KELIBRON Guides
Courses中文Open the lab

Measurement basics

Sizes and tolerances on a drawing

2 min readUpdated Course: Measurement basics · Lesson 4

Key answer

A size on a drawing is a nominal size with its limit deviations, written above and below or as ±; each limit is the nominal size plus a deviation. A size with no tolerance written takes the general tolerance named on the drawing, such as GB/T 1804-m or ISO 2768-m.

On this page
  1. Reading a size
  2. Deviations and limits
  3. What “size” means
  4. Sizes without a tolerance
  5. In KELIBRON
  6. References
  7. Watch it done
  8. Practise in the lab
  9. Questions
Lab clipJudge an oversize shaft (rejected)Open in the lab →

Reading a size

On drawings to GB/T 4458.4, linear sizes in millimetres are written without the unit; any other unit is written out. Symbols in front of the number say what kind of size it is:

  • Ø: a diameter, such as Ø25.
  • R: a radius.
  • 6×Ø7: six holes of the same diameter, 7 mm.

Deviations and limits

On the drawing the deviations follow the nominal size in smaller figures, the upper deviation above and the lower below (GB/T 4458.5). Add each deviation to the nominal size to get the limits. The table uses parts from the lab and writes the deviations as upper / lower.

DrawingLower limit (mm)Upper limit (mm)Tolerance (mm)
Ø20 0 / −0.0519.9520.000.05
Ø40 0 / −0.0839.9240.000.08
Ø32 ±0.0531.9532.050.10
Ø28 +0.05 / 0 (bore)28.0028.050.05

A size can also carry a tolerance class instead of numbers, such as Ø20H7 or Ø20g6; the deviations come from the ISO 286 tables (see tolerances and fits).

What “size” means

A caliper or a micrometer measures a two-point size: the distance between two opposite points on the feature. ISO 14405-1 (2016 edition, adopted with modifications as GB/T 38762.1-2020; replaced by the 2025 edition) defines the two-point size and the other kinds of linear size a drawing can call for. A shaft can therefore be within its limits in one direction and out in another, which is why diameters are checked in more than one direction.

Sizes without a tolerance

Many sizes on a drawing have no tolerance written next to them. A note in or near the title block then names a general tolerance class, for example “GB/T 1804-m” or “ISO 2768-m”. GB/T 1804-2000 is equivalent to ISO 2768-1:1989 and defines four classes, f (fine), m (medium), c (coarse) and v (very coarse); the permitted deviation depends on the class and on the size range. Look up the value in the standard’s table for that class and size.

If a drawing has neither a tolerance nor a general tolerance note, ask whoever issued it which applies before judging the part.

In KELIBRON

Each part in the lab has its drawing size and deviations, and the panel shows the limits next to the reading. The rejected shaft (Ø20 0/−0.05, made at 20.07) is there to practise reading the limits and judging against them.

References

  1. GB/T 4458.4-2003《机械制图 尺寸注法》 · 国家质量监督检验检疫总局
  2. GB/T 4458.5-2003《机械制图 尺寸公差与配合注法》 · 国家质量监督检验检疫总局
  3. GB/T 1800.1-2020《产品几何技术规范（GPS）线性尺寸公差ISO代号体系 第1部分：公差、偏差和配合的基础》 (cited by number; no public page) · 国家市场监督管理总局、国家标准化管理委员会
  4. ISO 286-1:2010 — Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits · ISO
  5. ISO 14405-1:2025 — Geometrical product specifications (GPS) — Dimensional tolerancing — Part 1: Linear sizes (replaces the 2016 edition) · ISO
  6. GB/T 38762.1-2020《产品几何技术规范（GPS） 尺寸公差 第1部分：线性尺寸》 · 国家市场监督管理总局、国家标准化管理委员会
  7. GB/T 1804-2000《一般公差 未注公差的线性和角度尺寸的公差》 · 国家质量技术监督局
  8. ISO 2768-1:1989 — General tolerances — Part 1: Tolerances for linear and angular dimensions without individual tolerance indications · ISO

Watch it done

ReenactmentDigital caliper: outside diameter of Shaft Ø20 — inspect · 0:28Watch in the lab → Follow step by step → Download video (12.0 MB)

Played back in the lab from domain commands; the reading and verdict come from the measurement model.

Practise in the lab

Questions

How do I work out the limits from a drawing size?

Add each deviation to the nominal size: Ø20 0/−0.05 gives 20.00 mm and 19.95 mm.

What does “ISO 2768-m” on a drawing mean?

Sizes without their own tolerance take the general tolerances of class m (medium) in ISO 2768-1; in China the corresponding standard is GB/T 1804-2000.

Try it yourself

Open the same setup in the 3D lab and take the reading yourself.

Open in the lab →

Related guides