Tolerance Calculator
ISO fit bands in numbers — hole and shaft limits, worst-case clearance or interference, straight from the standard-tolerance arithmetic.
Tolerance Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Hole-basis H system; shaft-basis pairs not shown
- Offset presets match a 25 mm nominal — type others
In short: A 25 mm nominal with an H7 hole and a shaft sitting 7 µm below nominal gives 25.000–25.021 mm in the hole and 24.980–24.993 on the shaft — 0.007 to 0.041 mm of clearance, a sliding fit. The same hole over a shaft 35 µm above nominal becomes a press fit that never lets go. One hole, three behaviors: the whole fit lives in where the shaft's top edge sits relative to nominal.
Formula
i = 0.45·D1/3 + 0.001·D — IT7 = 16i, IT6 = 10i
The ISO fit system grows its tolerance widths from a standard tolerance unit i computed at the geometric mean of the size step — at 25 mm the step runs 18 to 30, so D is 23.24 and i is 1.307 µm. Grades multiply i: seven times for IT5, ten for IT6, sixteen for IT7, up to a hundred for IT11. The hole starts at nominal and grows upward; the shaft hangs its top edge es below (or above) nominal and grows down from there. The letters on a drawing — the g in H7/g6 — are shorthand for that offset.
Worked Example
- Find the size step and the tolerance unit i.
- Grow the hole up from nominal, the shaft down from es.
- Read worst-case clearance or interference.
Defaults: 25 mm, H7 over a g6-style shaft — 0.007–0.041 mm of clearance. Flip the offset to +35 and the same hole grips 0.001–0.035 mm of interference; the press did the drawing's talking.
Strengths & Limits Of This Model
Where this engine is strong
- Worst cases both ways, straight from the band
- The letter on the drawing becomes a number you can audit
Where it stops
- No surface, temperature or stress terms
Practical Use Cases
Fit selection
sliding, transition or press
Inspection limits
the two numbers the gauge holds
Drawing checks
what H7/g6 actually commits to
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Tolerance Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What does H7/g6 actually mean?
H sets the hole starting at nominal, 7 sets its width; the lowercase g drops the shaft's top edge a few microns below nominal and 6 sets the shaft's narrower width. This screen computes both parts from those two ideas — the letter becomes a number.
Why does the tolerance unit use the size step, not my size?
The ISO system grades sizes in bands and applies one formula per band at its geometric mean, so every size in the band shares tolerance tables. At a 25 mm nominal that mean is the root of 18 times 30, about 23.24 mm — which is why a 25 H7 hole opens 21 µm, not the 23 a naive plug would give.
Clearance, transition or interference?
Look at the two extremes. Both edges positive: the shaft always floats, a clearance fit. Both negative: the shaft never enters without force, an interference fit. Straddling zero: a transition fit — assembly feels thin, sometimes a tap, sometimes a press.
How much interference is safe?
Light press duties live near a thousandth of the diameter; heavy shrink fits push several times that. The number on this screen is the geometry; the stress in the hub and the press tonnage are a machine-design step that follows.
Can I use any offset I like?
The arithmetic does not care, but the drawing should: real letters map to real offsets that shift with the size band. The presets here match the common letters at a 25 mm nominal; at other sizes type the offset your standard tables quote.
Why is the shaft grade one tighter than the hole?
Convention and economics: holes are made with standard tools and carry the wider grade, shafts are turned or ground to the tighter one. H7/g6 and H7/h6 — grades seven and six — are the workhorse pairs of general engineering.
What do I measure with?
Bore gauges for the hole, micrometers for the shaft, and limits that bracket the computed band. The tolerances here are tenths of a hundredth — well inside a good mic, a real task for a caliper.
Does temperature change the fit?
Steel grows about 12 µm per metre per degree. A fifty-degree swing on this 25 mm pair moves both parts about 15 µm together — mostly harmless because they move alike, until one part is aluminum and the other is not.