A free tool for process engineers
This tolerance calculator gives the upper and lower deviation, the maximum and minimum size and the IT tolerance value for any diameter from 1 to 500 mm and any ISO 286 tolerance class. Enter the nominal size, choose hole or shaft, the fundamental deviation and the IT grade. The second tab works as a fit calculator for pairs such as H7/g6.
Values from the ISO 286-1 and ISO 286-2 tables. Check against the standard before releasing a drawing to production.
02Designation
Letter and number
A tolerance class is a letter plus a number. The letter is the fundamental deviation, which sets where the tolerance zone sits relative to the nominal size. The number is the IT grade, which sets its width. Capital letters are holes and lower-case letters are shafts: H7 and h7 are equally wide but sit on opposite sides of zero.
Fundamental deviation
For an H hole the lower deviation is zero and the zone lies above the nominal size. For an h shaft the upper deviation is zero and the zone lies below it. Letters a to g (A to G) make shafts smaller and holes larger than nominal, while k to zc move the zone towards interference. The js and JS classes are symmetrical: plus or minus half the IT value.
IT grade
The IT value grows with size. IT7 is 15 µm at Ø10, 25 µm at Ø50 and 35 µm at Ø100, so a tolerance means nothing without the size. Range limits matter too: Ø50 belongs to the range above 30 up to and including 50 mm, not to 50-80.
EI = 0, ES = +ITes = 0, ei = -IT±IT/203Chart
The ISO tolerance chart below lists upper and lower deviations in micrometres for the classes that appear most often on drawings: holes H7 and H8 and shafts h6, h7, g6, f7, k6 and p6. The values come from the ISO 286-2 tables and cover size ranges up to 500 mm.
| Size [mm] | H7 | H8 | h6 | h7 | g6 | f7 | k6 | p6 |
|---|---|---|---|---|---|---|---|---|
| up to 3 | +10 / 0 | +14 / 0 | 0 / -6 | 0 / -10 | -2 / -8 | -6 / -16 | +6 / 0 | +12 / +6 |
| over 3 to 6 | +12 / 0 | +18 / 0 | 0 / -8 | 0 / -12 | -4 / -12 | -10 / -22 | +9 / +1 | +20 / +12 |
| over 6 to 10 | +15 / 0 | +22 / 0 | 0 / -9 | 0 / -15 | -5 / -14 | -13 / -28 | +10 / +1 | +24 / +15 |
| over 10 to 18 | +18 / 0 | +27 / 0 | 0 / -11 | 0 / -18 | -6 / -17 | -16 / -34 | +12 / +1 | +29 / +18 |
| over 18 to 30 | +21 / 0 | +33 / 0 | 0 / -13 | 0 / -21 | -7 / -20 | -20 / -41 | +15 / +2 | +35 / +22 |
| over 30 to 50 | +25 / 0 | +39 / 0 | 0 / -16 | 0 / -25 | -9 / -25 | -25 / -50 | +18 / +2 | +42 / +26 |
| over 50 to 80 | +30 / 0 | +46 / 0 | 0 / -19 | 0 / -30 | -10 / -29 | -30 / -60 | +21 / +2 | +51 / +32 |
| over 80 to 120 | +35 / 0 | +54 / 0 | 0 / -22 | 0 / -35 | -12 / -34 | -36 / -71 | +25 / +3 | +59 / +37 |
| over 120 to 180 | +40 / 0 | +63 / 0 | 0 / -25 | 0 / -40 | -14 / -39 | -43 / -83 | +28 / +3 | +68 / +43 |
| over 180 to 250 | +46 / 0 | +72 / 0 | 0 / -29 | 0 / -46 | -15 / -44 | -50 / -96 | +33 / +4 | +79 / +50 |
| over 250 to 315 | +52 / 0 | +81 / 0 | 0 / -32 | 0 / -52 | -17 / -49 | -56 / -108 | +36 / +4 | +88 / +56 |
| over 315 to 400 | +57 / 0 | +89 / 0 | 0 / -36 | 0 / -57 | -18 / -54 | -62 / -119 | +40 / +4 | +98 / +62 |
| over 400 to 500 | +63 / 0 | +97 / 0 | 0 / -40 | 0 / -63 | -20 / -60 | -68 / -131 | +45 / +5 | +108 / +68 |
ISO 286-1:2010, Tables 1-3; ISO 286-2:2010. Range: above the first value up to and including the second.
Reading example: a Ø40 g6 shaft has deviations of -9 and -25 µm, so its size must lie between 39.975 and 39.991 mm. Other classes, including M, N, P, R, S and grades IT5 to IT11, are in the calculator above.
04Fits
Ø25 mm, hole-basis system
The light blue zone is the hole, the dark one the shaft, the line the nominal size Ø25. A shaft zone wholly below the hole zone always gives clearance (green band), wholly above it always interference (violet). Zones that share a height give clearance or interference, depending on the actual sizes (amber).
ES - eiMinimum clearanceEI - esIn the hole-basis system every hole is toleranced H and the fit is set by the choice of shaft: H7/g6 is a clearance fit, H7/k6 a transition fit and H7/p6 an interference fit. It is the default in machine building, because holes are finished with fixed-size tools while a shaft is easy to measure and turn to size.
The shaft-basis system, with an h shaft and holes of different letters, is used when several parts with different fits sit on one plain shaft, with drawn bar supplied to h tolerances and for the outer rings of rolling bearings.
05Process
The IT grade on a drawing decides the process, because each machining method has its own achievable accuracy. As a guide: roughing gives IT12-IT14, ordinary finish turning IT8-IT9, precision turning IT7 and grinding IT5-IT6. Every step down means slower parameters, an extra pass and often an extra operation and inspection.
An H7 hole is typically drilled and reamed or bored to size. An h6 bearing seat usually needs grinding or very careful turning with every part checked. You can estimate the effect on machining time with the cutting parameters calculator, and the link between accuracy and process is explained in our guide to machining.
06ISO 2768
Dimensions without an individual tolerance fall under general tolerances when the title block says ISO 2768. Class m (medium) is the most common: ±0.2 mm for 6-30 mm and ±0.3 mm for 30-120 mm. Class f is roughly twice as tight, while c and v are for coarse parts. The chart below and the third tab of the calculator cover sizes from 0.5 to 4000 mm.
| Class | 0.5-3 | 3-6 | 6-30 | 30-120 | 120-400 | 400-1000 | 1000-2000 | 2000-4000 |
|---|---|---|---|---|---|---|---|---|
| f - fine | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | ±0.3 | ±0.5 | - |
| m - medium | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 |
| c - coarse | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 | ±3 | ±4 |
| v - very coarse | - | ±0.5 | ±1 | ±1.5 | ±2.5 | ±4 | ±6 | ±8 |
ISO 2768-1:1989, Table 1. Range: above the first value up to and including the second, the first from 0.5 mm. A dash: no value specified.
07FAQ
Read on in the blog:
An h7 tolerance is a shaft tolerance zone with fundamental deviation h and grade IT7. The upper deviation is zero and the lower deviation equals minus the IT7 value for the size range. An h7 shaft is never larger than its nominal size and may be smaller by 10 to 63 µm, depending on the diameter.
For Ø20 mm the h7 tolerance is 0 upper and -21 µm lower, because Ø20 lies in the range above 18 up to 30 mm, where IT7 is 21 µm. The shaft must therefore measure between 19.979 and 20.000 mm. An H7 hole of the same diameter lies between 20.000 and 20.021 mm.
H7 applies to a hole and h7 to a shaft. Both zones have the same IT7 width, but H7 lies above the nominal size (from 0 upwards) and h7 below it (from 0 downwards). The H7/h7 pair gives a clearance fit with a minimum clearance of zero, used for located parts assembled by hand.
H7/g6 is a hole-basis clearance fit: an H7 hole with a g6 shaft. At Ø25 the clearance ranges from 7 to 41 µm. It is used for precise guidance with small clearance, such as guide pins, change gears and locating bushings, where parts must slide without noticeable play.
Maximum clearance is the hole's upper deviation minus the shaft's lower deviation, and minimum clearance is the hole's lower deviation minus the shaft's upper deviation. A negative result means interference. If both results are positive the fit is a clearance fit, if both are negative an interference fit, and with mixed signs a transition fit.
The standard gives rounded IT values, and those are what apply on a drawing. The tolerance unit formula differs from the tables by 1-2 µm, and the fundamental deviations have exceptions, such as the delta added for K, M, N and P to S holes. The calculator uses the ISO 286 tables only.
Every IT grade tighter means longer finishing and sometimes grinding. In MetronQ the customer marks fitted holes and surfaces with a tighter tolerance on the 3D model, and the engine adds the time to finish and inspect them. The process engineer sees fits and surface tolerances as separate lines in the price breakdown before approving it. How tightening tolerances changes machining time and price is explained in tolerances vs CNC machining cost.