ImgToSTL logoImgToSTL

STEP to STL Converter for 3D Printing

Turn a .step or .stp file from SolidWorks, Fusion, Onshape or FreeCAD into a binary STL at its real size, with assembly parts kept apart if you want.

Loading 3D preview
Drag to rotate

Converted in your browser. Files are never uploaded.

Next steps

Continue with your model

Inspect the CAD file first, or prepare the STL parts for your slicer.

STEP viewer

STEP and STP files, size, print check

STL to 3MF

Pack parts for Bambu and Prusa

IGES to STL

IGS surfaces and solids to STL

How it works

How to convert STEP to STL

  1. 1

    Add your STEP files

    Drop .step or .stp files, as many as you like. The CAD reader loads once, with the first file.

  2. 2

    Check size and parts

    The size reads in millimeters from the file's own unit. Assemblies list their parts, and you can resize the longest side.

  3. 3

    Download the STL

    Save one binary STL per file, or a ZIP with one STL per part, resting on the build plate.

Features

Why use this STEP to STL converter

Made for CAD parts on their way to a 3D printer.

Private, in your browser

Your STEP file is converted on your own computer by OpenCascade in WebAssembly and never uploaded.

True size in millimeters

Inches, centimeters or meters in the file become millimeters in the STL, the unit every slicer assumes.

One STL or one per part

Merge an assembly into a single STL, or download a ZIP with a separate STL for each solid, named after the part.

Round holes and fillets

Curved faces are split finely: a 5.5 mm hole gets about 60 straight segments, so it prints round.

Format comparison

What carries over from STEP to STL

STEP describes a product for engineering. A slicer only needs the outer surface, so the shape and size come through and the engineering data stays behind.

Kept

  • Every solid, shell and surface in the file
  • Part positions in the assembly
  • Real size, converted to millimeters
  • Part names, as file names when split

Left behind

  • Exact curves, replaced by fine triangles
  • Part and face colors
  • Loose curves, points and sketches
  • PMI annotations, tolerances and metadata
Print checks

Four checks before you slice

A STEP part converts cleanly, but a model designed for machining is not always ready to print.

Real size

Compare the size with your build plate. Parts drawn for machining or sheet metal can be larger than a desktop printer.

Print orientation

The STL keeps the CAD orientation with its lowest point on the plate. Rotate it in the slicer so the largest flat face sits down.

Holes and fits

FDM printers tend to print holes slightly small. Leave about 0.2 mm of clearance for pins and screws, or drill holes out after printing.

Watertight check

Solids from STEP are normally closed. Open the file in the STEP viewer to confirm the watertight check before a long print.

Guide

STEP to STL guide

Formats, tessellation, units, assemblies and what to do when a file will not convert.

STEP and STL compared

STEP (ISO 10303) is the exchange format of the CAD world. It stores a part as a boundary representation: faces bounded by edges, each face an exact plane, cylinder, cone or NURBS surface, plus the unit, the assembly structure, part names and colors. STL stores none of that. It is a flat list of triangles that slicers read as millimeters.

Converting means keeping the shape and the size and letting everything else go. The table shows what each format can hold.

  • Geometry

    STEP
    Exact surfaces and solids
    STL
    Flat triangles

  • Units

    STEP
    Stored in the file
    STL
    Not stored, read as mm

  • Parts

    STEP
    Assembly with names and positions
    STL
    One unnamed surface

  • Color

    STEP
    Per part and per face
    STL
    None

  • Opened by

    STEP
    CAD and CAM programs, some slicers
    STL
    Every slicer and 3D app
Tessellation: from exact surfaces to triangles

A CAD surface is exact: a 10 mm hole is a true circle at any zoom. An STL can only hold flat triangles, so every curved face is tessellated, which means it is covered with triangles whose corners sit on the true surface. The gap between a triangle and the curve it replaces is the chord error. Flat faces and straight edges carry no error at all.

This converter keeps the chord error within 0.05 percent of the part's average size, and limits the angle between neighboring facets so small holes and fillets stay round. The result is far finer than a 0.4 mm nozzle can print, while the triangle count stays modest.

  • 20 mm

    Largest chord error
    0.01 mm
    Compared with a 0.4 mm nozzle
    40 times smaller

  • 100 mm

    Largest chord error
    0.05 mm
    Compared with a 0.4 mm nozzle
    8 times smaller

  • 500 mm

    Largest chord error
    0.25 mm
    Compared with a 0.4 mm nozzle
    Still smaller
Units: why the STL comes out the right size

A STEP file declares its unit, usually millimeters, sometimes inches or meters. An STL has no unit field, and every slicer reads its numbers as millimeters. The converter reads the declared unit and writes millimeters, so a 2 inch bracket arrives as 50.8 mm instead of 2 mm.

If the size readout still looks wrong, the part was modeled at the wrong scale in CAD. Turn on Resize longest side and enter the size you want in millimeters before you download.

Assemblies and multi-part prints

Each solid in a STEP assembly becomes a separate object, and the file list shows how many there are. With Files set to One per model, all of them merge into one STL in their assembled positions, which suits a single part or a display piece printed in one go.

Choose One per object for a ZIP with one STL per solid, named after the part. Each STL keeps its place in the assembly, so loading them together in Bambu Studio or PrusaSlicer rebuilds the layout, and you can drop the screws and nuts you plan to buy. The STL to 3MF converter packs the parts into one 3MF.

When a STEP file will not convert

If you see No triangles found, the file holds only curves or points, such as a sketch or a wireframe export, with no surface to fill. Export the solid body instead. If the file cannot be read at all, it may be damaged, for example cut short by a failed download. Export it again from the original program.

Large assemblies take longer, because every face is tessellated on your own computer. Files up to 300 MB are accepted. If a file still fails, export it again as AP214 or AP242 if your CAD program offers a choice.

FAQ

STEP to STL questions

Answers about accuracy, units, assemblies, slicers and privacy.

How accurate is the STL?
The triangles stay within 0.05 percent of the part's average size from the true surface: 0.05 mm on a 100 mm part. Flat faces and straight edges are exact.
Why is my STL the wrong size?
The converter reads the unit from the STEP file and writes millimeters, so the size matches the CAD model. If the part itself was modeled at the wrong scale, turn on Resize longest side and type the size you want in millimeters.
Can I convert a STEP assembly into separate STL files?
Yes. Choose One per object under Files to get a ZIP with one STL per solid, named after the part. Every STL keeps its place in the assembly, so the parts line up again when loaded together.
Do I need to convert STEP to STL for my slicer?
PrusaSlicer, Bambu Studio and OrcaSlicer import STEP directly. Cura and many other slicers need STL, and an STL is also the safest file to send to a print service.
Is the STEP to STL converter free and private?
Yes. It is free with no sign-up, and the conversion runs in your browser. Your file is never uploaded; only the OpenCascade reader is downloaded the first time.