The 25.4 problem: a part that is far too small
An STL stores bare coordinates. It has no field that says millimetres or inches, so every program guesses, and this viewer shows the numbers as millimetres. A 1 inch cube exported from an inch drawing therefore reads 1 × 1 × 1 mm, far too small in every direction.
Edit → Size → Inches → mm (×25.4) multiplies every coordinate by the inch factor, and the cube reads 25.4 × 25.4 × 25.4 mm. The opposite button, mm → inches (÷25.4), fixes a part that came in that much too large. STEP files are different: they declare a unit, and the STEP viewer converts it for you.
Three ways to resize an STL file
| Control in Edit → Size | What it does | When to use it |
|---|---|---|
| Scale … % | multiplies X, Y and Z by the same factor | you know the ratio: 50 % for half size, 200 % for double |
| Set X / Y / Z to … mm | sets one axis to that length, the other two follow in proportion | the part must be exactly 120 mm long or 35 mm tall |
| Inches → mm / mm → inches | multiply or divide by the inch factor | a unit mix-up between programs |
All three keep the triangle count. Scaling multiplies coordinates, so detail and smoothness stay as they were.
How to scale an STL to an exact size
- Drop the model on the viewer. The Info panel shows the current X × Y × Z.
- Press Edit; the Size section is open at the top.
- Pick the axis in Set X/Y/Z to, type the target in mm and press Apply.
- Read Info again, then Convert → STL or 3MF to download.
Set Z to 50 mm on a 20 mm cube and it becomes 50 × 50 × 50 mm. The factor is 2.5, so its volume grows from 8 cm³ to 125 cm³.
Volume and weight grow faster than size
Length scales with the factor, area with its square and volume with its cube. At 200 % a part is twice as long, but it needs 8 times the plastic and a much longer print. Better to see that before the spool runs out halfway through the job. Price the new size in the 3D print cost calculator before you start the job.
Going the other way, a 50 % copy keeps one eighth of the volume. Thin walls and small text shrink too, and a wall narrower than your nozzle line may vanish in the slicer.
Stretching one axis and fitting the bed
The Size tools always keep proportions. To stretch a part along one axis only, use the non-uniform scale in your slicer or change the CAD model. When a part is too big for the bed, scale it down here or split it with the STL splitter. Our slicer refuses a model larger than the bed and shows both sizes, so you see how far off it is.
Saving as 3MF writes the unit, millimetres, into the file. That keeps the corrected size intact when you hand the part to someone else. The STL format guide explains why STL cannot do that.
Questions and answers
Can you scale STL files without losing quality?
Yes. Scaling multiplies the coordinates of every vertex; the number of triangles and the shape stay the same.
How do I scale down an STL file?
Open it, press Edit and type a value below 100 in Scale %, for example 50 for half size. Or set one axis to the smaller size in mm, and the other two shrink with it.
My model imports far too big, about 25 times. Which button do I press?
Edit → Size → mm → inches (÷25.4). The part was drawn in millimetres and read as inches somewhere along the way.
Is the model uploaded to resize it?
No. The file is read and scaled by JavaScript in your browser tab, and the result is saved straight from there.
Does the scaled STL remember its new units?
STL stores no units, so the next program shows the numbers in its own default. Save as 3MF if you want millimetres recorded in the file.