How to Change Units from Inches to Millimeters in AutoCAD

Short Answer

To change units from inches to millimeters in AutoCAD, use the -DWGUNITS command and choose millimeters as the new drawing unit. This is the most common professional method because it can also scale existing geometry correctly during conversion. Limitation: this can affect blocks, dimensions, and xrefs if they were inserted with inconsistent units.

What You Need to Know Before

Warning: Changing units is not just a display change in AutoCAD—your actual geometry may need to be scaled. A common failure is switching units in the drawing settings without converting existing objects, which leaves a 10-inch part still measured as 10 millimeters.

How to Change Units from Inches to Millimeters in AutoCAD

• Command: -DWGUNITS

• Shortcut: No default shortcut

• Quick Steps:

  1. Type -DWGUNITS in the command line and press Enter.
  2. When prompted for drawing units, choose Millimeters.
  3. Accept the prompt to scale objects in current drawing by selecting Yes, then review prompts for matching inserted objects if shown.

Variables & Settings

• System Variable: INSUNITS (Default: 1 in many imperial templates)

• Expert Setting: INSUNITS controls the insertion scale used for blocks and xrefs. After converting from inches to millimeters, set INSUNITS to 4 so future content inserts at the correct scale instead of coming in 25.4 times too large or too small.

Why it Fails

• Cause 1 (Geometry): Users run UNITS and change the insertion/display unit to millimeters, but existing geometry is not scaled, so object sizes become numerically wrong.

• Cause 2 (layers/Locks): Geometry inside locked blocks, xrefs, or externally referenced files may not match the converted host drawing, causing apparent scale errors.

• Cause 3 (Command/Logic): The drawing was created unitless or with the wrong INSUNITS value, so AutoCAD cannot reliably interpret how inserted content should scale.

Quick Fix & Best Practice

• Quick Fix: Run -DWGUNITS, convert to millimeters, answer Yes to scaling, then verify INSUNITS = 4 with the UNITS command or system variable.

• Manager’s Verdict: Use -DWGUNITS when you need to truly convert an inch-based drawing to millimeters. Avoid using UNITS alone for this task unless you only want to change insertion settings for future content.

FAQ

Can I use UNITS to change inches to millimeters in AutoCAD?

No, UNITS mainly changes settings and insertion behavior, not existing geometry scale.

What is the millimeter value for INSUNITS in AutoCAD?

It is 4.

Will changing drawing units also scale dimensions correctly?

Usually yes if the drawing and dimension style were set up properly, but you should always verify dimension results after conversion.

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