Dimensioning technical drawings to ISO 129-1

A dimension under ISO 129-1 is made of extension lines, a dimension line, terminators (usually arrowheads) and a value written above the line, typically 3.5 mm high. Use Ø for holes and R for arcs, and dimension each feature only once. This guide explains each part with a diagram and shows when to use chain and when to use baseline dimensioning.

The parts of a dimension

Every dimension has four parts that must all be right at the same time. Get one wrong and the drawing is read slowly — or, worse, read wrongly.

1. Extension lines

Thin lines that carry the measured edge or point out of the view, at right angles to the distance being given. By convention they start with a small gap from the outline and run about 2 mm past the dimension line, so it is clear where the dimension line ends.

2. Dimension line

A thin line parallel to the measured distance, drawn between the two extension lines. Place it at least about 8–10 mm (on paper) from the outline, and when several are stacked, keep the spacing between them even. Never use an outline or a centre line as a dimension line.

3. Terminators

Mechanical drawings normally use filled, pointed arrowheads with an included angle of roughly 15–30°. Architectural drawings often use 45° oblique strokes or dots. ISO 129-1 allows several kinds, but a sheet should use just one. When the space is too narrow for arrows, put them outside the extension lines, pointing in.

4. Dimension value

The value sits in the middle of the dimension line, slightly above it without touching, and reads from the bottom or the right-hand side of the sheet. The usual text height for general drawings is 3.5 mm, one of the standard ISO 3098 heights (2.5, 3.5, 5, 7, 10 mm). Values are in millimetres without writing "mm" after each one; state the unit once in the title block.

Symbols written with the value

In practice: a full circle or a hole gets Ø, because the machinist measures it with calipers or picks a drill by diameter. An arc that is not a full circle, such as a rounded corner, gets R. When several holes are the same size, write it once — 4 × Ø6 — instead of dimensioning every hole.

Rules drafters often break

Dimension each feature only once

Give each feature one dimension, in the view where it shows most clearly. If you give the overall length and also every partial length, the drawing is over-dimensioned: once tolerances are applied, the parts no longer add up to the total and the machinist cannot tell which number wins. If you want the total for convenience, put it in brackets as a reference dimension, e.g. (120).

Give enough dimensions to make the part

The opposite mistake is a missing dimension, which forces someone to scale the drawing with a ruler. A simple test: imagine you are the one making the part and check that the position and size of every edge and every hole can be read from the numbers.

Avoid dimensioning to hidden lines

Dashed lines are hidden edges. To dimension an internal feature, draw a section view and dimension the real edge there.

Chain versus baseline dimensioning

Chain dimensioning strings dimensions end to end, from one hole to the next. It is easy to read and suits cases where the distance between neighbouring features matters, but tolerances add up: if each step is ±0.1, the fourth hole may be up to ±0.4 away from the edge.

Baseline dimensioning measures every feature from the same datum edge. Tolerances do not accumulate, and it matches how CNC machines and lasers work from a zero point, so it suits plates whose holes must line up with another part. The drawback is many stacked dimension lines; the fix is running dimensions — a single line with values measured from zero at each point.

Which to use: baseline when position relative to a datum matters (assemblies, CNC work); chain when the spacing between neighbours matters more (for example the pitch of chain-mounting holes). Do not mix both on the same feature and create duplicate dimensions.

Dimensioning holes and angles

Always dimension hole positions to the centre, not the edge: the machinist marks the centre and drills, and a CNC machine moves to the centre coordinate. Holes on a circle in a flange get a pitch circle diameter (PCD), the number of holes and the angle between them, rather than X–Y coordinates for each hole.

Angles are given in degrees with an arc-shaped dimension line centred on the vertex. A 45° edge chamfer is usually written compactly — 2 × 45° means a 2 mm chamfer at 45°. For any other angle, give the length and the angle separately.

Dimensioning in DXCAD2D Online

1. Draw the part at real size, e.g. press R and type 120,80 Enter.
2. Add a DIM layer in the layer panel and make it current, so dimensions stay separate from the part geometry.
3. Press D, click two points with object snap (F3) and move the mouse out to place the dimension line. Tab switches between horizontal, vertical and aligned.
4. For holes, add a leader (LE) with text such as 4 × Ø6.
5. On spec sheets that list dimensions in a table, a dimension can show a letter (A, B, C…) instead of its value — see named dimensions and dimension tables.
6. When exporting a DXF for cutting, hide the DIM layer or tell the shop not to cut it.

Summary

A well-dimensioned drawing can be made without a phone call. Extension and dimension lines are thin, values are 3.5 mm high and sit above the line, holes get Ø and arcs get R, each feature is dimensioned once, and baseline dimensioning is used when position against a datum matters. For the finer points — angles, arcs and tolerances — see ISO 129-1 and ISO 2768.