What is an STL file in 3D printing?

An STL file describes the surface of a 3D model as a mesh of triangles. It is the standard file that 3D printing services accept for quoting and printing. It stores the shape only. No units, no colors, no material. The printing world reads it in mm.

What resolution should I use when exporting an STL?

Set the deviation, sometimes called chord height or tolerance, between 0.01 and 0.05 mm. That keeps curves smooth on the printed part without bloating the file. Finer settings add file size and nothing else. FDM lays plastic in lines about 0.4 mm wide, so it cannot show detail below that.

Why is my STL file so large?

The export resolution is set too fine. A deviation under 0.01 mm makes millions of triangles. They hold detail far smaller than the printer can print. Re-export near 0.05 mm and the file usually shrinks by ten times or more with no visible change to the part.

Should I send an STL or a STEP file to a 3D printing service?

Send the STL when you want an instant price, since it can be measured automatically. Send the STEP when the part needs a review, an edit, or one measurement held. STEP carries the exact CAD math, not a triangle stand-in. Sending both is fine, and it is often fastest.

Can you print a 3MF file?

Yes. A 3MF uploads like any other file and gets an instant price. We convert it to an STL first, then it rides the same path. A 3MF also carries its own units, which removes the most common scaling mistake. If the file carries more than one color, it gets flagged, since we print one color per part.

Why does my part show up tiny in the viewer?

It was drawn in inches and exported without scaling. An STL carries no unit label, so quote tools read it as mm. That makes an inch-based part 25.4 times too small. Export it again with the units set to mm, or scale the model by 25.4 first.

What do I do if my mesh is not watertight?

Try the repair option in your slicer first. If that fails, run the file through Microsoft 3D Builder or Autodesk Meshmixer. Both are free. The best fix is to export again from the CAD file. That kills the problem at the source. You can also upload as is, and small holes get repaired on our side.

STL files for 3D printing: export settings that just work

By the 3D Printer on Demand team · Updated July 2026

An STL file holds the outside surface of a part as a mesh of small triangles. It is the standard upload format for 3D printing. An STL stores the shape and nothing else. No units, no colors, no material. The printing world reads it in mm. So a part drawn in inches has to be scaled at export, or it shows up 25 times too small. Every CAD tool can write one in a few clicks. This page shows where that button lives, what to set, and how to fix a file that will not quote.

How to export an STL from your CAD tool

Every mainstream CAD tool has STL export built in. The menu name changes, the settings do not. Find your tool below.

Where the STL export lives in the CAD tools our customers use most.
CAD tool	Where the export lives
Fusion 360	Right click the body, then Save as Mesh
SolidWorks	File, Save As, pick STL, then open Options
Onshape	Right click the part or the tab, then Export
Autodesk Inventor	File, Export, CAD Format, pick STL, then Options
Creo	File, Save As, Save a Copy, pick STL
Solid Edge	File, Save As, Save Copy As, pick STL
FreeCAD	Select the body, then File, Export, pick STL
Rhino	File, Export Selected, pick STL
Blender	File, Export, pick STL, then set the scale
SketchUp	Add the free STL extension, then File, Export STL
Tinkercad	Export, then .STL

Set the units to mm. This is the setting that goes wrong most often.
Set the deviation, sometimes called chord height or tolerance, to 0.01 to 0.05 mm.
Pick binary, not ASCII, when the tool asks. Same shape, much smaller file.
Export one part per file unless you want the parts printed as one piece.
Name the file with the part number and the revision. It saves a mix-up later.

STL resolution: how fine is fine enough

Resolution sets how closely the triangles hug your curves. Set it too coarse and a round face prints with flat facets you can feel. Set it too fine and you get a huge file for no gain at all.

A deviation of 0.01 to 0.05 mm is the sweet spot for FDM. Here is why. The printer lays plastic in lines about 0.4 mm wide. Detail finer than that line simply cannot reach the part. Chasing it just makes the file bigger.

If the export tops 100 MB, back the deviation off a step. The shape you give up is smaller than the plastic can show anyway. Very heavy meshes also slow the quote down, so a lighter file gets you a price faster.

What each STL export resolution setting is good for.
Deviation setting	What you get	Use it for
0.1 mm and coarser	Visible flats on curves	Blocky parts with no round faces
0.05 mm	Smooth to the eye and the hand	The safe default for most parts
0.01 mm	Smooth, larger file	Small parts with fine curves
Under 0.01 mm	Millions of triangles, no visible gain	Nothing on an FDM part

Units: why your part uploads 25 times too small

An STL carries no unit label at all. Printers and quote tools read it as mm. So a part drawn in inches and exported without scaling comes in 25.4 times smaller than you meant.

The tell is instant. Your 3 inch bracket shows up as a 3 mm speck in the viewer. The fix is just as fast. Export it again with the units set to mm, or scale the model by 25.4 first.

Our upload viewer shows the measured size in both mm and inches, right under the part. Check that number before you order. If the size looks wrong, the units are wrong.

The reverse happens too. A model scaled up by mistake lands far past our 220 mm build plate. The quote page flags it as oversized.

Broken meshes: holes, flipped faces, and what to do

A printable mesh has to be watertight. That means every edge joins exactly two triangles, with no gaps and no faces pointing the wrong way. Think of it as a balloon that holds air.

CAD exports are usually clean. Files edited by hand, pulled off a model site, or made from a 3D scan often are not. These are the four problems we see.

Free ways to repair a broken mesh before you upload it.
Tool	Good at	Cost
Your slicer's repair option	Small holes and flipped faces	Free
Microsoft 3D Builder	One-click repair on Windows	Free
Autodesk Meshmixer	Inspector tool, hole filling, loose shells	Free
Blender 3D Print Toolbox	Finding non-manifold edges	Free
Export again from CAD	Fixing it at the source	Free and always best

Holes in the mesh. A triangle is missing, so the surface has an opening and no real inside.
Flipped faces. A triangle points inward, so the software cannot tell inside from outside.
Loose shells. Extra floating bits sit near the part, usually left over from an edit.
A surface that crosses itself. Two parts of the shell pass through each other.

What happens when we cannot measure your file

Nothing bad, and nothing silent. Our quote page tells you in plain words.

When the checker cannot measure a solid volume, it says so and prices from the outside box as a fallback. That price is usually a little high, since a hollow part looks solid to a box. Small holes we can repair on our side. A badly broken mesh routes to a human quote, and our team gets back to you within four business hours.

Either way, you see the flag next to the price before you spend anything. Our 7 design checks page lists everything the checker looks at.

STL vs 3MF vs STEP vs OBJ: what to upload

We accept all four. Two of them price instantly, and two route to a human. The difference is whether we can measure the shape on the spot.

The four file types we accept, what each one stores, and how it gets priced.
Format	What it stores	Instant price?	Best for
STL	Surface triangles only, no units	Yes	The fastest path to a price
3MF	Triangles plus units and color, zipped	Yes, we convert it first	Anyone exporting from a modern slicer
STEP or STP	Exact CAD math with true curves	No, human quote	Design edits and critical measurements
OBJ	Triangles plus color and texture	No, human quote	Scans and art files

Upload the STL or the 3MF when you want a number right now.
Keep the STEP in your back pocket. It is the right file when a measurement has to be reviewed or the part needs an edit.
Sending both is fine and often the fastest route on a fussy part.
A 3MF that carries more than one color gets flagged, since we print one color per part.

A 60 second check before you upload

Run this list and most quote problems never happen.

Units are mm, and the longest side reads the size you expect.
Longest side is under 220 mm, or you are ready to ask for a large-format review.
Deviation was set near 0.05 mm, and the file is well under 100 MB.
The mesh is watertight, with no holes, flipped faces, or floating bits.
Walls are 1.2 mm or thicker. Our design guide covers the rest.
The file name says which part and which revision it is.
Any measurement that truly has to land is written in the order notes. See our tolerances guide for what FDM can hold.

Upload and get your price

Drop the file on our instant quote page. No login is needed to see a number.

PLA and PETG price instantly. ABS, PA-CF, and TPU get a fast human quote. Either way it is one flat price per part, held for 30 days, with the per-part price stepping down at 10, 50, 250, and 1,000 units. The tiers are on our pricing page.

Parts ship in 2 to 5 days, made in the USA. If a term on the quote is new to you, our glossary defines it in plain words.

Quick takeaways

Export STL in mm, binary, with the deviation set to 0.01 to 0.05 mm.
A part that uploads 25 times too small was drawn in inches. Export it again in mm.
A file over 100 MB has more triangles than it needs. Back the deviation off a step.
A printable mesh is watertight. Holes, flipped faces, and floating shells all break the quote.
STL and 3MF get an instant price. STEP and OBJ route to a fast human quote.
STEP is the file to send when a measurement has to be reviewed or the part needs an edit.
Check the measured size in the viewer before you order. If the size looks wrong, the units are wrong.

Have a part to print? Get an instant price.

Instant Quote

Common questions

What is an STL file in 3D printing?: An STL file describes the surface of a 3D model as a mesh of triangles. It is the standard file that 3D printing services accept for quoting and printing. It stores the shape only. No units, no colors, no material. The printing world reads it in mm.
What resolution should I use when exporting an STL?: Set the deviation, sometimes called chord height or tolerance, between 0.01 and 0.05 mm. That keeps curves smooth on the printed part without bloating the file. Finer settings add file size and nothing else. FDM lays plastic in lines about 0.4 mm wide, so it cannot show detail below that.
Why is my STL file so large?: The export resolution is set too fine. A deviation under 0.01 mm makes millions of triangles. They hold detail far smaller than the printer can print. Re-export near 0.05 mm and the file usually shrinks by ten times or more with no visible change to the part.
Should I send an STL or a STEP file to a 3D printing service?: Send the STL when you want an instant price, since it can be measured automatically. Send the STEP when the part needs a review, an edit, or one measurement held. STEP carries the exact CAD math, not a triangle stand-in. Sending both is fine, and it is often fastest.
Can you print a 3MF file?: Yes. A 3MF uploads like any other file and gets an instant price. We convert it to an STL first, then it rides the same path. A 3MF also carries its own units, which removes the most common scaling mistake. If the file carries more than one color, it gets flagged, since we print one color per part.
Why does my part show up tiny in the viewer?: It was drawn in inches and exported without scaling. An STL carries no unit label, so quote tools read it as mm. That makes an inch-based part 25.4 times too small. Export it again with the units set to mm, or scale the model by 25.4 first.
What do I do if my mesh is not watertight?: Try the repair option in your slicer first. If that fails, run the file through Microsoft 3D Builder or Autodesk Meshmixer. Both are free. The best fix is to export again from the CAD file. That kills the problem at the source. You can also upload as is, and small holes get repaired on our side.