ISO limits and fits, explained: what H7/g6 actually means

A fit like H7/g6 packs a hole tolerance, a shaft tolerance and the gap between them into four characters. Once you see that the letter sets where the band sits and the number sets how wide it is, the whole ISO 286 system reads on sight.

Letter = position, number = width

The nominal size (25 mm in 25 H7/g6) is the common reference. Each part gets a tolerance band placed relative to it:

letter → fundamental deviation (band position) · IT grade → tolerance width

Capital letters are holes, lower-case are shafts. H and h put the band right on the nominal line; g, f, d step a shaft progressively below it for more clearance. The IT grade is the width: at 25 mm, IT6 = 13 µm, IT7 = 21 µm, IT9 = 52 µm — lower number, tighter part. The hole & shaft fit calculator computes the limits and clearance for any size and fit.

Worked example — 25 mm H7/g6

The classic sliding fit:

H7 hole: 25.000 / 25.021 (band 0…+21 µm) · g6 shaft: 24.980 / 24.993 (band −20…−7 µm) · clearance 0.007–0.041 mm

The hole's band starts at nominal and opens up 21 µm. The shaft's band sits entirely below nominal — top at −7 µm, bottom at −20 µm — so the shaft is always smaller than the hole, and the gap ranges from a snug 7 µm to a loose 41 µm. That is exactly the behaviour you want for a part that slides or rotates slowly with a film of oil.

Three families of fit

• Clearance — always a gap. H7/g6 (sliding), H8/f7 (running, 0.020–0.074 mm at 25 mm), H9/d9 (loose running, 0.064–0.168 mm). Pick looser as speed, heat or contamination rise.
• Transition — H7/h6 sits line-to-line (clearance from 0 to 0.034 mm); H7/k6 and H7/n6 may come out slightly tight. Use for accurate location that still comes apart by hand or light tap.
• Interference — H7/p6, H7/s6: the shaft is bigger than the hole and must be pressed or shrunk in. Those are the press-fit interference calculator's territory.

Common mistakes

• Reading the grade as the gap. IT7 is the hole's tolerance width, not the clearance — the clearance comes from the shaft letter's position plus both bands.
• Over-tightening the fit. A tighter grade (IT5/IT6 everywhere) multiplies cost without function; only tighten the dimensions that control the fit. The tolerance stack-up calculator shows where tolerance actually matters.
• Mixing hole basis and shaft basis. Stay on hole basis (vary the shaft letter) unless one shaft deliberately runs through several fits.
• Specifying a press fit as a clearance fit. If the shaft letter reaches p, s or u, the parts interfere — size the assembly force and stress in the press-fit calculator, do not treat it as a slip-together joint.

Frequently asked questions

What does H7/g6 mean?

It is a hole-basis ISO 286 fit. H7 is the hole: letter H means its lower limit sits exactly on the nominal size, and grade 7 sets the tolerance band — 21 µm at 25 mm, so the hole runs 25.000–25.021. g6 is the shaft: letter g places its band just below nominal, grade 6 makes it 13 µm wide, giving 24.980–24.993. The result is a sliding fit with 0.007–0.041 mm clearance.

What is the difference between the letter and the number in a fit?

The letter sets position — where the tolerance band sits relative to the nominal size — and the number (IT grade) sets width — how tight the band is. H/h sit on the nominal line; g, f, d step progressively below it for more clearance. A lower grade number means a tighter tolerance: IT6 is 13 µm at 25 mm, IT7 is 21 µm, IT9 is 52 µm.

What are clearance, transition and interference fits?

Clearance fits always leave a gap (the shaft is always smaller than the hole) — H7/g6, H8/f7, H9/d9. Transition fits may end up slightly loose or slightly tight depending on where each part lands in its tolerance (H7/k6, H7/n6). Interference (press) fits are always tight — the shaft is larger than the hole and must be pressed or shrunk in (H7/p6, H7/s6).

Why is it called hole basis?

Because the hole is held to a fixed letter (H, lower limit on the nominal) and the shaft letter is varied to set the fit. Holes are cut with fixed-size tooling (drills, reamers), so it is cheaper to standardize the hole and adjust the more easily-turned shaft. Shaft-basis (lower-case h hole-letter varied) exists but is used mainly when one shaft runs through several different fits.