Tolerance calculator for ISO fits
Tolerance calculator for ISO 286 fits: enter a nominal size, pick a hole class and a shaft class, and you get the limits of both parts and the clearance or interference between them.
- Up to 500 mmHole and shaft letters c to u, grades IT1 to IT18. Set one class to None to look up a single hole or shaft.
- Checked twiceCompared with two published tables before launch. What we found.
- Links that keep the resultThe address bar follows your inputs, so a link opens the same fit.
Preferred fits at 25 mm
These ten hole-basis fits are the usual first choices, from loosest to tightest. Names and uses follow the tables in Wikipedia's engineering fit article. The clearance column is worked out for the size in the calculator and changes with it. A negative number is interference.
| Fit | Name | Clearance (µm) | Typical use |
|---|---|---|---|
| H11/c11 | Loose running | +110 to +370 | Pivots, latches, parts exposed to dirt, heat or corrosion |
| H9/d9 | Free running | +65 to +169 | High speeds, big temperature swings, heavy journal pressure |
| H8/f7 | Close running | +20 to +74 | Spindles, shafts and sliding rods at moderate speed |
| H7/g6 | Sliding | +7 to +41 | Turns and slides freely: guided shafts, sliding gears |
| H7/h6 | Location | 0 to +34 | Assembles by hand, slides when oiled: precise shaft guides |
| H7/k6 | Similar (transition) | -15 to +19 | Fitted with a rubber mallet: hubs, gears, pulleys, bearings |
| H7/n6 | Fixed (transition) | -28 to +6 | Light pressing force: plugs, driven bushes |
| H7/p6 | Press | -35 to -1 | Cold pressed: hubs, bearings, bushings |
| H7/s6 | Driving | -48 to -14 | Hot pressed or large force: permanent gears and bushes |
| H7/u6 | Forced | -61 to -27 | Shrink fit with a large temperature difference, not meant to come apart |
Working out H7/g6 at 25 mm by hand
The calculator only reads two tables from ISO 286-1: standard tolerances (IT grades) and fundamental deviations. Here is the same H7/g6 result done on paper.
- Find the size step. 25 mm falls in the step over 18 up to and including 30 mm. Tables split that step at 24 mm, but only letters such as t and u change there, so H and g read the same on both sides.
- Read the zone widths. In that step IT7 is 21 µm and IT6 is 13 µm. The 7 in H7 and the 6 in g6 pick these grades.
- Place the hole. Letter H puts the lower deviation at 0, so H7 runs from 0 to +21 µm: 25.000 to 25.021 mm.
- Place the shaft. Letter g puts the upper deviation at -7 µm for this size, so g6 runs from -7 down to -7 - 13 = -20 µm: 24.980 to 24.993 mm.
- Subtract. Smallest clearance is the smallest hole minus the largest shaft: 25.000 - 24.993 = 0.007 mm. Largest clearance is 25.021 - 24.980 = 0.041 mm. Both are positive, so H7/g6 is a clearance fit at 25 mm.
Holes K to U are not a mirror image of the shaft
For holes A to H you flip the sign of the matching shaft deviation. Holes K, M and N up to IT8, and P to U up to IT7, also get a small correction, Δ, which is the step from the grade below to this one. At 25 mm, shaft p6 starts at +22 µm, so a plain mirror would put P7 at -22 µm on top. Adding Δ = IT7 - IT6 = 8 µm gives the real P7: -14 / -35 µm.
How we checked the numbers
Every limit on this page is computed from the ISO 286-1 tables, using one data file that the calculator and the tables share. Before publishing we compared the output with two independent published tables.
- isofits, an open-source ISO 286-2 table with 37 hole and 37 shaft classes from 3 to 400 mm. For the 67 of those classes the calculator covers, 1,334 of 1,340 cells match.
- Mitsubishi Materials' fit tolerance tables for holes and shafts. We checked 59 values there, picked to cover what isofits lacks: classes such as c, s, t and u, and sizes up to 3 mm and from 400 to 500 mm. All 59 match.
The 6 cells that differ are errors in isofits. In each one the upper minus the lower deviation is not the IT value, so the zone has the wrong width:
| Class and sizes | isofits prints | Width vs IT | Mitsubishi and this calculator |
|---|---|---|---|
| E7, 315 to 400 mm | +185 / +125 | 60, IT7 is 57 | +182 / +125 |
| K6, 6 to 10 mm | +2 / -6 | 8, IT6 is 9 | +2 / -7 |
| f6, 120 to 180 mm | -43 / -48 | 5, IT6 is 25 | -43 / -68 |
One difference is a real disagreement, not a typo. For js7 and JS7, when IT7 is an odd number, Mitsubishi rounds the half down to a whole micron (±10 at 18 to 30 mm) and isofits prints the exact half (±10.5). This calculator shows the exact half, IT7 / 2.