft; minimum clearance = smallest hole - largest shaft. A negative number is interference.
The standard's published tables are rounded and, for the smallest sizes, set by hand. Where they differ from the formula, the calculator uses the table value. Confirm limits on critical parts against ISO 286-2, and remember that a fit also depends on form, surface finish and temperature: a bore with 0.010 mm of taper or a rough reamed finish will not behave like the numbers say. See the surface finish chart for the finish that suits each grade.
IT6 and IT7 work is turning, reaming and grinding territory, and it has to be measured to match. Send the drawing and U.S. shops that hold these tolerances quote it, or browse CNC machine shops.
Frequently asked questions
- What does H7/g6 mean?
- The capital letter and number are the hole class and the lower-case pair is the shaft class. The letter places the tolerance zone relative to the nominal size (the fundamental deviation) and the number is the IT grade that sets how wide the zone is. H puts the bottom of the hole zone on nominal; g puts the shaft zone just under it. At 25 mm, H7 is +0.021/0 mm and g6 is -0.007/-0.020 mm, so every pair has between 0.007 and 0.041 mm of clearance.
- Hole basis or shaft basis?
- Most drawings use hole basis: the hole stays at H and the shaft letter changes. Holes are sized with fixed tools such as reamers and plug gauges, so keeping one hole size is cheaper. Shaft basis (an h shaft with the hole letter changing) suits ground bar used as-is and long shafts that carry several parts with different fits. ISO sets the hole deviations so that G7/h6 gives the same clearances as H7/g6, and the calculator lets you switch between the two sets of presets.
- Which fit should I pick?
- Running fits (c11, d9, f7 shafts in an H hole) turn freely and loosen from c to f. g6 slides and h6 locates without play but still assembles by hand. k6 and n6 are transition fits for accurate location that goes together with light force. p6, s6 and u6 are interference fits, from a locating press fit to a shrink fit that carries torque. The preferred fits chart lists what each is for.
- How does the calculator get its numbers?
- It computes them from the ISO 286-1 formulas: the tolerance factor i = 0.45 x cube root of D + 0.001 x D, IT grades as multiples of i, the fundamental deviation formulas for each letter, and the delta rule for K to U holes. The results are rounded the way the standard rounds. Where the published ISO 286-2 table differs from the rounded formula (mostly sizes up to 3 mm), the table value is used, and every letter offered was checked against the published table.
- Can I use ISO fits on inch parts?
- Yes. Enter the size in inches and the calculator converts it to millimeters and applies the ISO size step, then shows the limits in both units. Inch drawings more often call ANSI B4.1 classes (RC, LC, LT, LN and FN), which play the same roles but are tabulated in thousandths of an inch and do not match ISO numbers exactly.
- Which classes does it cover?
- Holes C, D, E, F, G, H, JS, K, M, N, P, R, S and U, and the matching shafts c to u, for sizes up to 500 mm. H and h run from IT01 to IT18, JS and js from IT1 to IT18, and the other letters from IT3 to IT13. IT14 to IT18 start above 1 mm. The rarely used letters a, b, j, t and v to zc are left out rather than approximated. For those, and for anything safety critical, check ISO 286-2.
