You've been modeling for three hours. And the assembly looks clean. That said, the mates are solid. You hit "Save" and send the STEP file to the vendor.
Then the email comes back: *"Your part is 25.But 4 times too small. Did you model in inches?
You modeled in millimeters. Here's the thing — the vendor's system defaulted to inches. Nobody checked.
This happens more than anyone admits. Not because SolidWorks makes units difficult — it doesn't. But because the unit system lives in three different places, and changing one doesn't always change the others. If you've ever scratched your head wondering why your drawing shows dual dimensions but your model properties say something else, you're not alone.
Let's fix that once and for all.
What Is the Unit System in SolidWorks
SolidWorks doesn't have a single "units setting." It has layers.
At the top level, you've got document units — what the current part, assembly, or drawing uses. This controls what you see in the status bar, what dimensions display, and what gets exported in neutral formats.
Below that, there's template units. That said, every new file starts from a template. If your template is set to IPS (inch-pound-second), every new part defaults to inches. Change the template, and you change the default for every future file Simple, but easy to overlook..
Then there's application-level units — the fallback SolidWorks uses when a document doesn't specify. Most people never touch this. They should Small thing, real impact..
And in drawings? There's a whole separate drawing units setting that can override the model. Dual dimensions? That's yet another toggle.
The short version: units aren't a setting. They're a chain. Break one link and the whole thing drifts.
Why It Matters / Why People Care
A unit mismatch doesn't just look wrong. It ships wrong.
I've seen CNC programs cut parts 25.4x undersized because the CAM software read the STEP file in inches while the model was metric. Still, i've seen BOMs show "0. Now, 039 in" for a 1 mm hole because the drawing template forced dual dimensions with the wrong primary. I've watched simulation results report stress in psi on a model built in MPa — and the engineer didn't catch it until the safety factor looked "too good to be true.
It sounds simple, but the gap is usually here.
Units are the silent killer of downstream workflows. Also, cAM, FEA, PDM, ERP, 3D printing slicers — they all trust what the file says. If the file lies, everything downstream inherits the lie It's one of those things that adds up..
And here's the thing: SolidWorks tries to help. Some default to their own settings. It embeds unit metadata in Parasolid, STEP, and SLDPRT files. But not every reader respects it. Some strip it entirely.
So the only reliable strategy? Make sure your files are unambiguous. Every time.
How to Set Units in SolidWorks
Changing Units in an Open Document
This is the one everyone knows — and the one that causes the most confusion Practical, not theoretical..
With a part or assembly open, look at the bottom-right status bar. In practice, see "MMGS" or "IPS"? Because of that, click it. A menu pops up with the common systems: MMGS, CGS, MKS, IPS, and "Custom.
Pick one. Still, mass properties recalculate. And the model updates instantly. Dimensions on screen flip. The status bar reflects the new system.
But — and this trips people up — this only changes this document. It doesn't update your templates. It doesn't change future files. It doesn't touch drawings that reference this model unless you rebuild them.
Also: if you have equations, global variables, or design tables driving dimensions, those values don't auto-convert. Also, a "10" in an equation stays "10" — but now it means 10 mm instead of 10 inches. SolidWorks doesn't rewrite your math. You have to audit it.
Setting Default Units via Templates
This is where the make use of lives Worth keeping that in mind..
Open a blank part. Set units how you want them (status bar method above). Practically speaking, then go File > Save As, change "Save as type" to Part Template (. Here's the thing — prtdot). On the flip side, name it something clear: "MMGS_Standard. Day to day, prtdot" or "IPS_SheetMetal. Plus, prtdot. " Save it to your template folder — ideally a network location your whole team uses.
Repeat for assemblies (.asmdot) and drawings (.drwdot).
Now go Tools > Options > System Options > File Locations > Document Templates. Add that folder. Move it to the top.
Next time someone hits File > New, your template appears first. They get the right units. That's why they pick it. No status bar clicking required That's the part that actually makes a difference..
Pro tip: bake more than units into templates. Custom properties, material, annotation styles, sheet formats — all of it. Because of that, a template isn't a unit setting. It's a standard That's the part that actually makes a difference..
Application-Level Default Units
Buried in Tools > Options > System Options > General, there's a "Default units" dropdown. Most people leave it at "IPS" (the install default) and forget it exists Nothing fancy..
Here's why it matters: if you open a file that has no unit metadata — some imported STEP, a corrupted SLDPRT, a file from a system that stripped it — SolidWorks falls back to this setting. Think about it: if it's wrong, your imported geometry gets interpreted wrong. No warning. No prompt Simple, but easy to overlook. That alone is useful..
Set it to your shop standard. IPS for imperial. MMGS for metric shops. Done.
Drawing Units — The Separate Beast
Drawings have their own unit logic. Open a drawing. Right-click the sheet > Properties. There's a "Units" section with "Use document units" (the model's units) and "Override units Most people skip this — try not to..
If you override, the drawing displays in that system — but the model stays untouched. Dimensions show the converted values. Dual dimensions can show both.
It's powerful. It's also dangerous.
Say your model is metric. The vendor sees inches. That's why you override the drawing to IPS for a US vendor. Your shop sees mm. In real terms, you add dual dimensions: primary inches, secondary mm. Everyone's happy — until someone edits a dimension in the drawing Worth keeping that in mind..
Editing a dimension in a drawing with overridden units writes the value back to the model in the drawing's units. That 25.4 mm hole? And you type "1. 0" in the drawing. The model now stores 1.Here's the thing — 0 mm. Not 1.0 inch. The model just shrank 25.4x.
Never edit model dimensions from a drawing with overridden units. Add a note. Call out the conversion. But don't type in the dimension box.
Dual Dimensions and Alternate Unit Display
Drawings can show two unit systems simultaneously. Tools > Options > Document Properties > Dimensions > Dual Dimensions. Turn it on. Pick primary and secondary. Set formatting — precision, trailing zeros, brackets, whatever your standard demands Simple as that..
This is great for drawings that go to mixed audiences. But it's a drawing-only setting. Also, it doesn't affect the model. Now, it doesn't export to STEP. It's purely visual Simple, but easy to overlook..
Use it intentionally. Document it in your drawing standard. Don't let it become a surprise Easy to understand, harder to ignore..
Units in Equations, Design Tables, and Global Variables
This is the silent trap Most people skip this — try not to..
Equations store numbers, not unit-qualified values. If you switch the document from MMGS to IPS, that equation now drives 50 inches. So "D1@Sketch1 = 50" means 50 current document units. The sketch explodes Simple, but easy to overlook. Worth knowing..
Design tables (Excel-driven) are worse. The spreadsheet has no unit awareness. You type "10" in a cell
Equations and global variables are another layer where the document’s unit context can silently corrupt a design. An equation such as
WallThickness = 0.5 * Length
stores only the numeric factor 0.5. The actual thickness that SolidWorks applies is 0.In real terms, 5 × the current document unit. Even so, if the part file is opened in a metric environment, the same equation now yields a thickness of 0. Practically speaking, 5 mm instead of 0. 5 in, and the sketch may become under‑defined or over‑defined in ways that are not obvious from the equation itself.
Design tables operate on the same principle. A spreadsheet row that reads
PartLength, 12.0
does not carry any unit qualifier. 0 is interpreted in the new system, potentially inflating or shrinking every feature that references that table entry. When the part file is saved with a different unit system, the value 12.The safest approach is to keep a dedicated “unit” column in the table or to embed the unit directly in the cell (e.g., “12 in”) and then use a custom property or a separate conversion factor in the model to translate that value into the document’s native units.
Global variables behave similarly. A variable defined as
Diameter = 100
will always represent 100 of whatever unit the part is currently set to. Day to day, if the part file is later saved as a different unit system, the diameter changes accordingly, which can break downstream assemblies that expect a fixed size. In practice, to prevent this, many teams store the unit explicitly in the variable name (e. Plus, g. , Diameter_mm) or create a separate conversion variable (e.g., Diameter_in = Diameter_mm / 25.4) and reference the conversion variable in all downstream equations.
Best‑Practice Checklist for Units
- Set the document units once – verify the “Default units” dropdown matches the shop’s standard before opening any files.
- Lock drawing units – keep the “Use document units” option enabled unless a specific override is required, and never edit dimensions in a drawing that has an overridden unit setting.
- Document dual‑dimension intent – if a drawing must show both metric and imperial, note the primary system in the drawing title block and include a conversion note near any critical dimensions.
- Treat equations and tables as unit‑agnostic – always multiply or divide by a known conversion factor to translate between systems, and store the conversion factor as a separate global variable for easy updates.
- Audit imported files – after importing STEP, IGES, or other neutral formats, open the part’s property manager and confirm the units displayed match expectations; if not, re‑import with the correct unit setting or apply a manual scale correction.
Conclusion
Units are the invisible foundation of every SolidWorks model, drawing, equation, and table. Worth adding: a mismatched unit setting can silently reshape geometry, corrupt design intent, and generate costly rework that could have been avoided with a simple checklist. By establishing a clear unit standard, confirming it on every file open, respecting the separation between model and drawing units, and explicitly handling conversions in equations, design tables, and global variables, engineers eliminate the most common source of hidden errors. When units are managed deliberately, the full power of SolidWorks—precise modeling, accurate documentation, and seamless collaboration—can be realized without surprise.
