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 3D printing services accept for quoting and printing. It stores only the shape, with no units, colors, or materials, and the printing convention is to read it in millimeters.

What resolution should I use when exporting an STL?

Set the deviation, sometimes called chord height, between 0.01 and 0.05 mm. That keeps curves smooth on the printed part without bloating the file. Finer settings add file size but no visible quality, because FDM lines are 0.4 mm wide.

Why is my STL file so large?

The export resolution is set too fine. A deviation below 0.01 mm creates millions of triangles that describe detail smaller than the printer can make. Re-export with a deviation near 0.05 mm and the file usually shrinks by 10 times or more with no visible change.

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 engineering review, design edits, or a critical dimension held, because STEP carries the exact CAD geometry instead of a triangle approximation.

Guide

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

An STL file describes the outside surface of your part as a mesh of small triangles. It is the standard upload format for 3D printing. STL stores no units and no colors, just the shape. The printing convention is millimeters, so a part drawn in inches must be scaled at export or it arrives 25 times too small. Every CAD tool can export one in a few clicks.

How to export an STL from your CAD tool

Every mainstream CAD tool has STL export built in. In Fusion 360, right click the body and pick Save as Mesh. In SolidWorks, use Save As and pick STL, then check the options for units and quality. Onshape exports from the tab menu, and FreeCAD from File then Export.

Two settings matter: units and resolution. Set units to millimeters. Set resolution to fine or set the deviation to about 0.01 to 0.05 mm.

Export binary STL, not ASCII, when the tool asks. Same shape, much smaller file, faster upload.

STL resolution: how fine is fine enough

STL resolution controls how closely the triangles follow your curves. Too coarse and round features show flat facets on the printed part. Too fine and you get a 100 MB file with no visible gain.

A deviation (also called chord height) of 0.01 to 0.05 mm is the sweet spot. The printer lays plastic in 0.4 mm wide lines, so detail finer than that never reaches the part anyway.

If your export tops 100 MB, coarsen the deviation a step. The shape you lose is smaller than the plastic can show.

Units: why your part uploads 25 times too small

STL files carry no unit label. Printers and quote tools assume millimeters. A part drawn in inches and exported without scaling reads as 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. Re-export with units set to millimeters, or scale by 25.4 before export.

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

Broken meshes: holes, flipped faces, and what we do about them

A printable mesh must be watertight. Every edge joins exactly two triangles, with no holes and no faces pointing the wrong way. CAD exports are usually clean. Meshes edited by hand or downloaded from model sites often are not.

When our checker cannot measure a solid volume, it says so in plain words and prices from the bounding box as a fallback. Small holes we repair on our side. Badly broken meshes route to a human quote.

Want to fix it yourself first? Run the file through a free repair pass in your slicer or a tool like Meshmixer, then upload again.

STL vs STEP vs 3MF: what to upload

We accept STL, OBJ, 3MF, and STEP. STL gets you an instant price because we can measure it on the spot. STEP and 3MF route to a fast human quote instead.

Keep the STEP file in your back pocket either way. It is the exact CAD geometry, so it is the right file to send when a part needs design edits or a critical dimension checked.

For most orders the answer is simple. Upload the STL for the instant price. Attach the STEP in a review request when precision talk is needed.

Quick takeaways

Export STL in millimeters at 0.01 to 0.05 mm deviation, binary format.
A part that uploads 25 times too small was drawn in inches. Re-export in mm.
Files over 100 MB are over-resolved. Coarsen the export a step.
Watertight meshes quote instantly. Broken ones fall back to a human check.
STL gets the instant price. STEP is the file for design edits and critical dimensions.

Have a part to print? Get an instant price.

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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 3D printing services accept for quoting and printing. It stores only the shape, with no units, colors, or materials, and the printing convention is to read it in millimeters.
What resolution should I use when exporting an STL?: Set the deviation, sometimes called chord height, between 0.01 and 0.05 mm. That keeps curves smooth on the printed part without bloating the file. Finer settings add file size but no visible quality, because FDM lines are 0.4 mm wide.
Why is my STL file so large?: The export resolution is set too fine. A deviation below 0.01 mm creates millions of triangles that describe detail smaller than the printer can make. Re-export with a deviation near 0.05 mm and the file usually shrinks by 10 times or more with no visible change.
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 engineering review, design edits, or a critical dimension held, because STEP carries the exact CAD geometry instead of a triangle approximation.