Guides

Tolerance block on drawings: both styles, with a worked example of each

A tolerance block on drawings states the general tolerance: the limits that apply to every dimension with no tolerance of its own. There are two ways to write it. Metric ISO drawings name an ISO 2768 class in the title block (for example ISO 2768-m). Inch and ASME drawings often use a decimal-places block instead, where the number of places on a dimension picks its tolerance (X.X ±0.1, X.XX ±0.01).

Both styles do the same job. They save you writing a tolerance on every figure, and they tell the machinist or inspector what "no tolerance shown" actually means. Below is a worked example of each, how to read them, and when to use which.

What the general tolerance covers

Under ISO 2768-1, a dimension with no tolerance written on it is held to the general tolerance class the title block names. That's the whole rule. The block doesn't replace tolerances on the dimensions that matter. It covers the rest: overall sizes, chamfers, the outline of a bracket, the length of a slot that only needs to clear something.

Some dimensions sit outside it altogether:

• A dimension with its own tolerance. What's written on the figure wins.
• A theoretically exact dimension. It's boxed, and its tolerance comes from the geometric tolerance that refers to it.
• An auxiliary dimension. It's bracketed and carries no tolerance. It's there for information, not for inspection.

Form, orientation and location aren't toleranced by the general linear block either, and never with a note. ISO 1101 puts them in a frame with the geometric symbols, with datums lettered in a box on a triangle.

Style 1: an ISO 2768 class in the title block

On a metric ISO drawing, the general tolerance is one entry in the title block. ISO 2768-1 gives four classes:

• f: fine
• m: medium
• c: coarse
• v: very coarse

You write the standard and the class, such as ISO 2768-m, and the standard's tables do the rest. The tolerance grows with the size of the feature, so you don't pick a number yourself. You pick a class that suits the process and the part.

Worked example: reading ISO 2768-m

These are the linear tolerances for class m, in millimetres:

To read an untoleranced dimension, find the band its nominal size falls in and take the deviation. A length in the 6 to 30 band may be off by ±0.2. A length in the 30 to 120 band may be off by ±0.3. A length in the 120 to 400 band may be off by ±0.5. The figure on the drawing stays clean; the inspector looks up the band.

This is the style in the drawing above. Its title block names ISO 2768-mK. The overall sizes carry no tolerance of their own, so they're held to class m. The bore needs to be tighter than that, so it carries its own tolerance, +0.021 and 0, stacked because the two halves differ. The hole centres carry ±0.1, written once because the halves are equal.

Where it goes in the title block

ISO 7200 puts the identification and title at the bottom right of the block, reading the way the sheet reads. The scale, the projection symbol, the unit and the general tolerance class go on the block too, so a reader finds them in one place. The block is at most 180 mm wide and sits against the frame at the bottom right. For the full list of mandatory and optional fields, see the ISO 7200 title block guide.

Style 2: a decimal-places block

The decimal-places style is common on inch and ASME drawings. Instead of naming a standard, the block lists a tolerance for each number of decimal places. The way you write a dimension then selects its tolerance.

Worked example: a decimal-places block

A typical block reads like this:

A dimension written to one place, X.X, holds ±0.1. Write the same size to two places, X.XX, and it holds ±0.01. The figure is the same size; the extra digit is the instruction. So on this kind of drawing, trailing zeros aren't decoration. Dropping one loosens the part, and adding one tightens it.

The important point: the values in a decimal-places block are the company's own, not a standard's. Two companies can both use X.X and X.XX and mean different things. Always read the block on the sheet in front of you, and never assume it matches the last drawing you worked from.

Which style to use

Use ISO 2768 when you draw to ISO, in millimetres, and want a tolerance that scales with size without thinking about it. A supplier anywhere can look up the class. Use a decimal-places block when you draw in inches to ASME, or when your company already has one and its suppliers know it. Whichever you pick, state it once, in the title block, and don't mix the two on one sheet.

Writing a tolerance on the figure

Whatever the general block says, some dimensions need their own. ISO 129-1 sets out how:

• Deviations go after the figure, upper above lower, both with their sign, in the same unit as the figure. Where the halves differ, they're stacked, like the bore's +0.021 over 0 in the drawing above.
• Equal deviations are written once with ±, like the hole centres' ±0.1.
• Limits give the two sizes instead, larger above.

A common mistake is writing a geometric requirement as a note ("holes to be square to face"). That belongs in a feature control frame, not in text.

Building drawings are different

If you draw buildings rather than parts, you won't find a tolerance block in the title block at all. ISO 2768-1 is clear that building work isn't toleranced on the drawing; the permitted deviations sit in the specification. A building title block carries other things instead, such as a sheet number by discipline and series and a status like for approval or for construction.

How isogram handles it

In isogram, the title block carries the drawing's general tolerance, and a dimension that needs its own writes it on the figure, stacked where the halves differ. Dimensions follow ISO 129-1 or ASME, whichever your office works to, and geometric tolerances and datums are drawn to ASME Y14.5 in proper frames. Auto Dim proposes the dimensions for a whole view of a part; you review them and keep what you want, then add tolerances to the ones that matter.

Before a set is issued, isogram reads every sheet for what a senior drafter would catch, including an empty title block field. A missing tolerance class is exactly the kind of gap that gets a drawing sent back. If you want to try it on your own parts, the public beta opens in December 2026 on Mac and Windows, with Linux following in an update, and it's free to test: join the list.

Questions

8 November 2026