What an IT grade means, how the deviation letter positions a tolerance zone, why hole-basis is the shop default — and why no limit tables appear on this page.
A fit is the relationship between the tolerance zones of a hole and a shaft. In a call-out like H7/g6, the number is the IT grade — how wide the tolerance is — and the letter is the fundamental deviation — where that zone sits relative to the basic size. The drawing gives both because neither alone says whether the parts will assemble. This page explains the system. It does not print the limit values, and the section why the numbers are not on this page says why that is deliberate.
Every mating dimension starts from a basic size — the nominal figure on the drawing. Nothing is ever made exactly to it, so each part is given a tolerance zone: a band of permitted sizes between an upper and a lower limit. The zone has two independent properties, and the call-out names both.
The two documents behind this are ISO 286-1:2010, adopted in India as IS 919 (Part 1) : 2014, which sets out the basis of tolerances, deviations and fits; and ISO 286-2:2010, adopted as IS 919 (Part 2) : 2014, which holds the tables of standard tolerance grades and limit deviations for holes and shafts.
IT is the standard tolerance grade defined by the standard. The series runs from IT01 up to IT18: a lower number is a tighter tolerance, a higher number a looser one. A grade is a relative width, not a fixed distance — the same grade is a wider band on a large diameter than on a small one, because the grades are defined size-range by size-range.
The conventional division of labour between the grades is worth knowing, because it tells you what a drawing is asking for:
The standard uses the same alphabet twice. Uppercase letters are holes, lowercase are shafts, which is why H7 is a hole and h6 is a shaft even though the two read almost identically on a drawing.
For a hole, the letters A through G place the tolerance zone above the basic size, which always leaves a clearance against a shaft made to h. H is the boundary case: its lower deviation is zero, so the zone begins at the basic size. The letters from J onward shift the zone downward, producing transition and interference fits against that same shaft.
For a shaft, the pattern mirrors in lowercase: a through g sit below the basic size, h is the boundary case with a zero upper deviation, and j through zc sit at or above it. This is why H7/h6 is a clearance fit (the two zones fall on opposite sides of the basic size) while H7/p6 is an interference fit (the shaft zone has crossed to the other side of the hole zone).
Because there are two letters in every call-out, one of them has to be held constant for a workshop to keep its tooling sane. Which one you hold decides the system:
The hole is always H; the shaft letter carries the fit. Call-outs read H7/g6, H7/h6, H7/k6, H7/p6. One reamer and one plug gauge per nominal size then covers every fit at that size, which is why it dominates general engineering.
The shaft is always h; the hole letter carries the fit. Call-outs read G7/h6, F8/h6. Used when the shaft is a purchased or cold-drawn standard — ground bar stock that cannot be changed — so the hole has to absorb the fit instead.
Every fit falls into one of three families, and the family is decided by whether the two zones overlap.
The hole is always larger than the shaft, so there is play in every assembly. Used where parts must slide, rotate, or be located without force — sliding gears, spigots, and running fits.
The zones overlap, so the same pair may come out with a small clearance or a small interference depending on where each part fell inside its own tolerance. Used for accurate location that is still assembled by hand or with light pressure — dowels, keys, and gear location.
The hole is always smaller than the shaft, so the joint holds by elastic deformation and friction. Used for permanent joints — bearing rings, bushings, and press-fit pins — and usually assembled with force, heat, or cold.
The boundary between transition and interference is not fixed: the same letters can give a light interference at one size and a transition at another, because the deviation is defined per size range. That is one more reason the family should be read from the table for your size, not from the letters alone.
A fit is a functional decision before it is a table lookup. The questions that settle it:
Only after those are answered does the call-out get read out of the standard's table for the size and grade range in question.
The limit values live in the standard's Part 2 tables, and this page deliberately does not reproduce them. Two reasons, and both matter.
The first is ownership. The IS tables are BIS copyright and the ISO tables are ISO copyright. A reference page that retypes them is either licensed or it is not, and this one is not.
The second is arithmetic. The published limits are a rounded preferred-number series, not the raw output of the grade formula. Measured across a sample of ISO 286-1 cells, the formula disagrees with the published table in roughly half of them, because the table has been rounded to a convenient series and the formula has not. A page that computed its own values would therefore be wrong about half the time on the only numbers that give it value. For a tolerance, that is not a lost ranking — it is a scrapped batch or a rejected assembly at a customer's works.
So: get IS 919 (Part 2) : 2014 (or ISO 286-2:2010) from the source, quote the edition on the drawing, and work from that table. If you would rather not work through it, send us the drawing and the fit class and we will apply it to the gauge.
Related reading: IS 919 and IS 3455 explained — what each standard covers, which edition is current, and the H/Y/Z gauge tolerance and wear framework.
Once the limits are set, something has to check them at the machine. That is limit gauging: a full-form GO member that checks the assembly condition over the full length, and a short NO-GO member that checks size at a point — the Taylor principle as IS 3455 states it. A gauge is made to its own tolerance, placed inside the workpiece limit, and allowed to wear to a defined limit rather than to the part limit.
DSN Enterprises manufactures plain plug, ring and snap gauges to the fit classes on your drawing — H7 bores and their shaft counterparts among them — with matched GO/NO-GO members and traceable calibration certificates.
H7 is a hole tolerance. H is the fundamental deviation: the lower deviation is zero, so the tolerance zone begins at the basic size and runs upward into the clearance side. 7 is the IT grade, which sets the width of that zone. A hole made to H7 is therefore never smaller than the basic size.
The capital H is a hole; the lowercase h is a shaft. Both are the boundary case where the fundamental deviation is zero at the basic size, but H7 places the hole's zone above the basic size while h6 places the shaft's zone below it. Together, H7/h6 is the classic close-running clearance fit.
IT is the standard tolerance grade defined by ISO 286 and, identically in India, by IS 919. The series runs from IT01 to IT18. A lower number is a tighter tolerance, and the same grade is a wider zone on a larger diameter, because the grades are defined size-range by size-range rather than as a single absolute value.
In a hole-basis system the hole is always H and the shaft letter changes the fit — H7/g6, H7/h6, H7/p6. In a shaft-basis system the shaft is always h and the hole letter changes — G7/h6. Hole-basis is the shop default because one reamer and one plug gauge per nominal size then covers every fit at that size.
IS 919 (Part 1) : 2014 is identical to ISO 286-1:2010, and IS 919 (Part 2) : 2014 is identical to ISO 286-2:2010. An H7/g6 call-out means the same thing whether the drawing cites the IS number or the ISO number, which is what lets one part be supplied to an Indian and an export customer from the same drawing.
Because the published limits are a transcription, not a calculation. They are a rounded preferred-number series that does not reproduce exactly from the grade formula, so a value computed from the formula can disagree with the standard it is attributed to. A wrong limit is not a lost ranking, it is a scrapped batch — so this page explains the system and sends you to the source tables instead.
Send the drawing, the fit call-out and the size. We will make the gauge to the limits you specify and issue the calibration certificate against them.