H7 Tolerance and ISO Fit Calculator
Hole and shaft limits, fit type and clearance for ISO 286 fits
An ISO fit pairs a hole class such as H7 with a shaft class such as g6; a 25 mm H7 hole is 25.000 to 25.021 mm. Pick a size and the two classes below to get both sets of limits, the clearance or interference between them, and whether the fit slides, locates or presses.
ISO 286 hole and shaft fit
Nominal size in millimetres, up to 500 mm. Capital letters are holes, lower case are shafts.
| Hole tolerance class | |
| Shaft tolerance class |
| Hole H7 deviations | +21 / 0 µm |
| Hole H7 limits | 25.000 to 25.021 mm |
| Shaft g6 deviations | -7 / -20 µm |
| Shaft g6 limits | 24.980 to 24.993 mm |
| Clearance (+) or interference (−) | +7 to +41 µm |
| Clearance fit: the shaft is always smaller than the hole. | |
What is H7 tolerance?
H7 is the most common hole tolerance. The H puts the lower limit exactly on the nominal size, and the 7 sets the width of the zone: one IT7 grade, which grows with the diameter. A 25 mm H7 hole is 0 to +21 micrometres over size, so it measures 25.000 to 25.021 mm.
A reamed hole holds H7 comfortably, which is why it is the default hole in the hole-basis system: the shop makes every hole H7 with a standard reamer and gauge, and the shaft tolerance decides how tight the fit is.
What does an H7 tolerance chart look like?
The chart gives the H7 hole limits for each standard size range, with three shafts it is often paired with. Values are in micrometres (µm, 0.001 mm), upper deviation first; a size on a range boundary belongs to the lower range, so 30 mm uses the 18–30 row.
| Size (mm) | H7 hole | g6 shaft | k6 shaft | p6 shaft |
|---|---|---|---|---|
| 0–3 | +10 / 0 | -2 / -8 | +6 / 0 | +12 / +6 |
| 3–6 | +12 / 0 | -4 / -12 | +9 / +1 | +20 / +12 |
| 6–10 | +15 / 0 | -5 / -14 | +10 / +1 | +24 / +15 |
| 10–18 | +18 / 0 | -6 / -17 | +12 / +1 | +29 / +18 |
| 18–30 | +21 / 0 | -7 / -20 | +15 / +2 | +35 / +22 |
| 30–50 | +25 / 0 | -9 / -25 | +18 / +2 | +42 / +26 |
| 50–80 | +30 / 0 | -10 / -29 | +21 / +2 | +51 / +32 |
| 80–120 | +35 / 0 | -12 / -34 | +25 / +3 | +59 / +37 |
| 120–180 | +40 / 0 | -14 / -39 | +28 / +3 | +68 / +43 |
| 180–250 | +46 / 0 | -15 / -44 | +33 / +4 | +79 / +50 |
| 250–315 | +52 / 0 | -17 / -49 | +36 / +4 | +88 / +56 |
| 315–400 | +57 / 0 | -18 / -54 | +40 / +4 | +98 / +62 |
| 400–500 | +63 / 0 | -20 / -60 | +45 / +5 | +108 / +68 |
Which ISO fit should I use?
Start from the job the joint does. Parts that move need a clearance fit, parts that must locate accurately but come apart need a transition fit, and parts that carry load without fasteners need an interference fit. The ten hole-basis fits below cover most machine design.
| Fit | Type | At Ø25 mm (µm) | Typical use |
|---|---|---|---|
| H11/c11 Loose running | Clearance | +110 to +370 | Wide clearance for dirty or hot parts, pins in clevises |
| H9/d9 Free running | Clearance | +65 to +169 | Large temperature swings, high speeds, loose bearings |
| H8/f7 Close running | Clearance | +20 to +74 | Shafts in plain bearings at moderate speed |
| H7/g6 Sliding | Clearance | +7 to +41 | Parts that move and locate accurately: spigots, pistons |
| H7/h6 Locational clearance | Clearance | 0 to +34 | Snug fit that assembles by hand, still slides |
| H7/k6 Locational transition | Transition | -15 to +19 | Accurate location, a light tap to assemble |
| H7/n6 Locational transition | Transition | -28 to +6 | Tighter location, press or mallet to assemble |
| H7/p6 Locational interference | Interference | -35 to -1 | Light press fit: bushings, dowels in steel |
| H7/s6 Medium drive | Interference | -48 to -14 | Press fit that carries torque: gears, hubs |
| H7/u6 Force fit | Interference | -61 to -27 | Shrink or heavy press fit, permanent joints |
Positive numbers are clearance and negative numbers interference, from the tightest to the loosest pair of parts.
What is a good press fit tolerance?
For a steel shaft in a steel hub, H7/p6 is a light press fit, H7/s6 a medium drive fit that carries torque, and H7/u6 a heavy force or shrink fit. At 25 mm they give 1 to 35, 14 to 48 and 27 to 61 micrometres of interference.
Softer hub materials, thin walls and plastics need less interference, because the hub stretches instead of gripping. For 3D-printed parts, where ISO grades don’t apply, see press-fit and snap-fit design.
What is the difference between H7 and h7?
Capital letters are holes and lower-case letters are shafts. An H7 hole runs from the nominal size up; an h7 shaft runs from the nominal size down. At 25 mm, H7 is 25.000 to 25.021 mm and h7 is 24.979 to 25.000 mm: the same width, on opposite sides.
Letters early in the alphabet sit further from the nominal size on the loose side (c, d, f, g for shafts); letters after h move to the tight side (k, m, n, p, s, u). That is why H7 with a shaft from a to h always clears.
How is a fit written on a drawing?
A fit callout follows the diameter: Ø25 H7 on the hole, Ø25 g6 on the shaft, or Ø25 H7/g6 on an assembly. Many drawings add the limits in brackets, such as Ø25 H7 (+0.021/0), so the shop doesn’t need a chart.
For the other callouts on a print, see engineering drawing symbols, and for how tolerances add up along a chain of dimensions, see tolerance stack-up basics.
Frequently asked questions
What is the tolerance for H7?
H7 is a hole tolerance with a lower deviation of zero and an upper deviation of one IT7 grade. For a 25 mm hole that is 0 to +21 micrometres, so 25.000 to 25.021 mm; for 10 mm it is 0 to +15, and for 50 mm 0 to +25.
What is the difference between H7 and h7?
Capital H7 is a hole and lower-case h7 is a shaft. Both have one deviation at zero, but on opposite sides: an H7 hole is never smaller than the nominal size, and an h7 shaft is never larger. A 25 mm h7 shaft is 24.979 to 25.000 mm.
What is the best fit for a press fit?
H7/p6 is the usual light press fit, for bushings and dowels in steel. H7/s6 carries torque, as with gears and hubs, and H7/u6 is a heavy force or shrink fit. At 25 mm they give 1 to 35, 14 to 48 and 27 to 61 micrometres of interference.
Is H7/g6 a sliding fit?
Yes. H7/g6 is the standard sliding fit: always a small clearance, 7 to 41 micrometres at 25 mm, so the parts move freely but locate accurately. For a fit that assembles by hand and barely moves, use H7/h6.
What does the number in a tolerance class mean?
The number is the international tolerance grade, the width of the tolerance zone. A lower number is tighter: IT6 at 25 mm is 13 micrometres wide, IT7 21 and IT8 33. The letter sets where the zone sits relative to the nominal size.
Does the calculator work in inches?
ISO 286 fits are defined in millimetres, so enter the size in mm: multiply inches by 25.4. Divide a result in micrometres by 25,400 to get inches; 21 micrometres is about 0.0008 in.
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