ISO tolerance & fit calculator
Look up an ISO 286 tolerance or fit without opening the handbook. Enter a size, pick a hole and shaft class (or a common fit like H7/g6), and get the exact limits and the fit type.
How ISO fits and tolerances work
ISO 286 is the international system for fits and tolerances. A code like H7 or g6 says how far a hole or shaft may stray from its nominal size: the letter fixes where the tolerance band sits (a capital for holes, lowercase for shafts) and the number — an IT grade — fixes how wide it is. Pair a hole with a shaft, such as H7/g6, and you get a fit: how loosely or tightly the two go together. This tool gives you the exact upper and lower limits in millimetres and tells you whether the result is a clearance, transition or interference fit. The figures are the official ISO 286 values for sizes up to 120 mm — the numbers you would otherwise look up in a tolerance handbook.
What do the letter and number mean?
The letter sets the position of the tolerance zone relative to the nominal size — a capital letter for a hole, a lowercase letter for a shaft (H and h sit right on the nominal). The number is the IT grade, which sets the width of the zone: a smaller number is a tighter tolerance. So H7 is a hole on the nominal with a grade-7 spread; g6 is a shaft just below nominal with a slightly tighter grade-6 spread.
Clearance, transition, interference — what's the difference?
A clearance fit always leaves a gap, so the shaft turns or slides freely. An interference fit is always tight — the shaft is slightly bigger than the hole and has to be pressed or shrunk in. A transition fit sits in between: depending on where each part lands within its tolerance, it can end up slightly loose or slightly tight, which suits accurate location that you still want to assemble by hand.
Why is the hole usually H (like H7/g6)?
It is called the hole-basis system. Holes are harder and more expensive to adjust to size than shafts, so the hole is kept standard (H, right on the nominal) and the fit is chosen by varying the shaft. That is why the vast majority of drawings use H-something for the hole and change the shaft letter to get the fit they want.
Which sizes and classes are covered?
Nominal sizes up to 120 mm, which covers most machining work, and the common hole and shaft classes (H6–H11 and shafts from e to s). ISO 286 also defines larger sizes and more exotic classes; this tool focuses on the ones you actually reach for day to day.
Are the numbers exact?
Yes. They are the official published ISO 286 deviation values, not approximations from a formula — the formulas round to a few micrometres off, which matters for a tolerance. Every class in the tool was cross-checked against a reference handbook.
This calculator gives you a starting point — always check it against your machine's real limits and your tool manufacturer's data. Start with a light test pass, then gradually work your way up as you find the sweet spot: quiet running, no vibration, no chatter. Clamp your workpiece securely, and wear the right protective gear — safety glasses, hearing protection, gloves suited to the material, and clothing that won't catch in moving parts.
