Line Weight

By on November 2, 2009

Line weights, or the varying line thicknesses used in engineering drawing, are essential in creating a drawing that communicates efficiently.

While line weight is an important factor on drawings, it is only so on hardcopy documents.  Different line thicknesses on screen are of no value and in fact, if you were to use them, would be a hindrance.  Instead, different line types are shown in colors which can symbolize a number of attributes of the line work.  The CAD user soon subconsciously equates line color with plotted line weight and simultaneously any other attributes the color may represent in his or her office standard.  Our focus in this article set is on the relationship of color in a CAD drawing to line weight when that drawing is output on paper.

A engineering drawing is a highly stylized graphic representation of an idea.  The idea might be of something that we can see such a real or virtual object, space or environment.  In some cases, such as an electronic schematic diagram for example, the drawing will bear no visual resemblance to the physical object that will be built from the information it provides.

In every case with the possible exception of “3D” and rendered drawings, which communicate with a different graphics language, we can understand engineering drawings only because we can understand the basic language of technical graphics.

Line weights are a vital part of conventional technical graphics language.  They are embodied to the extent of being defined in national and international standards.

Line types and line weights allow drawings to communicate information that would otherwise be very difficult to convey.  For example:

  • Hidden outlines
  • Paths of motion
  • Planes of symmetry
  • Fictitious outlines such as major and minor diameters of screw threads
  • Dimensions and projections
  • Materials (hatching)
  • Centers and imaginary intersections

Conventional practice is that only two different line weights be used on any one drawing.  This is subject to discretion and some disciplines regularly use three, and occasionally four, different line weights.  Consistency and clarity of communication are the deciding factors.  You could use 10 line weights in a drawing provided everyone understood what they all meant and the document was consistent.

The thinnest line should be no less that 0.18mm. Finer lines are difficult to read and impossible to reproduce easily.

Line weight groups chosen for most engineering drawings are selected from adjacent pen thicknesses (in mm).  The table below indicates line weight groups for various sheet sizes.  Pen thickness are shown in mm.


0.18 A4, A2, A3
0.25 A1
0.35 A0

Typical Applications

Engineering drawings made on A4, A3 and A2-sized pages are at the smallest end of the range of document sizes that would reasonably be used. As such they would use a pen rage at the documents might use a pen group from the fine end of the scale. 0.18, 0.25 and 0.35mm pen widths.

Line type






Continuous thick 0.35 0.50 35 continuos Visible outlines, existing features, cut edges, general line work
Continuous medium 0.25 0.35 Used where another level of line weight would assist the delineation e.g. internal line work, notes
Continuous thin 0.18 0.25 Fictitious outlines, imaginary intersections and projections, hatching, dimensions, break lines
Dashed thick 0.35 0.50 Hidden outlines and edges
Dashed thin 0.18 0.25
Chain thick 0.35 0.50 Indication of special surface requirements or (sometimes with a text component) to indicate pipelines and services
Chain thin 0.18 0.25 Center lines, motion paths, indication of repeated detail
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  1. David Gabriel

    August 12, 2015 at 11:34 am

    Hello, I’m studying for an exam that has a few drafting questions. Really need help. If you would, please tell me what the drafting standard for a “thin” line is. The choices in the pretest book are 0.01, 0.015, 0.02, and 0.025. I have found that .01 =1/64 =.254 mm and .03 = 2/64 =.793 mm. so I can see which is smaller I just need to know what is the standard “thin” line. Thanks. Dave G.

    • Tony Zilles

      August 12, 2015 at 12:36 pm

      Hi Dave, Thanks for your question. The concept of standard in relation to “thin” is no so clear.

      Wikipeadia indicates a CAD standard thin line as 0.05mm. That seems ridiculously thin to me.

      A text book in Google Books suggests a lineweight of 0.3mm as a thin line. That’s a bit more like it.

      At they suggest that a thin line is relative to the sheet size (which is absolutely correct and the whole point behind teh ISO/DIN sizing system) and ranges from 3.35mm for A0 sheets, 0.25mm for A1 sheets size and 0.18mm for A2, A3, A4 sheet sizes.

      The standard metric pen set comprises 0.13mm, 0.18mm 0.25mm, 0.35mm, 0.5mm, 0.7mm, 1.0mm, 1.4mm & 2.0mm. As anyone who has used them will attest, 0.13 and 0.18 pen sizes are very fine and not robust in use either by hand or in a plotter. 0.25 would be my choice for a thing line. 0.18 if the choice is offered.

      Metric Pen sizes
      image courtesy of

      Hope this helps. Good luck with your exam. – Tony

  2. Brad

    October 14, 2014 at 8:48 pm


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