CAD Line Weights & Pen Colours: The Real ISO Chart

Last updated July 27, 2026

Complied by YZ.

Color your judgment

In a CAD system, colours are most commonly used to represent line weights. Selecting different colours is just like picking up pens with varying line thickness. We then tie these colours into layers to control the visibility of drawing components.

There are no right and wrong colouring systems — but there are some that have become more prevalent than others. Drafting managed quite well for centuries before CAD came onto the scene, so it is hardly surprising that the most common colour system has its roots in a well-established manual drafting system: the coloured bands on technical drafting pens.

AutoCAD Tip: Use the OPTIONS or CONFIG commands to change the background colour

The line width series

Before the colours, the widths — because the widths are the part that is actually standardised.

The current international standard is ISO 128-2:2022, Technical product documentation (TPD) — General principles of representation — Part 2: Basic conventions for lines. Its clause 5.1 sets out the permitted line widths:

0.13   0.18   0.25   0.35   0.5   0.7   1   1.4   2   mm

Those are not arbitrary. The standard states the series is based on a common ratio of 1:√2, about 1:1.4 — the same √2 that governs the A-series paper sizes. One step up is 1.4 times wider; two steps up is exactly double. That is why 0.25, 0.5 and 1.0 line up so neatly, and why a drawing reduced one paper size keeps its line hierarchy intact.

ISO 128-2 also gives the relationship between line classes: the widths of extra-wide, wide and narrow lines are in the ratio 4:2:1. Pick your narrow line and the other two follow.

The pen colour code, correctly

The coloured bands on technical pens are standardised, and this page has previously given that list slightly wrong — so here it is from the source. The standard is ISO 9175-1:1988, Tubular tips for hand-held technical pens using India ink on tracing paper, and its table of tubular tip dimensions carries a colour code column:

A footnote in that table is worth noting: the line thicknesses are stated to be according to ISO 128, and 0.13 mm is flagged as not considered in ISO 128. So the pen standard sizes the hardware to the drawing standard’s series and adds the barrel colours — the widths belong to ISO 128, the colours to ISO 9175-1.

What the CAD list actually is

The colour list drafting offices actually use in AutoCAD is close to the pen code but not the same:

Only three of the six match. The CAD version uses AutoCAD’s first seven index colours — the ones with single-digit ACI numbers, which are the easiest to see and to type — and assigns them in the same ascending order. It was inspired by the pen code rather than copied from it, and the practical reasons are visible in the list: brown and orange are hard to read on a black model space background, so magenta and green took their places.

Neither list is wrong to use. But calling the CAD one “the ISO/DIN system” is not right, and if you are handed an office standard that cites ISO for its colours, this is where the discrepancy comes from.

What the Australian standard actually says

Here is the part that surprises people: AS 1100.101 does not map colours to line widths at all. Colour-driven lineweight is a CAD and pen-plotter workflow convention, not an Australian Standard requirement.

What AS 1100.101-1992 (reconfirmed 2014), Technical drawing — General principles, does instead is define line groups. Each group is named for its thickest line and contains three widths taken from consecutive steps of the ISO series:

You pick a group to suit the drawing and its reproduction, then the line types — outlines, hidden, centre, dimension and so on — take the thick, medium or thin value from within that group. The mapping is line type to width, not colour to width.

Three further points from the same standard, all of which still earn their keep:

• 0.18 mm is the floor after reproduction. Whatever you plot, no line may end up thinner than that once the drawing has been copied. It is the reason the 0.35 group is the thinnest one offered.
• Two thicknesses is the ideal. The standard says it is desirable to restrict a drawing to two line thicknesses, while noting that some disciplines — structural and electrical among them — use a medium thickness as well for clarity.
• Give parallel lines room. The clear space between parallel lines should be at least twice the thickness of the thickest line, and never less than 1 mm.

Every value AS 1100.101 uses — 0.18, 0.25, 0.35, 0.5, 0.7, 1.0 — is a member of the ISO 128 series. It simply does not use 0.13, 1.4 or 2.0. The two standards agree; the Australian one just works from a narrower shelf.

How AutoCAD does the mapping

There are two ways to get from screen to plotted width, and an office generally commits to one.

• Colour-dependent plot style tables (CTB) — the traditional route, and the one this page is really about. The plot style table maps each colour to a plotted width, so colour carries the lineweight and the drawing looks like a pen plot waiting to happen.
• Named plot style tables (STB) — the alternative, where a plot style is assigned to a layer or object by name and colour is freed up to mean something else entirely.

You can also set lineweight directly on the layer and skip the mapping. That is the tidiest option on a new job, and the reason plenty of offices have moved to it. It is also the reason a drawing from one office can look wrong in another: if their widths live in a CTB you do not have, you are seeing colours with no widths attached.

“My lineweights are not showing”

The command is LWEIGHT — a command, not a system variable, which trips people up when they go looking. Autodesk describes it as setting “the current lineweight, lineweight display options, and lineweight units”, and it opens the Lineweight Settings dialog. Type -LWEIGHT for the command-line version.

The variable that governs whether you can see them is LWDISPLAY. It is a switch, it is saved in the drawing, and its initial value is 0 — lineweights not displayed. That default is the whole answer to the most common complaint about lineweights: they are not broken, the display toggle is simply off in that drawing.

Worth being clear that LWDISPLAY is a display setting. It changes nothing about what plots. Two more worth knowing: LWDEFAULT sets the default lineweight for layers, with an initial value of 0.25 mm, and LWUNITS controls whether widths are listed in millimetres or inches. The display scale slider in the dialog affects the Model tab only.

For sample structural drafting applications review the article What Line Width Where?

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Related on this site

What line width where? applies this to structural drafting. Reduction ratios covers the same √2 that drives the width series, applied to paper sizes. Text heights in drawings is the lettering equivalent.

References

1. ISO 128-2:2022 — Technical product documentation (TPD): General principles of representation, Part 2: Basic conventions for lines — ISO. Second edition, October 2022; clause 5.1 gives the line width series, the 1:√2 ratio and the 4:2:1 relationship. Supersedes ISO 128-2:2020.
2. ISO 9175-1:1988 — Tubular tips for hand-held technical pens using India ink on tracing paper, Part 1: Definitions, dimensions, designation and marking — ISO, prepared by ISO/TC 10. The tubular tip table and its colour code column.
3. AS 1100.101-1992 — Technical drawing, Part 101: General principles — Standards Australia, Committee ME-072; published 16 November 1992, reconfirmed 2014. Line groups, the 0.18 mm reproduction minimum, the two-thickness guidance and line spacing.
4. LWEIGHT (Command) — Autodesk AutoCAD Help.
5. Lineweight Settings Dialog Box — Autodesk AutoCAD Help.
6. LWDISPLAY (System Variable) — Autodesk AutoCAD Help. Switch, saved in drawing, initial value 0.