CNC / machinist calculator
ISO Limits and Fits Calculator (ISO 286)
The ISO system of limits and fits (ISO 286) turns a nominal size and a pair of tolerance classes, like 25 H7/g6, into the exact hole and shaft limits and the play or grip between them. Choose the nominal diameter, the hole class and the shaft class, and read the millimetre limits, the micron deviations, and whether the pair runs as a clearance, transition or interference fit. It is the quick way to turn a drawing callout into the numbers you set on the machine and the gauge.
| Min (mm) | Max (mm) | Deviations (µm) | |
|---|---|---|---|
| Hole | - | - | - |
| Shaft | - | - | - |
How it works
A tolerance class has two parts. The number is the IT grade, the standard tolerance grade, which fixes the width of the tolerance band: a smaller number is a tighter band, and the same grade gives a wider band as the diameter grows. The letter fixes the fundamental deviation, the position of that band relative to the nominal size: an upper-case letter is a hole, a lower-case letter is a shaft. For the basic hole H the lower deviation is zero, so the hole runs from nominal up by one IT grade.
The fit is what you get when a hole class meets a shaft class. If the smallest hole is still larger than the largest shaft, every pair has play and it is a clearance fit. If the largest shaft is bigger than the smallest hole for every pair, it is an interference (press) fit. When the ranges overlap so the assembly can be either, it is a transition fit. The calculator reports which one you have and the extreme clearance or interference in millimetres.
The numbers here are computed from ISO 286-1 using the published standard-tolerance grades, and were cross-checked against the open AmesWeb ISO 286 calculator and worked examples. Deviations are shown in micrometres (thousandths of a millimetre) the way the standard tabulates them; the limits are the nominal size plus those deviations.
Worked example
25 mm H7/g6: the hole runs 25.000 to 25.021 mm and the shaft 24.980 to 24.993 mm, so the pair always has 0.007 to 0.041 mm of clearance, the standard running fit for shafts in bushings and bearings.
Frequently asked questions
What does H7/g6 mean?
H7 is the hole: letter H (lower deviation zero, the basic hole) at IT grade 7. g6 is the shaft: letter g at IT grade 6. Together they make a common clearance running fit.
What is an IT grade?
The IT grade is the standard tolerance grade, which sets how wide the tolerance band is. A lower number is tighter; IT6 and IT7 are typical for precision fits, IT11 and coarser for general work.
What is the difference between clearance, transition and interference fits?
A clearance fit always leaves play between hole and shaft. An interference fit is always a press, the shaft larger than the hole. A transition fit overlaps, so a given pair may be slightly loose or slightly tight.
Is this calculator for metric or inch fits?
ISO 286 is a metric standard, so the nominal size is entered in millimetres and the limits come out in millimetres. Inch limits and fits follow the ANSI B4 tables, which use different values.
How accurate are these limit values?
They are computed from the ISO 286-1 formulas and standard-tolerance grades and cross-checked against the open AmesWeb ISO 286 calculator, so they match its published limits for the classes offered.
Related calculators
Coverage
Implements ISO 286 (limits and fits): Nominal sizes 1 to 500 mm, Standard tolerance grades IT5 to IT11, Clearance and transition fits: hole H G F K N, shaft c d e f g h k m n. Deliberately not offered yet:
- Interference fits (shaft p r s t u and beyond, hole P S and beyond) - their fundamental deviations use tabulated per-grade increments this engine does not yet encode; shipping an unverified press fit is worse than omitting it (revisit: when the interference deviation tables are transcribed and golden-vector tested)
- Inch / ANSI limits and fits - ISO 286 is a metric standard; inch fits follow ANSI B4.x with separate tables
Sources
- ISO 286-1:2010 - ISO code system for tolerances on linear sizes (the standard we implement; ISO text is paywalled, so values are verified against the open AmesWeb and Fractory tables below, not the paid standard)
- AmesWeb - ISO 286 limits and fits tolerance calculator (fundamental deviations by IT grade and size)
- Limits and fits explained (Fractory)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.