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BeginnerFree AutoCAD course about 12 min

Drawing units and working full size

AutoCAD doesn’t know what “1” means until you decide. Choose your units once, draw everything full size, and scale problems never happen.

In this lesson you will

  • Understand that one AutoCAD unit can mean any real unit
  • Set units with the UNITS dialog and templates
  • Choose between millimetres, metres and feet-inches
  • Fix blocks and references that arrive at the wrong size

One unit is whatever you decide

AutoCAD stores distances as plain numbers. If you decide 1 unit = 1 millimetre, a 3.6 m wall is drawn 3600 long. If you decide 1 unit = 1 metre, the same wall is 3.6. Neither is wrong — but everyone working on the drawing must use the same convention.

The industry habit in most countries is 1 unit = 1 mm for buildings, 1 unit = 1 m for civil and site work, and 1 unit = 1 inch for US architectural drawings (shown in feet and inches).

Always draw full size

Draw objects at their real size in model space, whatever the final printed scale. The scale (1:100, 1:50, 1/4" = 1'-0") is applied later, when you set up a layout viewport. Drawing "to scale" in model space causes wrong dimensions, wrong text sizes and wrong quantities.

A 10-metre wall drawn full size in model space appears 100 millimetres long on paper at 1:100 and 200 millimetres long at 1:50.Model: wall 10 000 mm longPaper at 1:100 → 100 mm10 000 ÷ 100 = 100 mmPaper at 1:50 → 200 mm10 000 ÷ 50 = 200 mm(paper lines shown at a reduced size)
Figure: How drawing scale works. Original KitWren diagram.

Setting units

  1. 1Type UNITS (UN) and press Enter.
  2. 2Set Length type to Decimal (for mm or m) or Architectural (feet and inches).
  3. 3Set Precision to what you need to see, e.g. 0 for millimetres.
  4. 4Set Insertion scale to your unit (Millimeters, Meters or Inches). This tells AutoCAD how to scale blocks and references inserted from drawings in other units.
  5. 5Leave angles as Decimal Degrees, measured counter-clockwise, with 0° pointing East, unless your office standard says otherwise.

Start metric drawings from acadiso.dwt and imperial drawings from acad.dwt. Better still, build your own template (see the Templates lesson) so units are always right.

When things arrive at the wrong size

A furniture block that appears 1000 times too big or too small means the source drawing and yours disagree about units. Check Insertion scale in both drawings (the INSUNITS system variable). To fix an object already in the drawing, use SCALE with the right factor: mm → m is 0.001, m → mm is 1000, inches → mm is 25.4.

Quick reference

Command / toolHow to start itWhat it does
UNITSUNSets length and angle format, precision and insertion scale
INSUNITSType the nameSystem variable for insertion scale (4 = mm, 6 = m, 1 = inches)
DISTDIMeasures the distance between two points

Common mistakes

  • Drawing a 1:100 plan by making everything 100 times smaller.

    Draw at 1:1 and set the scale in the layout viewport.

  • Precision set to 0 hides small errors.

    Temporarily raise precision to 3 decimals to check for lines that are 3599.97 instead of 3600.

Exercise

Set up a metric drawing

  1. Start a new drawing from acadiso.dwt.
  2. Open UNITS: Decimal, precision 0, insertion scale Millimeters.
  3. Draw a line 3600 long and check its length in Properties.
  4. Measure it with DIST.
  5. Save as units-practice.dwg.

You should end up with: A correctly set-up millimetre drawing.

Checklist

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Lesson quiz

1 / 4

You use 1 unit = 1 mm. How long do you draw a 4.2 m wall?

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Free KitWren tools that help

Free, independent learning material written for KitWren. AutoCAD and AutoCAD LT are registered trademarks of Autodesk, Inc. KitWren is not affiliated with, sponsored or endorsed by Autodesk. We never provide software downloads, licence keys or cracks — use the official sources.