ISO 286 Limits and Fits Calculator: Hole and Shaft Tolerances, Clearance and Interference

The upper and lower size limits of a hole and a shaft from their ISO 286 tolerance classes, and the clearance or interference of the fit.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A fit says how tight two mating parts are. In the ISO system each part has a tolerance class made of a letter and a number. The letter gives the position of the tolerance zone relative to the nominal size (the fundamental deviation: capital letters for holes, small letters for shafts) and the number is the grade of the standard tolerance IT, which sets the width of the zone. A bigger grade number is a wider, cheaper tolerance.

For a hole of class H7 at 25 mm the standard tolerance IT7 is 21 micrometres and the fundamental deviation is zero, so the hole is between 25.000 and 25.021 mm. A shaft g6 has a fundamental deviation of minus 7 micrometres and IT6 of 13 micrometres, so it is between 24.980 and 24.993 mm. The smallest clearance is 7 micrometres and the largest is 41.

The calculator shows each step: the size step the nominal size falls in, the IT grade, the fundamental deviation and how it is turned into the upper and lower deviations of the hole and the shaft, the size limits, the largest and smallest clearance, and whether the fit is a clearance, a transition or an interference fit. It also draws the two tolerance zones against the zero line.

Formulas used

  • ES = EI + IT_{hole}
  • ei = es - IT_{shaft}
  • C_{max} = ES - ei
  • C_{min} = EI - es
  • D_{max} = D + ES/1000

Worked example

With the default numbers in the calculator above:

  1. Size step: 18 to 30 mm
  2. Hole tolerance H7: standard tolerance: 21 µm
  3. Hole fundamental deviation: H: EI = 0 (the reference position)
  4. Hole upper deviation ES: +21 µm
  5. Hole lower deviation EI: 0 µm
  6. Hole size limits: 25.021 and 25 mm
  7. Shaft tolerance g6: standard tolerance: 13 µm
  8. Shaft fundamental deviation: g: es = -7 (table value), ei = es - IT
  9. Shaft upper deviation es: -7 µm
  10. Shaft lower deviation ei: -20 µm
  11. Shaft size limits: 24.993 and 24.98 mm
  12. Largest clearance (largest hole, smallest shaft): +41 µm

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What does H7/g6 mean?

It is a hole of class H7 with a shaft of class g6: a sliding fit with a small clearance, used for precise location where parts slide or turn slowly.

What is the difference between a clearance, a transition and an interference fit?

In a clearance fit the shaft is always smaller than the hole. In an interference fit the shaft is always larger, so it has to be pressed or shrunk in. In a transition fit either can happen, depending on the actual sizes.

Why is the hole usually H?

Holes are made with fixed-size tools such as drills and reamers, so it is easier to keep the hole to one class and change the shaft. This is the hole-basis system.

Which sizes and classes does the calculator cover?

Nominal sizes above 0 up to 400 mm, hole classes D to S and shaft classes c to u in the common grades. For a critical fit check the standard ISO 286 itself.

Related calculators

Further reading: ISO 286-1 and ISO 286-2 (the standards themselves are licensed); Shigley, Mechanical Engineering Design; Machinery's Handbook (limits and fits).

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