How to read a symbol like H7/g6
Each half of the symbol is a letter and a number, and they do different jobs. The number is the IT grade, and it sets how wide the tolerance band is — IT7 on a 40 mm feature is 25 µm wide, IT6 is 16 µm, IT11 is 160 µm. The letter is the fundamental deviation, and it sets where that band sits relative to the basic size. Capital letters are holes, lower-case letters are shafts.
H is special: it places the lower limit exactly on the basic size, so an H7 hole at ø40 runs from 40.000 to 40.025 and never goes undersize. That is what makes it the basis of the hole-basis system, which nearly all general engineering uses. You machine one standard hole size, usually with a standard reamer or boring tool, and vary the shaft to change the fit. Only where the shaft is a bought-in standard size — bearing journals, drawn bar, hydraulic rod — does the shaft-basis system with an h shaft make more sense.
Shaft letters run alphabetically from a to zc, and h is the pivot. Letters before h place the whole shaft band below basic size, so the fit is always clearance and the gap grows as you move backwards through the alphabet: g is a close sliding fit, f a running fit, d a loose running fit. Letters after h push the band upward, so k, m and n give transition fits that may go either way, and p, r and s give interference that always requires force.
The four numbers and where they come from
Every fit reduces to four deviations. The hole has an upper deviation ES and a lower deviation EI; the shaft has an upper deviation es and a lower deviation ei. All four are measured in micrometres from the basic size, and all four can be positive, negative or zero.
Maximum clearance occurs when the hole is at its largest and the shaft at its smallest: Cmax = ES − ei. Minimum clearance occurs the other way round: Cmin = EI − es. If Cmin is zero or positive the fit is a clearance fit; if Cmax is zero or negative it is an interference fit; anything in between is a transition fit, where two conforming parts may or may not press together depending on where each one landed inside its band.
The IT grade widths are not arbitrary. ISO 286 builds them from a standard tolerance unit i = 0.45 D1/3 + 0.001 D micrometres, with D the geometric mean of the size range, and then assigns each grade a multiple: IT5 = 7i, IT6 = 10i, IT7 = 16i, IT8 = 25i, and so on in a roughly geometric series, with the results rounded to preferred numbers. The cube-root term reflects manufacturing reality — holding a tolerance gets harder as a part gets bigger, but not in proportion. Below 3 mm the formula stops fitting and the standard simply tabulates the values, which is why the small-size numbers look slightly out of pattern.
The letters have their own formulas. Shaft f is es = −5.5 D0.41, shaft g is −2.5 D0.34, shaft n is +5 D0.34. Those constants are what make each letter behave consistently across sizes: an f shaft leaves roughly the same proportional running clearance at ø10 as at ø200.
Worked example: ø40 H7/g6
A 40 mm shaft that must slide freely in a bore but with almost no play — a gauge plunger, a locating pin, a lightly loaded plain bearing.
- Find the size range. 40 mm falls in the 30–50 mm band of ISO 286.
- Hole band. IT7 in that range is 25 µm. H puts EI = 0, so ES = +25 µm and the hole is 40.000 / 40.025 mm.
- Shaft band. IT6 in that range is 16 µm. The fundamental deviation of g at 30–50 mm is es = −9 µm, so ei = −9 − 16 = −25 µm and the shaft is 39.975 / 39.991 mm.
- Maximum clearance. ES − ei = 25 − (−25) = 50 µm = 0.050 mm.
- Minimum clearance. EI − es = 0 − (−9) = 9 µm = 0.009 mm.
- Classification. The minimum is positive, so this is a clearance fit throughout, running from 9 to 50 µm.
Compare that with ø40 H7/p6, keeping the hole identical. The p deviation at 30–50 mm is ei = +26 µm, so the shaft becomes 40.026 / 40.042 mm. Maximum clearance is 25 − 26 = −1 µm and minimum clearance is 0 − 42 = −42 µm: both negative, so it is an interference fit of 1 to 42 µm. A single letter change turned a part you can push together by hand into one that needs a press or a temperature difference.
Choosing a fit, and reading the one you have
Start from function. If the parts must move relative to each other, you need clearance, and the amount depends on speed, load and lubrication: H7/g6 for a sliding location with minimal play, H7/f7 or H8/f7 for a running fit, H9/d9 or H11/d11 where alignment is loose and dirt is expected. If the parts must locate accurately but be separable, a transition fit — H7/k6 or H7/n6 — centres the part with almost no play and still allows assembly with a mallet or a light press. If the joint must transmit torque by friction alone, you need interference: H7/p6 for a light press, H7/s6 for a shrink fit on a hub or a gear.
Then check the grade against how you will make the part. IT6 on a shaft means grinding or very good turning; IT7 in a hole means reaming, boring or honing; IT9 and above are ordinary turning and milling. Specifying IT5 where IT8 would work is one of the more expensive habits in mechanical design, because cost climbs sharply as the band narrows and inspection gets harder at the same rate.
Two practical checks are worth making on any interference fit. The first is whether the outer member can survive the hoop stress; ISO 286 tells you the dimensions and says nothing about stress. The second is what happens at temperature, because a steel hub on a steel shaft holds its interference while an aluminium hub on a steel shaft loses it as the assembly warms. Neither is captured by the fit symbol.
Finally, remember that the symbol is a size specification, not a form specification. A hole can be perfectly within its H7 limits and still be bell-mouthed, tapered or lobed, and any of those will change how the assembly behaves. Where form matters, it needs its own geometric tolerance on the drawing, and the envelope requirement or a maximum-material condition is what ties size and form together.
Common hole-basis fits at ø40 mm
| Fit | Hole limits | Shaft limits | Clearance range | Character |
|---|---|---|---|---|
| H11/d11 | 40.000 / 40.160 | 39.760 / 39.920 | +0.080 to +0.400 | Loose running |
| H9/d9 | 40.000 / 40.062 | 39.858 / 39.920 | +0.080 to +0.204 | Free running |
| H8/f7 | 40.000 / 40.039 | 39.950 / 39.975 | +0.025 to +0.089 | Close running |
| H7/g6 | 40.000 / 40.025 | 39.975 / 39.991 | +0.009 to +0.050 | Sliding |
| H7/h6 | 40.000 / 40.025 | 39.984 / 40.000 | 0.000 to +0.041 | Locational clearance |
| H7/k6 | 40.000 / 40.025 | 40.002 / 40.018 | −0.018 to +0.023 | Transition |
| H7/n6 | 40.000 / 40.025 | 40.017 / 40.033 | −0.033 to +0.008 | Tight transition |
| H7/p6 | 40.000 / 40.025 | 40.026 / 40.042 | −0.042 to −0.001 | Light press |
| H7/s6 | 40.000 / 40.025 | 40.043 / 40.059 | −0.059 to −0.018 | Shrink / drive |
The hole is identical in five of these nine rows. That is the entire point of the hole-basis system: one reamer, one bore programme, and the fit is changed on the shaft.
ISO 286 standard tolerance grades, IT5 to IT11, in micrometres
| Nominal size (mm) | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 |
|---|---|---|---|---|---|---|---|
| over 3 to 6 | 5 | 8 | 12 | 18 | 30 | 48 | 75 |
| over 6 to 10 | 6 | 9 | 15 | 22 | 36 | 58 | 90 |
| over 10 to 18 | 8 | 11 | 18 | 27 | 43 | 70 | 110 |
| over 18 to 30 | 9 | 13 | 21 | 33 | 52 | 84 | 130 |
| over 30 to 50 | 11 | 16 | 25 | 39 | 62 | 100 | 160 |
| over 50 to 80 | 13 | 19 | 30 | 46 | 74 | 120 | 190 |
| over 80 to 120 | 15 | 22 | 35 | 54 | 87 | 140 | 220 |
| over 120 to 180 | 18 | 25 | 40 | 63 | 100 | 160 | 250 |
| over 180 to 250 | 20 | 29 | 46 | 72 | 115 | 185 | 290 |
Each grade is roughly 1.6 times the one below it, so IT7 is about 2.5 times IT5 and IT11 is about 6.4 times IT7. That geometric spacing is deliberate and matches the way process capability improves.
What the fit symbol does not tell you
- Nothing about form. A bore can be inside its H7 limits and still be oval, tapered or bell-mouthed. Roundness, cylindricity and straightness need their own geometric tolerances.
- Nothing about surface finish. A press fit into a rough bore loses interference as the asperities flatten during assembly, so an interference fit needs a specified surface roughness to behave as calculated.
- Nothing about temperature. The limits are defined at 20 °C. Dissimilar materials change the effective interference as the assembly heats or cools.
- Nothing about stress. An interference fit generates hoop stress in the outer part and contact pressure between the two; those are a Lamé thick-cylinder calculation, not a tolerance one.
- Nothing about how you measure it. A 25 µm band needs an instrument with resolution well under 5 µm and a controlled part temperature; a vernier caliper cannot verify an IT7 bore.
- Nothing about assembly force. Two interference fits with the same interference need very different forces depending on engagement length, materials and lubrication.
Where the ISO system sits alongside the alternatives
ISO 286 is the international limits-and-fits system and is what metric drawings use worldwide. The ANSI/ASME B4.1 inch system covers the same ground with a different vocabulary — RC for running and sliding clearance, LC and LT for locational clearance and transition, FN for force and shrink fits — and ANSI B4.2 restates the ISO system in preferred metric fits. The concepts translate directly: an RC4 is close in spirit to H8/f7, an FN2 to H7/s6. What does not translate is the numbers, so never mix a class from one system with a limit from the other.
Getting a part to an IT7 hole is a process decision as much as a specification. Drilling alone will not do it — a drilled hole is oversize, out of round and not straight — so the sequence is drill, then bore or ream, and the drilling speed and feed calculator handles the first step. Where the hole is threaded rather than fitted, a different rule applies entirely, and the tap drill size calculator sizes it from the thread rather than from a fit class.
Fits also determine how a joint carries load. A press fit transmits torque by friction, which is why gears and pulleys are often mounted H7/s6 with no key at all; a clearance fit relies on a fastener's clamp load instead, and the bolt torque calculator works out what that clamp load actually is. Where a shaft carries a belt or gear drive, the mounting fit and the drive geometry are usually specified together — see the gear ratio calculator for the drive side of that decision.