# STL to 3MF converter

> Convert STL files to 3MF in your browser: a smaller, compressed file that records millimetres, so the model can't open at the wrong scale. Fix inch STLs on the way. Free, no upload.

Source: https://freestlviewer.com/stl-to-3mf/  
Updated: October 22, 2026

Convert .stl to 3MF — a compressed file that records its units, so it always opens at the right size. Converted in your browser, never uploaded. The tool is interactive: open https://freestlviewer.com/stl-to-3mf/ and drop a file onto the page.

## STL's two weaknesses, fixed

**No units.** STL stores numbers without saying what they mean. Slicers assume
millimetres, so a part exported in inches opens 25.4 times too small. 3MF
writes the unit into the file.

**Size.** STL repeats every corner for every triangle and is uncompressed.
3MF stores each vertex once and packs the whole model in a ZIP container.

## How to convert

1. Drop the `.stl` on the box above.
2. Look at the size in the summary. If a part that should be 60 mm long reads
   2.36 mm, it was exported in inches — set **The file is in** to inches. The
   converter suggests this when the model looks suspiciously small.
3. Keep **3MF** selected and click **Convert and download**.

The 3MF contains one object, in millimetres, with the same triangles as the
STL.

## Is the conversion lossless?

For geometry, yes: every triangle is kept at the same position (after the unit
scaling you choose). Identical corners are merged into shared vertices, which
is how 3MF stores meshes; that changes the file's structure, not its shape.

## When to keep STL

Send STL as well when the receiver uses old software, a CNC or CAM tool, or an
online service that asks for it. For your own printing and for sharing with
people on current slicers, 3MF is the better default.

## On a phone

STL to 3MF works in a phone browser. The Mes3D app for iPhone, iPad, Mac and
Android also sets a file's unit with one tap — including an inch-to-millimetre
fix — and exports 3MF, STL or OBJ.

## Frequently asked questions

### Why convert STL to 3MF?

3MF records the unit (millimetres), so the model cannot open 25.4 times too small; it is compressed, so it is usually several times smaller than the STL; and it can carry colours and several objects. All current slicers read it.

### How do I fix an STL that was exported in inches?

Drop it here, set The file is in to inches, choose 3MF and convert. The coordinates are multiplied by 25.4 and the 3MF records millimetres, so every slicer opens it at the right size from then on.

### How much smaller is the 3MF?

It depends on the mesh, but a 3MF is commonly a third to a fifth of the size of the binary STL, because shared vertices are stored once and the package is ZIP-compressed.

### Can every slicer open 3MF?

PrusaSlicer, OrcaSlicer, Bambu Studio, Cura and most other current slicers open 3MF. If someone is on very old software, send them the STL as well.