ISO tolerance fit calculator
Enter a nominal size, then pick the hole and shaft tolerance classes. Limit deviations come out in micrometres along with the clearance or interference range the fit produces.
Scope of this tool, so nobody quotes it past what it covers. It implements the ISO 286-1 standard tolerance formula and the general fundamental-deviation rule for letters A through H plus JS, and shaft letters a through h plus js, k, m and n, for sizes from 0 to 500 mm at IT01 to IT16. It leaves out the special convention that ISO 286 applies to certain letters at high tolerance grades. Check any result you intend to certify against the ISO 286-2 tables or your own gauging software.
Reading a fit call
A fit written as H7/g6 packs a lot into five characters. The capital letter marks the hole and the lower case marks the shaft. The letter fixes where the tolerance zone sits relative to zero, and the number fixes how wide that zone is.
- H means the hole’s lower deviation is exactly zero, so the hole can only grow from nominal size. It is the default basis for the hole-basis system.
- h means the shaft’s upper deviation is zero, so the shaft can only sit below nominal.
- g places the shaft zone below zero by a clearance allowance, which guarantees a gap even at the worst combination.
- k, m, n straddle or exceed zero, giving transition or interference depending on the size.
- The number is the IT grade. Smaller numbers mean tighter tolerance and higher cost.
Hole-basis fits dominate production because reamers, drills and boring bars come in fixed sizes. Changing shaft sizes costs nothing; changing hole sizes means buying another tool. Unless a drawing demands otherwise, keep the hole at H and vary the shaft.
Where the common fits land
| Fit | Type | Typical use | Assembly |
|---|---|---|---|
| H7/g6 | Clearance | Precision sliding spindles, valve spools | Hand, light |
| H7/h6 | Clearance, near zero | Locating fits that must still dismantle | Hand, often light drift |
| H7/f7 | Clearance | General rotating bearings, moderate speed | Hand |
| H8/f8 | Clearance, looser | General engineering, non-critical rotation | Hand |
| G7/h6 | Clearance | Precision slide where hole is ground to size | Hand |
| H7/k6 | Transition | Gears and pulleys needing accurate location | Light press or mallet |
| H7/n6 | Transition, tighter | Motor couplings, heavier keyed hubs | Press |
Two parts that share a nominal size can still assemble very differently depending on where their tolerance zones fall. That is why the fit call carries two codes rather than one tolerance value. The zone positions decide how the assembly behaves while the grade numbers decide how much it costs to hold.
IT grades and what they imply
Standard tolerance follows a single function of size, scaled by the grade. The tolerance unit itself grows with the cube root of diameter, which is why the same grade tolerates more movement on a large bore than on a small one. Each grade multiplies that unit by a fixed factor.
| Grade | Multiple of i | Usually reached by |
|---|---|---|
| IT5 | 7 i | Precision grinding, honing, superfinishing |
| IT6 | 10 i | Fine grinding, quality internal grinding |
| IT7 | 16 i | Reaming, fine boring, precision turning |
| IT8 | 25 i | Standard turning and boring, broaching |
| IT9 | 40 i | General milling and turning |
| IT10 | 64 i | Casting and general fabrication |
| IT11 | 100 i | Rough work, sand casting |
Jumping one grade tighter does not cost one step more. Below about IT6 the cost climbs steeply because each step often demands a different process, a temperature-controlled inspection room, and higher scrap exposure. Specifying IT5 across a whole assembly when one bearing seat needs it is the most common way drawings become expensive.
Questions that come up
Why does the calculator use a different diameter than I typed?
ISO 286 does not evaluate tolerance at the exact size you enter. It groups sizes into steps and evaluates every size in that step at the step’s geometric mean, which is why a 25 mm bore and a 24 mm bore can quote identical limits if they fall in the same band. The tool prints the step it used so you can see which one applied.
Should I design hole-basis or shaft-basis?
Hole-basis, almost always. Tooling for holes comes in standard sizes, so keeping every hole at H lets you buy cutters once. Shaft-basis appears mainly when a single ground shaft carries several different hubs along its length, or in some hydraulic cylinder work built around standard piston rod stock.
What about general tolerances for untoleranced dimensions?
ISO 2768 covers those, split into classes such as fine, medium and coarse, with separate letter designations for linear dimensions versus geometric ones. Putting a blanket general tolerance note on the drawing keeps untoleranced features from becoming undefined. Machining centres in normal setup generally hold the medium class without special effort.
Do limits change with temperature?
They do, and it catches people inspecting large aluminium parts. ISO 286 limits apply at 20 °C. Aluminium moves roughly twice as much per degree as steel, so a 300 mm aluminium dimension measured at 28 °C reads measurably larger than it will at reference temperature. Let parts stabilise before final gauging.
Which standard should I cite?
ISO 286-1 defines the system and ISO 286-2 publishes the tables for holes and shafts up to 500 mm. Refer to ISO 286-1 for the basis and to the ISO 286-2 table set for the numbers that go on a drawing. Standard symbols for surface texture indication are set out under ISO 1302.
