Change your drawing units in AutoCAD through the Drawing Units dialog

To change units in AutoCAD, open the Drawing Units dialog by typing UNITS at the command line and pressing Enter, or by going to Format > Units in the menu bar. The dialog shows your current unit system — inches, millimeters, feet, or another standard — and lets you switch to a different one. Once you select a new unit and click OK, AutoCAD converts the display but does not change the actual size of objects already drawn. This matters: if you switch from inches to millimeters without planning, a line that was 10 inches long will now display as 10 millimeters, which is much smaller.

The unit change affects only how AutoCAD displays and interprets new measurements you enter. Objects you have already drawn keep their real-world size — AutoCAD just shows them in the new unit system. If you need to change units for a drawing that already has content, you should do this before adding more geometry, or be prepared to scale everything afterward.

Key Takeaways

  • Open the Drawing Units dialog by typing UNITS at the command line or using Format > Units from the menu.
  • Changing units changes how AutoCAD displays measurements, not the actual size of objects already in your drawing.
  • Set the unit type (inches, millimeters, feet, centimeters) and the precision level you need before you start drawing.
  • If you switch units on a drawing with existing objects, use the SCALE command to resize everything proportionally.

Access the Drawing Units dialog

The fastest way to reach the units settings is through the command line. Type UNITS and press Enter. The Drawing Units dialog opens immediately. If you prefer the menu, click Format in the menu bar, then select Units from the dropdown.

The dialog displays your current unit system in the top section labeled "Length". Below that, you see "Angle" settings, which control how AutoCAD measures rotation and direction. Most users only need to change the Length units, but you can adjust angle units the same way if your project requires it.

Select a new unit type

In the Drawing Units dialog, the "Length" section shows a dropdown list of unit systems. Click the dropdown to see all available options: Decimal (generic units with no real-world size), Architectural (feet and inches), Engineering (feet and decimal inches), Fractional (inches with fractions), and Scientific (exponential notation). Select the one that matches your project.

Most users working in the United States choose Architectural for building drawings or Decimal for general work. Users outside the US typically select Decimal and set the precision to millimeters. The unit type you choose does not lock you in — you can change it again at any time, but switching between incompatible systems (like Architectural to Decimal) on a drawing with existing objects requires scaling afterward.

Set the precision level

Below the unit type dropdown, you see a "Precision" field showing how many decimal places or fractional divisions AutoCAD displays. For Decimal units, precision might be set to 0.00 (two decimal places) or 0.0000 (four decimal places). For Architectural units, it might show 0'-0 1/16" (sixteenths of an inch) or 0'-0 1/8" (eighths).

Higher precision means more decimal places or smaller fractions, which makes measurements more exact but also harder to read on screen. For most architectural and engineering work, two to four decimal places is standard. If you are working with very small parts or high-tolerance manufacturing, you may need more precision. Click the Precision dropdown and select the level you need.

Understand how unit changes affect existing drawings

When you change units in a drawing that already contains objects, AutoCAD reinterprets the numbers but does not resize the geometry. For example, a line 10 units long in inches will still be 10 units long after you switch to millimeters — but now those 10 units represent millimeters instead of inches, so the line appears much smaller on screen.

If you need the line to actually be 10 millimeters (which is much shorter than 10 inches), you must scale the drawing. Use the SCALE command: type SCALE at the command line, select all objects with Ctrl+A, then enter a scale factor. To convert from inches to millimeters, the scale factor is 25.4 (since one inch equals 25.4 millimeters). After scaling, the drawing displays at the correct size in the new unit system.

Set up units before you start drawing

The best practice is to set your units and precision before you create any geometry. Open a new drawing, go to Format > Units, choose your unit system and precision, click OK, and then start drawing. This way, every measurement you enter is interpreted in the correct unit from the start, and you avoid the confusion and extra work of converting later.

If you work on multiple projects with different unit systems, consider creating template files with the correct units already set. Save a blank drawing with inches and architectural precision as one template, and another with millimeters and decimal precision as a second template. When you start a new project, open the template that matches your needs, and the units are already correct.

Frequently Asked Questions

What happens to my drawing if I change units without scaling?

The objects stay the same size in the file, but AutoCAD reinterprets the numbers in the new unit system. A 10-inch line becomes a 10-millimeter line on screen, appearing much smaller. Use the SCALE command with the correct conversion factor to resize everything proportionally.

How do I convert a drawing from inches to millimeters?

Change the units to millimeters in the Drawing Units dialog, then use the SCALE command. Select all objects, and enter 25.4 as the scale factor (one inch equals 25.4 millimeters). This resizes the entire drawing so that measurements in millimeters match the original size in inches.

Can I use decimal units without specifying inches or millimeters?

Yes. Decimal units are generic and do not represent any real-world measurement. You can use them for abstract drawings or when the actual unit does not matter. Just be consistent — if one unit equals one foot in your mind, treat it that way throughout the drawing.

Does changing units affect dimensions and text in my drawing?

Dimensions update to show the new unit system, but the text content does not change automatically. If you have text that says "10 inches", it will still say that even after you switch to millimeters. You must edit text manually if the unit label needs to change.