How To Change Units In SolidWorks: A Complete Guide For Precision Modeling
Changing units in SolidWorks requires updating the active document settings or modifying default templates to ensure every linear dimension, mass property, and tolerance complies with ASME or ISO manufacturing standards. Mastering this workflow prevents costly machining errors, incorrect bill of materials (BOM) weights, and revision bottlenecks during cross-functional collaboration.
Pre-Operation & Planning Checklist
Before modifying measurement parameters in any CAD file, you must understand how SolidWorks handles unit systems across parts, assemblies, and drawings. Unit configurations dictate whether your part is evaluated in millimeters, grams, and seconds (MIPS), or inches, pounds, and seconds (IPS), directly affecting finite element analysis (FEA) results and CNC machining G-code generation.
- Essential Software Setup:
- SolidWorks 2020 or later installed with active drafting licenses.
- Administrative access to save custom part and assembly templates to your designated file locations.
- Access to an existing part, assembly, or drawing file requiring unit conversion.
- Mandatory Standards & Knowledge:
- Familiarity with ASME Y14.5 (inch-based GD&T) or ISO 128 (metric technical drawings) specifications.
- Understanding of the difference between changing units for a single active document versus altering system-wide default templates.
- Awareness of mass property recalculations and density impacts when shifting from metric to imperial mass units.
- Operational Benchmarks:
- Estimated duration: 2 to 5 minutes for active documents; 10 minutes for custom template creation.
- Expected output: Fully converted parametric models, precise drafting dimensions, and updated mass property readouts.
Step-by-Step Guide to Changing Units in SolidWorks
Step 1: Access the Document Properties Menu
Open your target part, assembly, or drawing file in SolidWorks. Navigate to the top menu bar, click on Tools, select Options from the dropdown menu, and click the Document Properties tab. Alternatively, you can look at the bottom right corner of the SolidWorks status bar, where the current unit system is displayed, and click it directly to open the quick-selection flyout menu.
Pro-Tip: Clicking the status bar unit display at the bottom right of the graphics window is the fastest way to switch units on the fly without navigating through multi-layered menu systems.
Step 2: Select Your Preferred Unit System
Inside the Document Properties dialog box, click on the Units category on the left-hand panel. You will see a list of standard unit systems, including MKS (Meter, Kilogram, Second), CKS (Centimeter, Gram, Second), MMGS (Millimeter, Gram, Second), and IPS (Inch, Pound, Second). Select the system that matches your project requirements.
Warning: Changing the unit system of an active file directly converts existing numerical values rather than scaling the physical geometry. A part drawn as a 100 mm cube will become a 100-inch cube if you switch from MMGS to IPS without scaling the geometry first.
Step 3: Customize Custom Units and Decimal Precision
If standard unit presets do not meet your manufacturing tolerances, configure individual parameters at the bottom of the Units dialog box. You can modify units independently for lengths, angles, mass, time, and force. Set the decimal place precision for each parameter—ranging from whole numbers to five decimal places (e.g., 0.12345)—to align with your shop floor machining capabilities and quality control requirements.
Step 4: Update Mass Properties and Drafting Standards
Verify that your mass properties reflect the new unit system by going to Evaluate > Mass Properties. Ensure the material density assigned to the part is compatible with your newly selected unit system (e.g., grams per cubic centimeter versus pounds per cubic inch). If your file is a drawing sheet, navigate to Document Properties > Drafting Standard and Dimensions to ensure dimension text heights, arrow styles, and tolerances automatically update to match your newly selected standard.
Step 5: Save as a Template for Future Projects
To prevent having to change units for every new design, save your modified file as a template. Go to File > Save As, select *Part Templates (.prt; *.sldprt)**, *Assembly Templates (.asm; *.sldasm)**, or *Drawing Templates (.drw; .slddrw)* from the file type dropdown. Save this file into your designated company template folder. Go to Tools > Options > System Options > File Locations and ensure your template directory is linked so SolidWorks loads your custom units automatically for every new session.
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Comparison of Standard SolidWorks Unit Systems
| Unit System | Length Unit | Mass Unit | Time Unit | Common Industry Application |
|---|---|---|---|---|
| MMGS | Millimeter (mm) | Gram (g) | Second (s) | Precision mechanical design, injection molding, ISO compliance |
| IPS | Inch (in) | Pound (lb) | Second (s) | Aerospace, heavy machinery, ASME Y14.5 drafting |
| MKS | Meter (m) | Kilogram (kg) | Second (s) | Large-scale civil engineering, structural analysis |
| CKS | Centimeter (cm) | Gram (g) | Second (s) | Biomedical devices, academic research modeling |
Troubleshooting Unit Conversion Issues in SolidWorks
- Symptom: The physical geometry of the model appears massively oversized or microscopic after switching units.
- Root Cause: You converted the unit label (e.g., changing 10 mm to 10 inches) instead of scaling the actual parametric geometry to match the new measurement system.
- Actionable Fix: Undo the unit change, use the Scale feature located under Insert > Features > Scale to resize the geometry uniformly by the conversion factor (e.g., scale factor of 0.0393701 for mm to inches), and then update your document properties.
- Symptom: Dimension text on drawing sheets fails to update when the part file units are modified.
- Root Cause: The drawing file retains its own independent document properties that override or do not automatically sync with imported part file units.
- Actionable Fix: Open the drawing file, navigate to Tools > Options > Document Properties > Units, and manually match the drawing unit settings to the source part or assembly file.
- Symptom: Mass properties output impossible values like 5,000 kilograms for a small plastic bracket.
- Root Cause: A mismatch between the selected unit system (e.g., MMGS) and the material density units assigned in the Material editor.
- Actionable Fix: Right-click Material in the FeatureManager design tree, select Edit Material, and verify that the density value uses the correct volumetric mass units matching your active document settings.
Frequently Asked Questions
Can I convert a part from inches to millimeters without changing its physical size?
Yes. To convert a part's units without altering its physical dimensions, simply open the active document, go to Tools > Options > Document Properties > Units, and select MMGS. SolidWorks will automatically convert all existing numerical dimensions to their metric equivalents while maintaining the exact physical size of the model.
Why do my drawing dimensions still show inches after I changed the part to millimeters?
SolidWorks drawing documents maintain independent unit settings separate from the 3D part or assembly files referenced within them. To fix this, open your drawing document, navigate to Tools > Options > Document Properties > Units, and change the unit system to match your part file.
How do I set my custom units as the default for every new part I create?
Once you configure your preferred units in a blank part file, go to File > Save As and choose Part Template (*.prt) as the file type. Save this file in your company template folder, then update your file locations in Tools > Options > System Options > File Locations so SolidWorks references this template by default.
Does changing units affect existing simulation and FEA results?
Yes. Changing the unit system alters the underlying mathematical evaluation of loads, pressures, and material elasticity constants. If you alter units after running a simulation study, you must clear old results and re-run the static or dynamic study to ensure structural integrity data remains accurate.
Master Your CAD Workflow and Manufacturing Precision
Optimizing your SolidWorks unit settings and custom templates eliminates repetitive setup tasks and guarantees seamless communication between your engineering team and manufacturing facilities. Implement these standardized workflows today to streamline your design pipeline and ensure zero-defect production cycles.
