Unit mismatch is the most common reason a STEP or IGES model imports at the wrong size. A part designed in inches opens 25.4× too large in a millimeter-based tool. A millimeter model appears 1000× too small in a tool expecting meters. In both cases the geometry is correct - the coordinate values are simply being interpreted in the wrong unit.
The fix is to re-export the STEP or IGES file with the correct unit declaration. Autoconverter opens STEP and IGES files, lets you change the model units with the Convert Units function, and exports the result in the target unit - rescaling all coordinate values correctly so the model opens at the right physical dimensions in the receiving tool.
Why STEP and IGES Files Have Unit Problems
STEP and IGES both store the unit of measurement in the file header. When a tool opens the file, it reads the header unit and interprets the coordinate values accordingly. Unit problems occur in three situations:
- The header unit is wrong - the exporting CAD tool wrote the wrong unit declaration; the coordinate values are in millimeters but the header says inches. The receiving tool imports at the stated unit, producing a model 25.4× the wrong size.
- The receiving tool ignores the header - some older tools import STEP/IGES coordinates as-is and assume a default unit (usually millimeters). An inch-based STEP file with correct header produces a model 25.4× too small in these tools.
- Mixed-unit pipelines - mechanical CAD (SolidWorks, CATIA) typically works in millimeters; architectural/civil CAD often works in meters; game engines and rendering tools default to meters. Converting between these pipelines requires explicit unit rescaling.
Supported Unit Conversions
Autoconverter supports the following units for STEP and IGES import and export:
| Unit | Symbol | Typical Use Case |
|---|---|---|
| Millimeters | mm | Mechanical engineering, manufacturing, 3D printing (SolidWorks, CATIA, Creo default) |
| Centimeters | cm | Some architectural tools, product design |
| Meters | m | Civil/infrastructure engineering, game engines, rendering (Unreal Engine, Blender default) |
| Inches | in | US mechanical engineering, legacy CAD (Imperial ANSI standard) |
| Feet | ft | US architectural and civil engineering |
How to Change Units in a STEP or IGES File
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📥 Download and Install
Install and launch Autoconverter from the Windows Start menu. The interface shows the file menu, ribbon bar, properties manager on the right, and 3D model viewer.
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📂 Open your STEP or IGES File
Go to File > Open…, select your .stp, .step, .igs, or .iges file. The model loads into the 3D viewer and the Properties Manager populates with file properties including the detected units.
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📐 Check the Current Units
Check the current units in the Properties Manager. The Units field shows what Autoconverter read from the file header. If this matches what you expect but the model is wrong-sized, the source tool wrote incorrect unit data.
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📏 Change the Units
Click Convert Units and select the target unit. Autoconverter rescales all coordinate values to match the new unit declaration. The model's physical dimensions remain the same; the numerical values change to express those dimensions in the new unit.
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📤 Export the Updated File
Go to File > Save As…, select STEP (.step), IGES (.igs), or any other supported output format. The exported file contains the new unit declaration in the header and rescaled coordinate values.
Unit Conversion Scale Factors
When changing units, Autoconverter multiplies all coordinate values by the appropriate scale factor:
| From | To | Scale Factor |
|---|---|---|
| Inches | Millimeters | × 25.4 |
| Millimeters | Inches | × 0.03937 |
| Millimeters | Meters | × 0.001 |
| Meters | Millimeters | × 1000 |
| Inches | Feet | × 0.0833 |
| Feet | Millimeters | × 304.8 |
Frequently Asked Questions
Why does my STEP model import 25.4 times the correct size?
The model is stored in inches but the receiving tool is interpreting it as millimeters (or vice versa). Open the STEP file in Autoconverter, verify the current units in the Properties Manager, change to the correct target unit, and re-export. A 25.4× size discrepancy is always an inch/millimeter mismatch - 25.4 is the exact conversion factor.
Does changing units modify the geometry?
Yes - all coordinate values are rescaled so the model's physical dimensions remain correct in the new unit. A 100mm cube exported in meters becomes 0.1m - the same physical size expressed in different units. The geometry shape and proportions are unchanged; only the numerical coordinate values change.
What if the STEP file has the wrong unit declared in the header?
Open the file in Autoconverter. If the Properties Manager shows the wrong unit (e.g., "Inches" when the model was actually drawn in millimeters), first change it to the correct source unit, then if needed convert to the target unit for the output.
Is there a free trial?
Yes. The free evaluation version of Autoconverter supports up to 10 file conversions including unit conversion. The full licensed version provides unlimited conversions.
Summary
STEP and IGES unit mismatches cause models to import at the wrong scale - typically 25.4× (inch/millimeter) or 1000× (millimeter/meter). Autoconverter opens STEP and IGES files, allows you to convert the units, and exports the corrected file with rescaled coordinates and the correct unit header. The same unit change applies when exporting to any other format - OBJ, FBX, SKP, STL - enabling correct-scale conversion in a single step.
👉 Ready to fix your units? Download Autoconverter and try it free for up to 10 conversions.