H7 Tolerance

H7 tolerance puts the smallest allowed hole exactly on the nominal size and the largest one IT7 above it, so a 25 mm H7 hole measures 25.000 to 25.021 mm (0 to +21 µm). A hole that is within H7 is never smaller than the nominal size.

The capital letter matters. H7 is a hole class; lowercase h7 is a shaft class and points the other way, so a 25 mm h7 shaft measures 24.979 to 25.000 mm (0 to -21 µm). Both use the same IT7 width. The H7 and h7 values on this page are computed from the ISO 286-1 tables by the same code as our fit calculator.

H7 and h7 limits up to 500 mm

H7 only changes at the 13 main size steps of ISO 286, so this one table covers every diameter up to 500 mm. Read across from the step your diameter falls in: the hole column gives the H7 deviations, the shaft column the h7 deviations.

Print this page or save it as PDF; the size input is left out of the print layout. The same data as a spreadsheet: h7-tolerance.csv.

Bore sizes for a 3 to 30 mm H7 hole

A 10 mm H7 hole measures 10.000 to 10.015 mm. A 20 mm or 25 mm H7 hole gets 21 µm of tolerance, because both sizes sit in the over 18 to 30 mm step.

Reading H7 on a drawing

An H7 callout follows the diameter, as in Ø25 H7. The letter H fixes the lower deviation of the hole at zero, which is why the minimum bore is the nominal size. The number 7 picks the tolerance grade, IT7, and the IT7 value for that size becomes the upper deviation.

So working out H7 by hand takes one lookup: find the IT7 value for the size step and add it to the nominal size. At 40 mm, IT7 is 25 µm, so the hole runs from 40.000 to 40.025 mm.

Seven of the ten common hole-basis fits in our calculator's table use an H7 hole. In a hole-basis system the hole class stays the same and the shaft class changes to give a loose, located or pressed fit.

Reaming an H7 hole

M.A. Ford's TrueSize Series 270 and 272 reamers are not ground to the nominal size. The catalog lists their metric sizes at the DIN 1420 H7 tolerance, which puts the reamer diameter a few microns above nominal, inside the H7 zone.

Where an H7 reamer sits in the H7 zone

The reamer tolerances below are the DIN 1420 H7 values printed in M.A. Ford's TrueSize reamer catalog for Series 270 and 272 (pages 376 and 379). Set against the H7 limits, the reamer's largest size stays 2 to 3 µm below the top of the zone, and its smallest size is 4 to 8 µm above the nominal size.

Drill size before an H7 reamer

The drill leaves stock for the reamer to remove. M.A. Ford's metric total stock allowance chart (page 404) pairs these drill sizes with its reamers and lists a separate stock allowance for each material. The catalog calls that chart a starting point for reaming operations.

Which shaft goes in an H7 hole

H7 on its own is not a fit. The shaft class decides whether the parts slide, locate or press together. This table pairs a 10 mm H7 hole with the usual shafts, tightest last. Each class links to the calculator with that fit loaded.

The preferred fits table calls H7/p6 a press fit, but for sizes over 3 mm and up to 18 mm its minimum interference is 0 µm: a hole at the top of H7 and a shaft at the bottom of p6 meet exactly, with no grip. At 10 mm the range is 0 to 24 µm. Up to and including 3 mm it is not even an interference fit: at 2 mm it runs from 4 µm clearance to 12 µm interference. If a pressed part over 3 mm and up to 18 mm must never turn, a tighter shaft such as r6 or s6 keeps some interference even at the worst-case limits.