Lineweights in AutoCAD explained

The two ways AutoCAD controls printed thickness

This is the concept everything else hangs on. There are two independent systems, and a drawing uses one or the other:

- Object lineweight (the modern, explicit way). Every object and every layer has a Lineweight property measured in real units, typically millimetres — 0.13, 0.18, 0.25, 0.35, 0.50, 0.70 and so on. Set it ByLayer and the object prints at its layer's lineweight. What you assign is what prints. - Colour-dependent plot styles (the older, AutoCAD-heritage way). Here, line colour controls plotted thickness via a .ctb plot style table: colour 1 (red) might plot at 0.25 mm, colour 7 (white) at 0.35 mm, and so on. The object's own lineweight is ignored at plot time; the colour-to-pen mapping wins.

Knowing which system a drawing uses tells you where to go to change a line's thickness. If lineweights look ignored when you plot, you're almost certainly in a colour-dependent (.ctb) drawing and need to edit the plot style table, not the layer lineweight.

Assigning lineweight ByLayer (the clean method)

The tidy, modern approach is to set lineweight ByLayer and let the layer do the work. In the Layer Properties Manager, click each layer's Lineweight column and pick a value: walls might get 0.35 mm or 0.50 mm, doors and windows 0.25 mm, furniture and dimensions 0.13 mm or 0.18 mm. Keep every object's own lineweight at ByLayer (the default) so it simply inherits.

This mirrors the ByLayer discipline you use for colour and linetype, and it has the same benefit: one change to the layer updates every object on it. It also makes the drawing self-documenting — anyone can open the Layer Properties Manager and read the entire lineweight hierarchy in one table, instead of hunting through individual objects.

Why lineweights often don't show on screen

A frequent panic: 'I set 0.50 mm walls but they look the same as everything else.' Almost always the on-screen display of lineweights is simply switched off. The LWT button on the status bar (or the LWDISPLAY system variable) toggles whether lineweights render in model space. Turn it on and the heavy lines thicken visibly.

There are two more subtleties. First, on-screen lineweight display is approximate and scaled by a display setting (in the Lineweight Settings dialog, LWEIGHT command) — it's a preview, not a precise print. Second, in a layout's paper space the lines show at their true plotted thickness relative to the sheet, which is the more reliable preview. So if you want to judge the real hierarchy, check it in a layout viewport or in the plot preview rather than trusting the model-space display.

Plot style tables: CTB vs STB

Plot style tables are how AutoCAD maps your drawing to physical pen widths and effects at print time, and there are two flavours:

- Colour-dependent (.ctb): pen behaviour is keyed to the 255 ACI colours. This is the traditional system, where the colour of a line decides its plotted weight, screening and whether it prints in black or colour. It's subtle but powerful — many long-established offices run entirely on a house .ctb where 'colour 3 = 0.25 mm black'. - Named (.stb): plot styles are named (like 'Heavy', 'Light') and assigned to layers or objects independently of colour, so two red lines can plot differently. More flexible, but less common in legacy files.

A drawing is committed to one system. If you open a colour-dependent drawing, you edit thicknesses in the .ctb via the Plot Style Table Editor; if it's a named-style drawing, you assign .stb styles. Mixing them up is why imported lineweights sometimes behave unexpectedly.

Choosing a sensible lineweight scale

You don't need every available value. A clean, conventional set for architectural work runs roughly: 0.13 mm for hatching, very fine detail and centrelines; 0.18 mm for dimensions, text and furniture; 0.25 mm for objects in elevation and secondary detail; 0.35 mm for general outlines and doors; 0.50 mm for walls and primary cut lines; 0.70 mm and above for section cuts and titles you really want to read.

The golden ratio of about 1.4 between adjacent weights (0.18 → 0.25 → 0.35 → 0.50 → 0.70) gives a visible step at each level, which is why those values recur in standard pen sets. Build them into your template's layers once, and every new drawing starts with a working hierarchy instead of a wall of identical 0.25 mm lines.

Lineweights in downloaded blocks

Blocks you download carry their own lineweight information, and the result depends on how they were drawn. A block whose geometry is on layer 0 with lineweight ByLayer will adopt the lineweight of the layer you insert it onto — so a furniture block lands at your FURNITURE layer's thin weight automatically. A block drawn with fixed object lineweights keeps them regardless of the host layer.

If an inserted block plots too heavy or too light, check whether its lineweights are ByLayer (inheriting) or hard-set. You can select the block's geometry (via the Block Editor) and reset it to ByLayer so it obeys your layer scheme. The blocks on this site are drawn so their line hierarchy reads sensibly, but in a colour-dependent (.ctb) office it's the colour of the block's lines — not their lineweight property — that will drive the printed thickness, so map those colours into your house pen table.