Limits & Fits: Tolerance Chart + Fit Calculator

How to Read a Fit Like 25 H7/g6

Three parts: the nominal size (25 mm), the hole tolerance (capital letter + grade: H7 = bore at 25.000/+0.021), and the shaft tolerance (lowercase: g6 = shaft at 24.993/−0.013 below nominal). The letter sets where the tolerance zone sits relative to nominal; the number (IT grade) sets how wide it is. Because the H hole always starts exactly at nominal and shaft families range from loose (d) to heavy interference (s), one standard H7 bore can produce every fit type just by changing the shaft grade — which is why hole basis is the industry default and the system this page uses.

Preferred Fits — Quick Selection Table

These eight hole-basis combinations cover almost every job we machine. The example column is computed at Ø25 mm — use the calculator above for your exact size.

Clearance, Transition, Interference — What Actually Changes

Clearance fits (shaft always smaller)

The shaft's largest allowed diameter is still below the hole's smallest. Motion is guaranteed; the question is how much play you can accept. On sliding spool valves and guide pins we machine to H7/g6; for rotating shafts in bronze or POM bushings, H8/f7 leaves room for an oil film. A typical build we see weekly: a hardened 304 stainless pin sliding in a bored 6061 housing — free after anodizing because we allowed for the coating growth up front.

Transition fits (could be either)

H7/k6 and H7/n6 straddle the line: depending on where each part lands inside its tolerance, you get a light press or a snug slip. That's the point — near-zero play for accurate location, but still assemblable with a mallet and removable for service. Bearing seats and keyed pulleys live here. If your assembly must never loosen under vibration, step up to interference rather than hoping k6 lands tight.

Interference fits (shaft always larger)

H7/p6 presses a bushing into a housing so it stays put; H7/s6 transmits real torque by friction alone. Practical machining notes: interference values assume similar materials — pressing steel into a 7075 aluminum housing needs less interference than steel-into-steel because aluminum yields first, and thin-wall bushings can close their bore when pressed, so we verify the installed ID, not just the free-state ID. This is the detail that separates a chart from a part that works.

Standard Tolerance Grades (IT6–IT11)

The IT grade is the total tolerance width in micrometres for a given size range — IT6 is precision grinding territory, IT7 a reamed or precision-bored hole, IT9 routine turning, IT11 rough work. Values per ISO 286:

Hole Deviations (H7, H8, H9, H11)

H holes are the easy ones: lower limit = nominal exactly, upper limit = nominal + IT grade. Cells show ES over EI in µm:

Shaft Deviations (d9 → s6)

Cells show upper deviation (es) over lower deviation (ei) in µm. Negative = below nominal (clearance families d–h), positive = above nominal (transition and interference grades k6–s6):

Hitting the Fit: How These Numbers Get Machined

A tolerance chart tells you the target; holding it is process. H7 bores under Ø20 we ream; larger ones we single-point bore and measure with three-point mics. Precision shafts to g6/h6 come off CNC turning with in-process gauging, ground where the drawing demands it. Every toleranced feature can ship with a dimensional report from our inspection department — and if your part is a worn shaft or a bushing with no drawing at all, reverse engineering recovers the original fit before we machine the replacement. The parts these fits live on are exactly what we build daily: custom shafts, spacers and bushings, and press-fit components — even without CAD.

Limits & Fits FAQ