Line weights for technical illustration

Line weight is how a line drawing shows what matters. A heavier line says "this is the edge of the thing"; a lighter one says "this is detail on it". Get the hierarchy right and a reader finds the part they're looking for without thinking about it.

The standard: ISO 128-2

Lines on technical drawings are covered by ISO 128-2. The current edition is ISO 128-2:2022; it revised the 2020 edition, which replaced the older separate parts ISO 128-20 to 128-25. ISO describes it as follows:

The standard is sold, not free to read; buyers in the UK are directed to the BSI shop. If your drawings must follow it to the letter, buy it. What follows is the widely repeated part of it and how it applies to illustration.

The width series, and why it steps by √2

Line widths come from a fixed series rather than any value you like. Wikipedia's article on technical pens gives the series, which it says matches the widths in ISO 128:

Each width is about √2 (1.41) times the one before. That is the same ratio as the A paper sizes, so when an A4 drawing is enlarged to A3, every line moves up exactly one step: 0.35 mm becomes about 0.5 mm. A drawing reduced or enlarged by one paper size still uses widths from the series.

A practical scheme for illustrations

Engineering drawings typically get by with a couple of weights. Illustrations benefit from three or four, assigned by what kind of edge it is. The table below is a starting point we use, not something the standard prescribes; every value is from the series above.

Rules that matter more than the exact numbers

• Weights are at final size. Decide where the drawing will appear (half an A4 page, a full A3 sheet) and set weights for that. A 0.35 mm line shrunk to a quarter page is under 0.2 mm.
• Keep a clear step between levels. Weights one step apart in the series are distinguishable; weights closer than that just look inconsistent.
• Be consistent across a document. The same kind of edge gets the same weight on every page, or the manual looks assembled from different sources.
• Don't go too thin for the medium. Very thin lines can drop out when printed small or viewed on a low-resolution screen. Check a proof at real size.
• Emphasis is a weight or one colour, not both and not many. A heavier line or a single accent colour on the parts that matter in this step is enough.

Setting weights in practice

In a vector editor, select by layer or class and set the stroke width in millimetres. That's only practical if the export kept edge types apart: FreeCAD's TechDraw has separate widths for visible and hidden lines on each view, and a scripted export (build123d, for example) can put each kind of edge on its own layer.

Then decide which edges to draw at all: hidden lines in technical illustration.