STL, STEP or 3MF: what each file is, and which to upload
Any of them works. Upload STEP if you designed the part in CAD software, 3MF if it comes from a slicer or a model site, and STL for anything else. Our quote page measures the file and shows your price in seconds. If the price looks wrong, the cause is usually one of the file problems below.
The formats at a glance
| Format | Usually comes from | What we read from it | Watch out for |
|---|---|---|---|
| STEP (.step, .stp) | CAD software: Fusion, SolidWorks, Onshape, FreeCAD | The exact solid shape, its units and its true volume | Exporting surfaces instead of a solid body. We can't measure those, so they go to a manual quote. |
| 3MF | Slicers such as Bambu Studio, PrusaSlicer and OrcaSlicer; model sites such as MakerWorld and Printables | The mesh, its units, and any supports you've already added | Nothing much: it's the most complete mesh format. If it contains supports, we can print with them. |
| STL | Everything: every CAD program, slicer and model site | The mesh only, with no units and no settings | Holes in the mesh, and parts drawn in inches or centimetres |
| OBJ | 3D scans, Blender and other modelling tools | The mesh only | The same as STL. Colour and texture data is ignored. |
Of the 37 most recent quotes on our system (October 2026), 20 came from STL files, 13 from STEP files and 4 from 3MF files.
What is an STL file?
An STL file describes the surface of a 3D object as a list of flat triangles, and nothing else: no units, no colour, no print settings. It is the oldest 3D printing format, named after stereolithography, and every CAD program, slicer and model site can export it. That is why it's still the file we get most.
For a fairfab order, we read an STL's numbers as millimetres. If the mesh is closed (watertight) and has up to 600,000 triangles, we price it from its measured volume. If it has holes, or is more detailed than that, we estimate from its size instead. To check an STL before you order, open it in our free STL viewer and cost calculator.
What is a STEP file?
A STEP file (.step or .stp) is the standard way to move a design between CAD programs. Unlike STL, it stores the part's exact shape: true curves, flat faces and edges, plus the units it was drawn in. Fusion, SolidWorks, Onshape, FreeCAD and Inventor all export STEP.
A slicer can't print a STEP file directly, because it needs triangles. We do that conversion ourselves and measure the volume from the CAD solid, so a STEP file gets the most accurate price we can give. To look inside one without CAD software, use our free STEP file viewer.
What is a 3MF file?
A 3MF file (3D Manufacturing Format) is a newer 3D printing file that packs the mesh together with its units, the objects to print and the slicer's print settings, all in one compressed file. Bambu Studio, OrcaSlicer and MakerWorld save 3MF by default, which is why so many more people now have one.
For a fairfab order, upload the 3MF as it is. We read its units, so there's no inches-or-millimetres guesswork, and we price a closed mesh from its measured volume, just like an STL. Everything in the file is priced together as one item, and you choose the material and colour when you order.
3MF vs STL: which is better?
For 3D printing, 3MF is the better format: it carries its units and settings and is usually a smaller file. STL's advantage is that almost every 3D program opens it. Both describe the part as triangles, so the printed shape is the same.
- Send us a 3MF if your slicer or the model site gives you one. It rules out unit mistakes.
- Send an STL if that's what you have. We price it the same way.
- Need an STL for a slicer or service that won't take 3MF? Our free STL converter converts 3MF in your browser, without uploading it.
STEP vs STL: which should you send?
Send the STEP file if you have one. An STL is an approximation of your part: every curve is replaced by flat triangles, and the units are lost. When our STL converter turns a STEP file into STL, a 20 mm round pin becomes 45 flat sides, each within 0.03 mm of the true curve. That is fine for printing, but you can't get the curve back from the STL. Keep the STEP as your original, and export STL only when a slicer or another service asks for it.
What happens when you upload
- File checks. We take .stl, .step, .stp, .3mf and .obj files up to 100 MB. Empty files and other file types are turned away straight away.
- Size. We measure the part's bounding box. Parts up to 180 mm in each direction get an instant price, and we try turning a part diagonally on the print bed if that makes it fit. We print up to 330 mm; anything bigger than 180 mm is quoted by hand.
- Volume. Most of the price is plastic and machine time, so we measure how much material the part needs. STEP files are measured from the CAD solid. STL, 3MF and OBJ files are measured from the mesh, as long as it's closed (watertight).
- Open meshes. If a mesh has holes, its volume can't be measured. We still price it, using a cautious estimate based on its size. Repairing the file gets you a measured, more accurate price.
- Files we can't read. If the file can't be analysed automatically, you can send it for a manual quote and we'll reply by email.
Common file problems and how to fix them
- The part is tiny or huge. STL and OBJ files have no units, so we read their numbers as millimetres. A part drawn in inches arrives 25.4 times too small; one drawn in centimetres, 10 times too small. Check the size on the quote page, then re-export in millimetres. 3MF and STEP files store their units, and we convert them for you.
- The mesh isn't closed. Gaps, flipped faces or overlapping shells are common in scans and in models stitched together from parts. Most slicers have a repair option, and Blender's free 3D-Print Toolbox can close holes too (“Make Manifold”). Exporting from your CAD program as STEP avoids the problem altogether.
- A STEP file won't analyse. This usually means it contains surfaces rather than a solid. Export the solid body, or send an STL or 3MF instead.
- The part is bigger than 180 mm. Upload it anyway and request a manual quote, or split it into pieces in your CAD program. Big flat parts sometimes fit diagonally.
- Several parts in one file. We price everything in the file together as one item. If you want parts in different colours or quantities, upload them as separate files. You can add several at once, and they share one checkout.
Which should you choose?
- You designed it in CAD: export STEP. It's the most accurate, and you won't get unit or mesh problems.
- You downloaded it, or you've set it up in a slicer: upload the 3MF if there is one, otherwise the STL.
- It's a scan: repair the mesh first, then upload STL, OBJ or 3MF.
Need an STL for another slicer or service? Our free STL converter turns STEP, 3MF, OBJ and GLB files into STL.
Not sure about a file? Message us on WhatsApp or call 07356 011581, and we'll have a look before you order.
Questions
Which file should I upload for 3D printing: STL, STEP or 3MF?
Any of them works. Upload STEP if you designed the part in CAD software, 3MF if it comes from a slicer or a model site, and STL for anything else. fairfab measures the file and shows a price in seconds.
What is the difference between 3MF and STL?
Both describe a part as triangles, so the printed shape is the same. A 3MF file also stores its units, the objects to print and slicer settings, and is usually smaller; an STL holds the triangles only. Send whichever you have: fairfab prices both from the measured volume when the mesh is closed.
Can I convert a STEP or 3MF file to STL?
Yes. fairfab's free STL converter at fairfab.org/tools/convert-to-stl turns STEP, 3MF, OBJ and GLB files into STL. You don't need to convert to order from us, though: we take STEP and 3MF files as they are.
Can you print from an STL file?
Yes. STL is the most common file we get. We read its numbers as millimetres and price closed meshes from their measured volume. If the mesh has holes or is very detailed (over 600,000 triangles), we estimate from its size instead, so a repaired file gets a more accurate price.
Do you accept STEP files?
Yes, .step and .stp. We measure the solid straight from the CAD data, which is the most accurate way to price a part. STEP files take a few seconds longer to analyse than mesh files.
What units should my file be in?
Millimetres. STL files carry no units, so a part drawn in inches arrives 25.4 times too small. 3MF and STEP files store their units, and we convert them. Always check the size shown on the quote page.
What is the largest file or part you can take?
Files up to 100 MB. Parts up to 180 mm in each direction get an instant price; we print up to 330 mm, and larger parts are quoted by hand.