If your CAD tool or 3D printer shows “UM” and your print shop asks for “MM,” don’t panic—you’re seeing the same distance, just written differently. 1,000 μm = 1 mm. Need a quick fix? Just divide microns by 1,000.
What’s going on here?
UM and MM are just two ways to write micrometers (μm) and millimeters (mm).
Here’s the deal: a micrometer is one-thousandth of a millimeter, which makes it perfect for super-fine measurements like dust specs, hair thickness, or 3D-print layer heights. The micron symbol “μm” can be a pain to type, so many engineers and printers just write “um” (lowercase) or “UM” (uppercase). Even in 2026, most CAD packages, slicers, and inspection software still flip between both formats.
How to fix mismatched units
Convert between UM and MM by dividing or multiplying by 1,000.
- Double-check the unit in your software
- In Fusion 360 2026: Head to File → Properties → Units to see the model’s default unit.
- In Ultimaker Cura 5.8: Check Settings → Printer → Printer Settings → Print head size—it uses millimeters by default, so any value over 1 mm is in millimeters.
- Do the math
- See 250 UM but need MM? 250 ÷ 1,000 = 0.25 mm.
- See 0.05 MM but need UM? 0.05 × 1,000 = 50 μm.
- Update your model or profile
- In SolidWorks 2026: Go to Tools → Options → Document Properties → Units to switch between MM and UM; pick MM to match what your print shop wants.
- Export with the right unit
- Most STL or STEP file exporters default to millimeters. If your contract requires microns, just add a note in the filename, like “bracket_025mm_250um.stl”.
Still stuck?
Try these troubleshooting steps if the conversion isn’t working.
- Check your slicer’s layer height If your 3D printer’s slicer (PrusaSlicer 2.8, Bambu Studio 1.9) shows layer height in UM, switch the display to millimeters in Printer Settings → Custom G-code → Layer height.
- Look at the drawing tolerance In AutoCAD 2026, open Annotate → Dimensions → Tolerance; if the tolerance is in microns, convert it by dividing by 1,000 before exporting a PDF for the machine shop.
- Use a free online converter Type “250 um to mm” into any search engine and copy the result (0.25 mm) into your CAM or slicer.
How to avoid this mess next time
Standardize your units to prevent future confusion.
- Pick one unit for your team—millimeters or micrometers—and stick with it. Enforce this in template files, printer profiles, and purchase orders.
- Add a unit hint to filenames. Just tack on “_mm” or “_um” so the next person knows what they’re dealing with.
- Set your CAD software’s default units to millimeters. (A human hair is ~70 μm, so anything smaller than 0.1 mm is probably worth writing as microns.)
- Before sending a drawing to a supplier, do a quick unit check: Edit → Find → Search for “um” or “mm” and make sure every dimension is in the right unit.
