How To Change Units In SolidWorks: The Professional Guide To Unit Systems And Document Properties

To change units in SolidWorks, navigate to the Unit System flyout menu located in the bottom-right corner of the Status Bar and select your preferred system, such as MMGS or IPS. For permanent changes to default templates or custom decimal precision, access the Document Properties under the Options gear icon and modify the Units category settings. These adjustments ensure your CAD geometry aligns with ISO or ASME standards for manufacturing and simulation accuracy.

Fundamental Unit Planning and Template Standardization

Before modifying unit systems in a live project, CAD managers and design engineers must distinguish between document-level changes and system-level defaults. Changing units in an active part file only affects that specific document. To ensure every new project begins in the correct measurement system, you must update your Part (.prtdot), Assembly (.asmdot), and Drawing (.drwdot) templates. Working in the wrong unit system can lead to catastrophic errors in downstream processes such as Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and CNC toolpath generation.

Effective unit management requires adherence to standardized drafting practices. The American Society of Mechanical Engineers (ASME) Y14.5 and the International Organization for Standardization (ISO) 1000 provide the framework for how these units are displayed and rounded.

Pre-Procedure Checklist and Technical Requirements:

• Software Version: SolidWorks 2018 or newer (for the modern Status Bar interface).
• Administrative Access: Necessary for modifying and saving system-level document templates in protected folders.
• Standard Knowledge: Familiarity with IPS (Inch, Pound, Second), MMGS (Millimeter, Gram, Second), CGS (Centimeter, Gram, Second), and MKS (Meter, Kilogram, Second).
• Precision Requirements: Determination of decimal place thresholds (e.g., four decimal places for aerospace tolerances vs. two for general fabrication).
• Estimated Duration: 2 minutes for active document changes; 15 minutes for global template overhaul.

Executing Unit Transitions in the SolidWorks Environment

Changing the unit system is a multi-tiered process. You can perform a "Quick Switch" for immediate feedback, a "Deep Configuration" for specific precision, or a "Template Lock" for long-term consistency.

Step 1: Rapid Unit Switching via the Status Bar

The fastest method to toggle between metric and imperial systems is using the Status Bar at the bottom of the SolidWorks window.

1. Locate the Unit System indicator in the bottom-right corner of the interface. It typically displays labels like MMGS or IPS.
2. Click the text to open a flyout menu.
3. Select the desired system from the list. The model will instantly update its internal calculations to reflect the new system.
4. Verify the change by clicking on a sketch dimension or an edge to view the updated values in the PropertyManager.

Pro-Tip: Switching units via the Status Bar does not scale your model; it only changes the measurement value. If a line was 1 inch long, it will become 25.4 millimeters long after switching to MMGS.

Step 2: Advanced Configuration via Document Properties

When you need custom units (such as mixing millimeters with pounds) or specific decimal precision, the Document Properties menu is required.

1. Click the Options (gear icon) in the top toolbar.
2. Navigate to the Document Properties tab. Ensure you are not on the System Options tab, as unit settings are document-specific.
3. Select Units from the left-hand menu tree.
4. Under the Unit System header, select the radio button for your primary system or choose Custom.
5. In the main table, adjust the "Decimals" column for Length, Dual Dimension Length, Angle, and Mass/Section Properties.
6. Modify the "Rounding" column to specify whether SolidWorks should truncate or round up at specific thresholds.
7. Click OK to apply the changes to the active document.

Step 3: Implementing Dual Dimensions for International Collaboration

In global manufacturing, it is common to display both Metric and Imperial units simultaneously on a drawing.

1. Go to Tools, then Options, and select Document Properties.
2. Click on Dimensions.
3. Check the box labeled Dual dimensions display.
4. Choose the position of the dual dimension (e.g., Top, Bottom, Right, or Left of the primary dimension).
5. Navigate to the Units sub-menu within Document Properties to define the secondary unit system used for these dual dimensions.

Warning: Excessive dual dimensioning can clutter complex engineering drawings. Use this feature selectively or only on critical-to-quality (CTQ) dimensions.

Step 4: Permanent Unit Changes via Template Modification

To prevent having to change units every time a new file is created, you must update the template files.

1. Create a new, blank Part file.
2. Follow the instructions in Step 2 to set your preferred units and precision.
3. Go to File > Save As.
4. Change the "Save as type" dropdown to Part Templates (*.prtdot).
5. SolidWorks will automatically navigate to the default template folder. Name the file "Standard_Metric" or "Standard_Imperial" to distinguish it.
6. Repeat this process for Assembly Templates (.asmdot) and Drawing Templates (.drwdot).
7. To make these the default, go to System Options > Default Templates and browse to your new files.

Step 5: Handling Units in Equations and Global Variables

If your model uses complex equations, changing units can sometimes cause syntax errors if units were hard-coded as strings.

1. Right-click the Equations folder in the FeatureManager Design Tree and select Manage Equations.
2. Examine the "Value/Equation" column.
3. Ensure that any constant values are followed by their unit designators (e.g., "12in" or "300mm").
4. If a variable is unitless, SolidWorks will apply the current document unit to it by default, which may alter the design intent if you switch systems. Explicitly labeling units in equations prevents unexpected geometry shifts.

Unit System Parameters and Precision Matrix

The following table outlines the standard unit configurations available in SolidWorks and their typical applications in industrial design.

Troubleshooting Unit Failures and Model Scaling Issues

Even with a robust understanding of the interface, technical conflicts can arise when merging files from different sources or converting legacy data.

• Scenario: The Model Appears 25.4 Times Too Large or Small

  
  
  
  • Root Cause: This occurs when a file is imported from a neutral format (like STL or IGES) that does not contain unit metadata, or when a user interprets "1 unit" differently during the import process.
  • Actionable Fix: Use the "Scale" feature (found under Insert > Features > Scale). Select the centroid as the scale point and enter a factor of 0.03937 (to shrink mm to inches) or 25.4 (to enlarge inches to mm).

• Scenario: Mass Properties Showing Zero or Incorrect Density

  
  
  
  • Root Cause: Switching units can sometimes lead to rounding errors in custom material properties, especially if the density was defined in a non-standard unit like slugs/cubic foot.
  • Actionable Fix: Open the Material Editor and verify that the units in the "Properties" tab match your document units. Re-apply the material to the part to force a recalculation of mass, volume, and surface area.

• Scenario: Drawing Dimensions Displaying "Inches" Instead of "Millimeters" Despite Changes

  
  
  
  • Root Cause: The Drawing document has its own independent Unit settings that do not automatically synchronize with the Part document's units.
  • Actionable Fix: Open the Drawing file, access Document Properties > Units, and change the setting there. To automate this, the Drawing Template must be updated separately from the Part Template.

• Scenario: Simulation Results Showing Impossible Stress Values

  
  
  
  • Root Cause: SolidWorks Simulation uses its own unit manager which can differ from the CAD environment.
  • Actionable Fix: Inside the Simulation study tree, right-click the "Properties" of the study. Ensure the unit system for results (e.g., SI vs. English/IPS) is set correctly before running the solver.

Frequently Asked Questions

Does changing units in SolidWorks change the physical size of the part?

No, changing the unit system only changes the units of measurement applied to the existing geometry. A 100mm line will remain the same physical length but will be labeled as 3.937 inches if you switch to the IPS system. To actually change the size, you must use the Scale tool.

How do I set SolidWorks to always open in Millimeters?

You must modify your default templates. Create a new part, set the units to MMGS in Document Properties, and save the file as a Part Template (.prtdot). Finally, set this new template as the default under System Options > Default Templates.

Can I show both Metric and Imperial units on the same dimension?

Yes, this is achieved through Dual Dimensions. In Document Properties > Dimensions, enable "Dual dimensions display." You can then configure which unit appears in brackets and the precision for both primary and secondary units.

Why are my units grayed out and unchangeable?

This typically happens when the document is being managed by a PDM (Product Data Management) system or if the file is "Read-Only." Check out the file from the vault or right-click the file in Windows Explorer to ensure the "Read-Only" attribute is unchecked in the properties menu.

How do I change the units for an entire assembly at once?

While you can change the assembly's display units via the Status Bar, this does not automatically change the internal units of every individual part within that assembly. Each part file must be opened and updated individually, or a macro must be used to batch-update the unit properties of all components.

Optimize Your Engineering Workflow

Mastering unit management is essential for eliminating manufacturing errors and ensuring cross-departmental compatibility. Implement these template standards today to guarantee your designs meet the rigorous precision required by modern industrial applications.