Understanding H7 g6 and Other Fit Callouts

By Bradley Taylor · August 2026

The first time a print says 25 H7/g6 and nobody explains it, the natural move is to nod and go look it up quietly. That callout is a complete recipe for how a shaft and a hole are supposed to fit together, packed into a few characters. Once you know how to read it, you can look at almost any fit on a metric print and know roughly what the assembly will feel like before you cut a chip.

What the letters and numbers encode

Start with the basic size, 25 in this case. That is the nominal dimension both parts are built around. Neither the hole nor the shaft is expected to land exactly on 25.000. Each one gets a tolerance zone, and the callout tells you two things about that zone. The letter tells you where the zone sits relative to basic size. The standard calls this the fundamental deviation. The number tells you how wide the zone is, which is the IT grade. A smaller grade number means a tighter tolerance, and the width also scales with the size of the part, so an IT7 on a 25 mm bore is a different number of microns than an IT7 on a 200 mm bore.

The other rule worth tattooing somewhere is that capital letters are holes and lowercase letters are shafts. H7 is a hole. g6 is a shaft. Letters early in the alphabet put the zone on the loose side of basic, letters late in the alphabet push it toward or past basic into interference. H is the special case sitting right at the pivot point. An H hole starts exactly at basic size and the entire tolerance zone runs upward from there. The hole is never smaller than nominal and never bigger than nominal plus the grade width.

Why everything is built around H holes

That special property of H is why the hole basis system exists. Holes are the hard half of the job. You make them with drills, reamers, and boring tools, and a reamer only comes in one size. Shafts are the easy half, because turning or grinding a shaft to a target diameter is just a matter of feeding in until you get there. So the system fixes the hole at H, lets the shop stock standard reamers and plug gauges for H7 bores in the common sizes, and moves the shaft tolerance around to create whatever fit the designer wants. One set of hole tooling covers everything from a loose running fit to a heavy press, and only the shaft letter changes. It is a system designed by people who had to pay for reamers.

Working the numbers for 25 H7/g6

Here is the actual arithmetic for the example. A 25 H7 hole is allowed to be anywhere from 25.000 to 25.021, so plus zero to plus 0.021 mm on basic. A 25 g6 shaft sits entirely below basic, from minus 0.007 to minus 0.020 mm, which is 24.993 down to 24.980. Put the biggest allowed shaft in the smallest allowed hole and you still have 0.007 mm of clearance. Put the smallest shaft in the biggest hole and you have 0.041 mm. So every legal combination of parts assembles with somewhere between 0.007 and 0.041 mm of room.

In the hand, that is a close sliding fit. The shaft always clears, so it slides home under its own weight or with a light push, but there is no shake or rattle you can feel. It is the fit you want for a part that has to locate accurately and still move, like a plunger, a spigot on a fixture, or a shaft that gets assembled and disassembled without a press. When the clearance gets down near that 0.007 mm end, a film of oil and matching part temperatures matter, because a warm shaft in a cold bore can turn a sliding fit into a stuck one in a hurry.

A quick tour of the common fits

The same logic covers the rest of the family. H7/h6 is a locational clearance fit, where the shaft zone also touches basic from below, so the parts range from line to line contact up to a small clearance. It locates precisely but you may need to coax it together. H7/k6 is a transition fit, meaning the tolerance zones overlap basic and any given pair of parts might have a whisker of clearance or a whisker of interference. That is your light press for keyed pulleys and bearings that need to stay put but come off with a puller. H7/p6 is a true interference fit, the classic press fit for dowel pins and bushings, and it takes an arbor press and some care with alignment to assemble. H7/s6 is a heavier interference for parts that are never coming apart on purpose, and at that level you are usually reaching for a hydraulic press or heating the hole and freezing the shaft so the parts do the work for you. The pattern is always the same. As the shaft letter marches from g toward s, the fit walks from sliding, through located, through gripping, to permanent.

What you actually do with a callout

At the machine, the letters stop mattering. The first thing I do with any fit callout is convert it to limit dimensions before I touch a handwheel, because you cannot measure a part against a letter. The H7 bore becomes 25.000 to 25.021 written on the traveler or in my head, and from that point it is just a toleranced diameter like any other. Then gauge to match the job. For bores in production, a go and no go plug gauge answers the only question that matters, in or out, faster than any measurement. For one offs or when you need the actual number, a bore gauge or tenths reading internals do the job, and the shaft gets a micrometer either way. Just remember that a 21 micron window does not leave room for sloppy technique or a part fresh off the machine and still warm.

One honest limitation. The deviations and grade widths come out of tables in the ISO limits and fits standard, and outside the handful of fits you use weekly, nobody carries them in memory. A limits and fits calculator for this site is on the list. Until it is live, the numbers come from the published tables or from your quality department, the same way the thread limits in the thread calculator come from theirs. Look them up, write them down as limits, and machine to the numbers.

As always, this is general practice, not a spec. Prints, customer requirements, and the governing standard win every argument.