Limits of Size and Tolerance – Definition and Examples

Educational explanation, not professional design advice. Last reviewed 24 October 2026. Disclaimer

In plain English

No machine can make a part to an exact size. So a drawing does not say “make this 25 mm”; it says how far from 25 mm the part may be and still work. The biggest acceptable size and the smallest acceptable size are the limits of size. The difference between them is the tolerance — the amount of variation the designer allows.

Technical definition

The limits of size are the two extreme permissible sizes of a feature of size (ISO 286-1): the upper limit of size and the lower limit of size. They are given relative to the nominal size by the upper and lower limit deviations. The tolerance is the difference between the upper and lower limits, which is always positive.

For holes the deviations are written ES (upper) and EI (lower); for shafts es and ei. Standard tolerance widths are arranged in IT grades (for example IT6, IT7): a lower grade number means a smaller tolerance for the same size.

How it works and where it is used

Limits can be written in several ways that mean the same thing:

Limit dimensioning: 25.021 / 25.000
Deviations from nominal: 25 +0.021 / 0 (unilateral, one side only) or 50 ± 0.05 (bilateral, both sides)
ISO code: 25 H7 — the letter sets where the tolerance zone sits, the number sets its width; the values come from ISO 286-2 tables.

The inspector checks every part against the limits, not against the nominal size. Parts anywhere inside the limits are equally good.

Formula

Upper limit = nominal + upper deviation Lower limit = nominal + lower deviation

Tolerance = upper limit − lower limit = upper deviation − lower deviation

Deviations are signed: a negative deviation means smaller than nominal. Mean size = (upper limit + lower limit) ÷ 2.

Worked example

A shaft is drawn as Ø25 g6. From ISO 286-2, the deviations for 25 mm g6 are es = −0.007 mm and ei = −0.020 mm.

Upper limit = 25 + (−0.007) = 24.993 mm
Lower limit = 25 + (−0.020) = 24.980 mm
Tolerance = −0.007 − (−0.020) = 0.013 mm (13 µm)

A shaft measuring 24.985 mm is good; one measuring 24.995 mm is too large, even though it is closer to 25 mm.

In the workshop and in practice

On the shop floor, limits decide which measuring instrument and which process you need. A 0.1 mm tolerance can be checked with a vernier caliper; a 13 µm tolerance needs a micrometer or comparator and a stable, temperature-controlled measurement. Limits are also the basis of fits between holes and shafts, of tolerance stack-ups in assemblies, and of go/no-go gauges.

Common mistakes and misconceptions

Aiming at the nominal size. The nominal size may lie outside the tolerance zone (as with 25 g6). Aim for the middle of the tolerance.
Losing the sign of a deviation. −0.020 is below nominal; mixing up signs puts the part out of tolerance.
Measuring warm parts. Sizes are specified at 20 °C. A steel part 25 mm long grows by about 3 µm for a 10 °C rise — a large share of a 13 µm tolerance.
Using an instrument that is too coarse. A common rule of thumb is that the measuring uncertainty should be a small fraction of the tolerance; see measurement uncertainty.

Related calculators

Limits of Size & Tolerance Calculator — Maximum and minimum limits, tolerance and mean size from nominal size and deviations.
Fits Calculator – Clearance, Transition & Interference — Fit type and maximum/minimum clearance or interference of a hole and shaft.
Tolerance Stack-Up Calculator (Worst Case) — Worst-case maximum and minimum gap of a 1-D dimension chain, with an RSS comparison.

References

ISO 286-1: Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits
ISO 286-2: Tables of standard tolerance classes and limit deviations for holes and shafts
ISO 1: GPS — Standard reference temperature for the specification of geometrical and dimensional properties (20 °C)
ISO 14405-1: GPS — Dimensional tolerancing — Linear sizes