Fit Calculator (ISO 286)
Choose a hole and a shaft tolerance, enter the size, and see the limits of size and the clearance or interference.
Deviations and tolerances (micrometers)
| Part | Upper deviation | Lower deviation | Tolerance |
|---|---|---|---|
| Hole | +21 | 0 | 21 |
| Shaft | -7 | -20 | 13 |
How it works
The calculator uses the ISO system of limits and fits (ISO 286, as published in the Machinery’s Handbook BS 4500 and ANSI B4.2 tables). For the basic size it finds the IT tolerance for each part’s grade, and the fundamental deviation for its letter, and from those the upper and lower limits. The clearance is the smallest hole minus the largest shaft (smallest clearance) and the largest hole minus the smallest shaft (largest clearance). A negative number is an interference. The theory is on the Fits - Metric (ISO System) page.
Worked example
Preferred fit H7/g6 at 25 mm: IT7 = 21 µm, IT6 = 13 µm, g has a fundamental deviation of −7 µm. The hole is 25.000 to 25.021 mm, the shaft 24.980 to 24.993 mm, and the clearance is 0.007 to 0.041 mm: a sliding fit.
Things to know
Sizes are over 3 mm and up to 500 mm, the range of the Handbook’s tables. A size on a range boundary belongs to the lower range (30 mm is in “over 18 to 30”).
The letters offered are the ones in the Handbook’s tables for common work: holes C to U, shafts c to u. Combinations that the tables do not give (for example the letter t below 24 mm) show a message.
Holes K to U use the standard rule for hole deviations (the mirror of the shaft value, plus a correction for fine grades). The results agree with the Handbook’s printed fit tables at 10, 25 and 60 mm.
The limits are at 20 °C. Temperature, coatings and form error change a real fit: see the Temperature, Coatings and Fit Problems page.
Common questions
Can I enter an inch size?
Yes. The size is converted to millimeters, the ISO fit is calculated, and the limits are shown in inches. For true inch fits (RC, LC, LT, LN, FN) use the inch fit tables.
Why does the hole letter list have H, JS and so on?
The capital letter places the hole’s tolerance zone relative to the basic size; H starts at the basic size, C to G are above it, K to U are below it.
What is a transition fit?
A fit that can be either a small clearance or a small interference, depending on where the two parts fall in their tolerances. The calculator shows the largest of each.
Is the result safe for a drawing?
It follows the Handbook’s published tables. For a contract or safety-critical drawing, confirm the limits against the current edition of ISO 286.
Sources
- Machinery’s Handbook, 31st edition, Industrial Press: “British Standard Metric ISO Limits and Fits” (Tables 4, 5a, 5b, 6a, 6b) and “ANSI/ASME Preferred Metric Limits and Fits”.
- ISO 286-1 and ISO 286-2; ANSI/ASME B4.2.
Last reviewed: October 22, 2026