ISO Fits and Tolerances Calculator
ISO 286 limit deviations for hole and shaft classes like H7, g6, k6 and p6 up to 500 mm, with max and min clearance or interference and the fit type.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the ISO Fits and Tolerances Calculator?
An ISO fit such as H7/g6 describes how a hole and a shaft of the same nominal size go together: whether the shaft always slides in, sometimes needs a tap, or has to be pressed or shrunk in. The letters and numbers come from ISO 286, and turning them into actual limits means reading two tables and doing some arithmetic. This ISO fits and tolerances calculator does it for any nominal size up to 500 mm.
Choose the hole class (F, G, H, JS, K, M, N or P with an IT grade) and the shaft class (f, g, h, js, k, m, n, p, r or s). The calculator returns the upper and lower deviations of each part in micrometres, the largest and smallest permitted sizes in millimetres, the maximum and minimum clearance, and whether the result is a clearance, transition or interference fit. A diagram draws both tolerance zones against the nominal size, and the common fits table shows the clearance range of a dozen preferred fits at your size.
- IT1 to IT16 grades and holes F, G, H, JS, K, M, N, P
- Shafts f, g, h, js, k, m, n, p, r, s with sub-ranges for r and s
- Δ correction for K, M, N and P holes and the M6 special case
- Clearance, transition or interference with max and min values
- Zone diagram, common fits table, µm or thou
- Free, private, shareable link
How to use the ISO Fits and Tolerances Calculator
- Enter the nominal size in millimetres (above 0 and up to 500).
- Pick the hole letter and grade, then the shaft letter and grade, or tap a common fit such as H7/g6 or H7/p6.
- Read the fit type and the clearance or interference range.
- Use the hole and shaft limits in mm for drawings, inspection and gauge sizes. Switch to thou and inches if you work in imperial.
- Compare alternatives in the common fits table at the same size.
Worked examples
Sliding fit H7/g6 at 25 mm
Input
25 mm, H7 and g6
Output
Hole +21/0 µm, shaft −7/−20 µm: clearance 7 to 41 µm
The g shaft sits 7 µm below the nominal size, so even the tightest pair still slides.
Press fit H7/p6 at 50 mm
Input
50 mm, H7 and p6
Output
Hole +25/0 µm, shaft +42/+26 µm: interference 1 to 42 µm
The smallest shaft is still 1 µm larger than the largest hole, so this is an interference fit every time.
Transition fit H7/k6 at 50 mm
Input
50 mm, H7 and k6
Output
Shaft +18/+2 µm: from 23 µm clearance to 18 µm interference
A transition fit can go either way depending on the actual parts. Gears and couplings on keyed shafts often use it.
Medium drive H7/s6 at 70 mm
Input
70 mm, H7 and s6
Output
Shaft +78/+59 µm: interference 29 to 78 µm
s uses the finer 65 to 80 mm step. Fits this tight are often assembled by heating the hub.
Keyway width N9 and P9 at 8 mm
Input
8 mm, N9 and P9
Output
N9 0/−36 µm, P9 −15/−51 µm
These are the hub and shaft keyway widths used with parallel keys for normal and tight fits.
Background and insight
ISO 286-1 defines a tolerance class by two things. The number is the international tolerance grade (IT grade): the width of the zone, which depends on the grade and the size range. For a size between 18 and 30 mm, IT6 is 13 µm and IT7 is 21 µm. The letter is the fundamental deviation: where the zone sits relative to the nominal size. Upper-case letters are holes and lower-case letters are shafts. H holes and h shafts start exactly at the nominal size; g, f and the letters before them sit below it (smaller shafts, so clearance), while k, m, n, p, r and s sit above it (larger shafts, so interference).
So 25 H7 is 25.000 to 25.021 mm and 25 g6 is 24.980 to 24.993 mm, which together give 7 to 41 µm of clearance. The hole-basis system (H holes with different shafts) is preferred because holes are made with fixed-size reamers and checked with plug gauges; the shaft-basis system (h shafts) suits ground bar stock. ISO 286-2 tabulates the resulting limits for each class and size range. Some hole letters need a correction term Δ, the difference between the hole grade and the next finer grade, so that hole and shaft fits stay equivalent; K7 at 25 mm is +6/−15 µm rather than a mirror of k7. The calculator applies that rule, and the special case for M6 from 250 to 315 mm.
Size ranges matter. ISO 286 splits sizes into steps (over 18 up to 30 mm, over 30 up to 50 mm, and so on), and every size in a step gets the same deviations, so 18.01 mm and 30 mm share limits. The r and s shafts use finer steps above 50 mm. The values here were checked against published ISO 286-2 tables for the classes listed. Standard ISO 286 covers sizes to 3150 mm and more letters (a to zc); this calculator covers the classes most used for machined fits up to 500 mm.
Common use cases
Drawing tolerances
Put the right limits on a bore or shaft dimension and check the fit before release.
Bearing and bushing seats
Check housing and shaft fits for bushings, pins and bearing seats against the maker's recommendation.
Machining and inspection
Turn a class on a drawing into the sizes to machine to and to set gauges at.
Design education
See how grades and letters move the zones and why H7/g6 slides while H7/p6 must be pressed.
Tips and limitations
- Hole-basis fits (H holes) are usually cheaper: standard reamers and plug gauges are made to H7 and H8.
- Each grade step is about 1.6 times wider. Do not ask for a finer grade than the function needs.
- Interference fits depend on materials and wall thickness. The interference tells you the press force and hub stress only after a Lamé (thick-cylinder) calculation.
- Thermal expansion changes clearance in service. An aluminium housing on a steel shaft loosens as it warms.
- Rolling bearings have their own tolerance classes; follow the bearing maker's tables for shaft and housing fits.
- Surface finish eats into small clearances. A fit with 7 µm minimum clearance needs a finish well under that.
Frequently asked questions
What is the tolerance of a 25 mm H7 hole?
From +0 to +21 µm, so the hole may measure 25.000 to 25.021 mm. IT7 for sizes over 18 up to 30 mm is 21 µm.
What kind of fit is H7/g6?
A close clearance fit. At 25 mm the clearance ranges from 7 to 41 µm, so parts slide and turn freely while locating accurately.
What does H7/p6 mean?
An H7 hole with a p6 shaft: a locational interference (light press) fit. At 50 mm the interference is 1 to 42 µm.
What is the difference between H7 and H8?
Both start at the nominal size, but H8 is one grade wider. At 25 mm H7 allows 21 µm and H8 allows 33 µm.
What is a transition fit?
A fit whose limits allow either a small clearance or a small interference, depending on the actual sizes. H7/k6 and H7/n6 are common examples.
What are IT grades?
International tolerance grades define the width of a tolerance zone for a size range. Lower numbers are tighter: IT6 is typical of grinding, IT7 of reaming and fine turning, IT9 to IT11 of general machining.
Why do hole letters K, M, N and P not mirror the shafts exactly?
ISO 286 adds a correction Δ for these holes at finer grades so that a hole-basis fit and the equivalent shaft-basis fit have the same clearances. That is why K7 at 25 mm is +6/−15 µm.
Is there an in-depth guide to the ISO Fits and Tolerances Calculator?
Yes. “ISO 286 Fits and Tolerances: Reading H7/g6 and Choosing a Fit” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
People also search for
- h7 tolerance for 25 mm hole
- h7 g6 fit clearance
- iso 286 tolerance table calculator
- h7 p6 press fit interference
- shaft tolerance g6 values
- difference between h7 and h8
- transition fit h7 k6
- it grade tolerance table
Related Tools You May Like
Free, in-browser tools that pair well with this one.