STL, 3MF or STEP: which file to download for your 3D printer

The short answer

The rest of the article explains why, and what to check in each case.

STL: just the shape

STL is the oldest and most universal format in 3D printing. It describes the surface of the part as a mesh of triangles, and nothing else: it stores no units, color, material or print settings.

Its big advantage is that simplicity. It opens in any slicer, from any era, and every model site offers it. Its limits come from the same simplicity:

• No units. The slicer assumes millimeters. If the part opens tiny, it was probably exported in inches: scale it by 2540% (25.4 times).
• Curves become polygons. A round hole is a polygon with many sides. If the author exported at low resolution, the facets show up on the printed part.
• Editing is surgery. Changing the size of a hole in a triangle mesh takes work and rarely comes out exact.
• Broken meshes. Holes and flipped faces happen. Slicers usually repair them on their own, but a missing layer or a strange wall in the preview may be a bad mesh, not a bad profile.

3MF: the shape and the project

3MF was created by an industry consortium precisely to fill STL's gaps. Inside, it's a compressed package (rename a copy to .zip and open it, if you're curious) that stores:

• the geometry, with defined units;
• several parts in a single file, with the position of each one on the bed;
• colors and materials, including per-face painting for multicolor printing;
• and, depending on the slicer, the whole project: printer, filament and process profiles, painted supports, modifiers and plates.

PrusaSlicer, Bambu Studio and OrcaSlicer save projects as 3MF; Cura opens the format too. But be aware: the settings part belongs to each slicer. A 3MF saved in Bambu Studio and opened in another program may bring only the geometry. That's normal, not a broken file.

Don't confuse a project 3MF with an already sliced file. Bambu Studio, for example, exports the sliced plate as .gcode.3mf: that's instructions for a specific printer model, not a model for you to slice again.

The profile inside a 3MF: what to check

The publisher's profile is a starting point, not an order. It was tuned for the author's machine, filament and taste. Before you hit print, check:

1. Printer and nozzle. A profile for a 0.6 mm nozzle won't work as is for a 0.4 mm nozzle.
2. Filament. PLA, PETG or something else? If your brand is different, the temperature that works for you may be different too.
3. Layer height, walls and infill. The author may have chosen a nice finish for the photo, and you need strength for the customer. Or the other way around.
4. Supports and brim. See whether they're on and where. Unnecessary supports are minutes of your finishing time.
5. Plates. Some 3MF files carry more than one plate. Make sure you're printing all the parts.
6. Time and grams. The slicer shows both after slicing. Take them to the cost calculator before deciding on a price.

When the project's printer differs from yours, the slicer usually asks whether you want to load the file's settings or just the geometry. If your machine is a different one, keep the geometry and apply the profile you've already tuned. A profile calibrated on your machine is worth more than any author's.

STEP: for changing dimensions

STEP (.step or .stp) is a CAD format, standardized as ISO 10303. Its difference from STL is one of nature: it stores exact surfaces. A hole is a true circle, a flat face is a plane, and you can measure and change them.

It opens in CAD programs such as Fusion, FreeCAD, Onshape and SolidWorks, and it's the right format when you need to:

• widen a hole for the screw you have in your drawer;
• adjust the clearance of a fit that came out tight on your machine;
• adapt the part to the size a customer asked for.

Two caveats. A STEP file usually doesn't carry the construction history: you edit the final geometry, not the author's feature tree. And it only really exists for mechanical and functional parts. Sculptures and organic shapes are born as meshes, and there's no useful STEP of them.

Some recent slicers already import STEP directly and convert it to a mesh on the fly. But for just printing, it has no advantage over STL; the gain is in editing.

Editing is also a matter of license rights, not just format. Many licenses forbid modifying the file. At STL Prime, the license provides the STEP, when the model has one, only on the plans that allow editing; the details are in the file license.

OBJ and the rest

OBJ is also a mesh, like STL, but it can carry color and texture in a material file alongside it. It's common in digital sculpting, 3D scanning and game art. For single-color FDM printing, it works just like an STL, and it doesn't store units either.

Others you'll come across:

• Native CAD files (.f3d, .sldprt, .FCStd): they carry the full history, but only open well in the program they came from.
• G-code: it's not a model, it's the list of commands for a specific machine. Don't print someone else's G-code blindly: temperature, speed and even the bed size may not match yours.

Which to download, case by case

For producers: save your own 3MF

This is where 3MF becomes a production tool. Once a part is dialed in, save your project as 3MF: the plate laid out with as many parts as fit, your profile, the filament, the supports. On the next order, it's open, slice and print. Time and grams repeat, and the cost in the calculator stays valid.

A few simple organization rules:

• Keep the original downloaded file in one folder and your projects in another.
• Put what matters for production in the name: coaster_PETG_nozzle04_8pc_v2.3mf.
• Changed the profile? Save a new version instead of overwriting the one that worked.
• Next to the folder, keep a note with time, grams and selling price.

One caution: the 3MF you saved is still the model's file. Sending that project to someone else is passing on the file, even with a profile you changed. If the license forbids passing it on, that applies to your project too.

Frequently asked questions

STL or 3MF, which is better?

To print on the same printer brand as the author, 3MF saves time. For any other machine, both work; what matters is your profile.

Can I convert an STL to STEP?

Yes, but the result is a solid made of the same triangles, hard to edit. To change dimensions precisely, you need to remodel it or have the original STEP.

Does 3MF open in Cura?

It does. The geometry always comes through; settings saved by another slicer may not.

The right format is the one that carries only what you'll use: the shape, the project or the dimensions.

Read next

• Commercial STL license: what you can sell (and what you can't)
• Common 3D printing failures and how to avoid them
• How to use the 3D printing cost calculator (and find out if a part is profitable)