ISO 2768 general tolerances calculator

The entry is the number of the standard followed by one or two class letters. The lower-case letter is the tolerance class from part 1 and governs linear and angular dimensions; the choices are f, m, c and v, from fine to very coarse. The upper-case letter is the class from part 2 and governs form and position; the choices are H, K and L, from tight to wide.

If only one letter appears — "ISO 2768-m" — the drawing invokes part 1 alone. Straightness, flatness, perpendicularity, symmetry and run-out then have no general specification at all, and anyone who needs one has to write it on the feature.

The case carries the whole meaning: "m" is a class for linear dimensions, "M" is nothing, and "K" is a class for form and position. An entry set in capitals, as some title-block templates force, is strictly speaking unreadable.

From part 1: every linear dimension of 0.5 mm and above that carries no tolerance of its own, plus external radii and chamfer heights on broken edges in a table of their own, and angular dimensions in a third. The angular table is the only one not banded by the dimension being toleranced but by the length of the shorter leg: the same angular error becomes a far larger departure at the far end of a long leg.

From part 2: straightness, flatness, perpendicularity, symmetry and circular run-out, each as the width of a tolerance zone. Run-out is the one value in the whole standard with no size ranges — it is the same at every size.

Four further characteristics are governed by part 2 without being tabulated, because they follow from values that already exist: roundness is limited to the value of the diameter tolerance, but never to more than the run-out value; parallelism equals the greater of the size tolerance and the flatness or straightness tolerance; coaxiality may be as large as the run-out value; and for cylindricity part 2 deliberately gives no value at all, since it is made up of roundness, straightness and parallelism.

Any dimension with a tolerance of its own. Where an ISO 286 class, a plus-minus figure or a geometrical tolerance frame stands on the dimension, that entry is what applies — the general tolerance steps back, and is never applied on top of it.

Nor does it cover auxiliary dimensions in brackets, theoretically exact dimensions in a box, which are read together with a tolerance frame, or dimensions below 0.5 mm, for which ISO 2768-1 expressly requires the deviation to be written beside the dimension.

And finally, a general tolerance describes no function. A feature that guides, centres, seals or is pressed into something belongs toleranced individually — not because the general figure is too wide, but because it says nothing about what matters at that particular place.

Part 1 has f (fine), m (medium), c (coarse) and v (very coarse). Class m is the ordinary case for machined parts and is what most tooling drawings carry. Class f belongs on parts whose dimensions are ground or finish-milled in any case. Classes c and v belong on flame-cut, sawn, welded and formed parts whose dimensions are machined later, if at all.

Part 2 has H, K and L, from tight to wide. The standard gives them no names; K is the usual companion to class m, H belongs with it where the faces are ground anyway, and L on welded or roughly machined assemblies.

The two letters are chosen as a pair, and they are chosen from the process the part is made by. A class is not a demand for care but a description of what a process holds anyway; its job is to close the drawing, not to tighten it.

The rule is simple and has no exception: a tolerance on the dimension always overrides the general tolerance in the title block. The two never apply to the same dimension at once, and they are never added together.

Why that is not a formality shows in the orders of magnitude. A 25 mm bore in class m has a band four tenths of a millimetre wide. The same bore in H7 has, per ISO 286, a band of twenty-one micrometres — barely a twentieth of it. Anyone resting a fit on the general tolerance has not really toleranced it at all.

The same holds for form and position: a fit assumes the bore is round and the shaft cylindrical, and the general figure from part 2 is too wide for a fitting feature in exactly the same way. Fitting dimensions get their class on the dimension, and their geometrical tolerances in a frame of their own.

The costliest is leaving a fitting feature to the general tolerance. It goes unnoticed at the drawing board, because nothing is missing from the drawing — it is noticed at assembly.

The second is writing f everywhere to be safe. That tightens every single linear dimension on the part without making any one of them more important, and turns every unimportant dimension into something to inspect. For broken edges and angular dimensions it buys nothing at all: ISO 2768-1 prints f and m in the same column there, and the values are identical.

The third is a misreading at a range boundary. The ranges read "over … up to and including …", so a 30 mm dimension is still in the range over 6 up to 30 and not in the next one. At every boundary in the table the value steps up a whole grade.

The fourth is the missing second letter. "ISO 2768-m" on its own says nothing about flatness, perpendicularity or run-out — and reading the drawing as though something still applied means assuming a specification nobody made.

A tighter class does not change the geometry of the part. It changes what has to be done to reach it, and what has to be measured afterwards to show that it was reached.

The relationship is qualitative, and it runs in one direction: narrower bands move work from one setup to several, from a simple fixture to a more accurate one, from an edge broken by eye to a controlled one; and they move inspection from a spot check to a documented measurement. With part 2 there is the further point that form and position are not checked with a caliper but against a datum system.

This page deliberately puts no numbers on that. What one step in class means depends on the part, the material, the process and the batch — and that belongs in a quotation and a control plan, not in a table.

The practical rule of thumb is therefore not a rule about saving anything but a rule about accuracy: the class should describe what the chosen process holds anyway. A class it does not hold turns one general specification into a set of individual requirements, applied to every dimension on the drawing at once.

ISO 2768-2 has been withdrawn and replaced by ISO 22081:2021. The change is not a different table but the abandonment of the table: instead of a class letter, ISO 22081 requires an explicit general geometrical specification in or beside the title block, referred to a datum system — a general profile tolerance to datums A, B and C, for instance — together with a general size specification. The reason was that the old class letters said nothing about what a feature is measured against: a perpendicularity with no named datum leaves open which of the two faces aligns the other.

Part 1 remains in force and is still what most drawings invoke for linear and angular dimensions. Drawings carrying "ISO 2768-mK" are in circulation in enormous numbers, are still being made, and remain readable — which is why this page covers both parts and not only the current one.