Model Scale and Units

Why the size comes out wrong

An STL stores coordinates as plain numbers. A vertex at 25.4 could mean 25.4 millimeters or 25.4 inches; the file does not say. The program that opens it has to assume a unit, and when that is not the unit the model was exported in, every dimension is off by the same factor. Exporting in millimeters, or as 3MF, removes the guesswork. See STL, 3MF, OBJ, STEP and G-code for what each format stores.

3MF fixes this. The 3MF core specification gives every model a unit attribute (micron, millimeter, centimeter, inch, foot or meter, with millimeter as the default) Maker spec, so the slicer knows how to read the numbers. When your software can export 3MF, use it.

Read the factor, find the cause

The amount a model is off by usually tells you what went wrong:

Model opens	Likely cause	Scale by Formula
25.4 times too small	Drawn or exported in inches, read as millimeters	2540 percent
10 times too small	Exported in centimeters	1000 percent
1000 times too small	Exported in meters	100,000 percent
25.4 times too large	Drawn in millimeters, read as inches by a program set to inches	3.937 percent

If the factor is something odd, such as 2 or 0.8, the file is not a unit problem. It was simply modeled at a different size; ask the designer or measure a part it has to fit.

Export at the right size

Fusion

Fusion's Save As Mesh (right-click a body or component in the Browser) asks for a format (3MF, STL binary, STL ASCII or OBJ) and a Unit Type: centimeter, millimeter, meter, inch or foot. Choose millimeter for STL, and confirm the size when it opens in the slicer.

Blender

Blender's STL exporter has a Scale value and a Scene Unit option that applies the scene's unit scale to the exported data. The importer has the same two options. Before exporting, check what one Blender unit means in your scene's Units settings, then confirm the exported size in the slicer. Exporting a known 10 mm test cube once tells you whether your scene and exporter agree.

Tinkercad

Tinkercad's Workspace settings switch between Metric (millimeters), Imperial (inches) and Bricks, where one Brick is 8 mm. Check the unit before you draw, so the numbers you type mean what you think.

Other CAD programs

Most CAD exporters have a unit setting in the export dialog. Look for it before you click save, and prefer millimeters or 3MF.

Rescale in the slicer

• Use the slicer's scale tool with the exact factor from the table, not by eye. Keep the scale uniform (locked) unless you mean to stretch the part.
• In PrusaSlicer 2.6 or newer, the Measurement tool can check a dimension and scale the object uniformly to the length you need by clicking the measured distance.
• Then measure a feature whose real size you know, such as a hole for a specific bolt.

What scaling does to function

Uniform scaling multiplies every dimension by the same factor, including the ones that should not change Formula:

• Walls get thinner when you scale down. A 1.2 mm wall at 50 percent is 0.6 mm, which may drop below what your printer can draw; see Wall Thickness and Minimum Feature Size.
• Clearances shrink too. A 0.3 mm gap at 50 percent is 0.15 mm, and a print-in-place joint may fuse.
• Threads and fastener holes stop fitting standard hardware, because the diameter and the pitch both change. Redraw them at the new size instead; see Threads and Snap Fits.

Correcting a unit mistake is safe: you are restoring the size the designer intended. Changing the size of a functional part is a redesign; do it in the source file, or check every fit after scaling and test with a dimensional accuracy check.