Enter a nominal size and a fit — 25 H7/g6 — and get the exact hole and shaft limits, the clearance or interference range, and what that fit actually behaves like on the bench. Then stack your dimensions and see the gap, worst case and RSS.
Free, no sign-up, and every number comes straight from the published standard. It runs entirely in your browser, so nothing you type leaves your machine.
The hole letter is upper case, the shaft lower case. The letter sets where the tolerance zone sits; the IT number sets how wide it is.
Build the chain that closes on the gap you care about. Mark a dimension + if it opens the gap and − if it closes it. The default chain is a shaft and two spacers inside a housing bore.
Hole-basis fits, in the order you meet them in practice. The bore is machined to H and the shaft is ground to suit — that is the cheaper way round, because a bore is harder to adjust than a shaft.
| Fit | Name | Behaviour, and when to reach for it |
|---|---|---|
| H11/c11 | Loose running | Wide, forgiving clearance. Pivots on farm and construction machinery, anywhere dirt, paint or a wide temperature swing matters more than precision. |
| H9/d9 | Free running | Generous clearance for long shafts, high speed or heavy heat. Good default when accuracy is not critical. |
| H8/f7 | Close running | The everyday journal-bearing fit. Runs freely with oil, holds reasonable location. Most rotating machinery lives here. |
| H7/g6 | Sliding | Small, controlled clearance — parts slide and locate accurately but do not run continuously at speed. Spigots, guides, location plugs. |
| H7/h6 | Location clearance | Zero to slight clearance. Assembles by hand, locates precisely, free to be taken apart. The standard for parts you want accurate but serviceable. |
| H7/k6 | Location transition | May be slightly tight or slightly loose. Accurate location with light pressure — keyed hubs, pulleys retained by a key. |
| H7/n6 | Tight transition | Usually interference. Needs a press; comes apart only with effort. Use where location must not shift but you may still need to service it. |
| H7/p6 | Light press | Genuine interference. A press fit for cast iron and steel that transmits light torque without a key. |
| H7/s6 | Medium drive | Heavy interference — heat the bore or chill the shaft. Permanent joints: shrunk-on gears, bushes, collars carrying real torque. |
The grade sets the width of the tolerance zone, and that maps directly onto process and price. Tightening a fit by two grades can double the cost of a feature — pick the loosest grade that still works.
| Grade | Typical process | Where it belongs |
|---|---|---|
| IT5–IT6 | Grinding, honing, lapping | Bearing seats, precision spindles, gauge work |
| IT7 | Reaming, fine turning, grinding | The workhorse grade for located, fitted features |
| IT8–IT9 | Turning, milling, boring | General machined features, running fits |
| IT10–IT11 | Drilling, rough milling, die casting | Clearance holes, non-critical dimensions |
| IT12–IT16 | Sawing, flame cutting, sand casting, forging | Raw stock and unmachined surfaces |
They answer different questions, and choosing the wrong one is how assemblies fail in the field or cost too much on the shop floor.
Rule of thumb: design with RSS, but check that the worst case is survivable. If worst case would cause damage rather than just a reject, use worst case.
Describe it in plain English and Volex Forge returns a verified STEP, STL or DXF — fits called out in your prompt are computed and reported with the geometry.